Every young academic will sooner or later take a shot at applying for an ERC grant. ERC grants are considered to be amongst the most prestigious grants out there. It provides you sufficient funding to establish a high risk/high gain research line. Obtaining one is considered a game changer in your academic career: it is a prestigious award, it greases the wheels for getting tenure and it can even be used as a bargaining chip to secure a position elsewhere.

In this blog article, I aim to give you a -as honestly as my memory allows it- recollection of all the attempts I undertook to obtain an ERC grant in the past decade. I guess most of you will think it has always worked out for me the first time right; well, I can tell you that has rarely been the case. In life like in science, failures are rarely reported (although I always honestly mention it in my personal discussions with colleagues and young academics). However, failures happen frequently to me, as you will read below; my story is far from unique if you ask around. My hope is that this account provides young academics with some more insight into my ERC journey and gives them hope for their own ERC mission.

The Starting Grant

Like many, I was eager to roll the dice and try early on in my independent academic career to apply for the Starting Grant (ERC-2013-StG). I prepared by collecting as many successful proposals as I could get my hands on and trying to discover an underlying structure for what a winning proposal might look like. I brainstormed with colleagues about potential ideas I had, and after a few sessions, I felt confident that I had a bold idea that fit the high risk/high gain category.

I focused for the next few weeks, fervently writing the proposal and I even managed to rope in some colleagues for proofreading duty. Either they were just too nice or didn’t have the heart to say otherwise, but the feedback was unanimous that it was at least good enough to get an interview. Full of confidence, I submitted both B1 and B2 parts and I waited …

A few months later I got an email … from the ERC. Excited and nervous, I opened it and my face immediately dropped: shit … a B grade, indicating that the proposal was of high quality but not sufficient to be invited for an interview. Back then, you still had to wait longer to get the evaluation reports so I couldn’t even tell why it is was deemed “not good enough” (thankfully, that has changed in the meantime and now the decision and the reports are sent together). After a few weeks of waiting, I got the feedback. “The project targets the development and implementation of photo-redox catalysis to be used in microreactors, which is argued to improve the efficiency of the photochemical processes. The ideas are interesting and reasonably well based on state-of-the-art. Unfortunately, the panel finds that the project is described in a rather generic fashion.”

OK, I thought, this feedback is actually not so bad. So I’ll try again next year and in the intervening time upgrade the proposal. I worked really hard to get more details in the proposal to avoid the “generic” criticism. After seeking further advice from a growing list of colleagues, I was happy with the upgrades and submitted it online (ERC-2014-StG).

A few months later, the verdict was again negative: another B grade! What the f*** I thought. The feedback left me equally confused: “The proposal clearly reports very clever and very ambitious objectives, with an impact on large range of fields. The proposal is therefore high risk and high gain. However, it does not go beyond the state of the art”. Re-reading this feedback today, it still sounds contradictory: ambitious but does not go beyond the state of the art… huh???

I must admit that the second time, the psychological blow was much harder to take. The amount of time I had invested into this project was significant and the feedback although it seemed kind of good, it clearly wasn’t good enough. It certainly was hard to accept, but at the end of the day what can you do?…

I had a long, fruitful discussion with my colleague (Dr. Martin Timmer, may he rest in peace) about what to do next. Resubmitting seemed like it was not an option and anyway, I was now restricted from submitting for one year (a new ERC rule: a B grade means excluded for one year, a C grade for two). We came to the conclusion that the proposal was good enough and that maybe the problem was that many of the details were in section B2. Which is -no joke- not even read if you do not reach the interview stage (I always felt frustrated about that, especially because it was the longest, most labor-intensive part).

We decided to repackage the proposal and submitted it to VIDI, a personal grant scheme from the Dutch Research Council (€800,000  vs €1,5 M from ERC StG). To my delight, it got great referee feedback and following a good interview the funding was approved on the first try! This process also gave me the confidence that the original ERC proposal was in fact not bad and the ideas were good enough to compete in prestigious grant schemes.

So, after sitting for one year on the ERC sidelines, I had one final chance for an ERC Starting Grant (ERC-2016-STG). Since my previous version was granted by NWO, I had to come up with a new idea and thus rewrite the entire proposal. The idea was to develop a new type of microreactor which could harvest solar energy efficiently to drive photocatalytic transformations. The reactor was made of a light-harvesting and light-guiding material, called luminescent solar concentrator. Once this reactor was developed we also wanted to develop an integrated reactor design, where all equipment (pumps, computers, etc.) were using solar energy as well, and thus could be utilized off-grid. This time we also had some preliminary results which demonstrated the feasibility of the concept.

After submitting the proposal, I got the news from ERC a few months later. Yet again, the news was negative, the result a B; thus no interview and another penalty of a one year-ban from submitting. “The panel found the ideas proposed in the project interesting; however questions were raised about the project neglecting the difficulties in using the solar radiation concentration. The stability of the luminescent solar concentrators under direct sunlight was also considered a critical issue.” This verdict meant that my chances to obtain an ERC starting grant were over…

However, since I was convinced of the strength of this research idea, I doubled down on it and tried to carry it out with other funding sources, which were however, less royal. Together with my PhD student Dario Cambié (MSCA ITN Photo4Future funding), we carried out the ERC idea and everything worked exactly as described in the proposal leading to one of the most successful research lines in my group: the luminescent solar concentrator-based photomicroreactor concept^1,2,3.

Once the idea was executed, we had one of the few solar-driven reactors in the world. Funnily enough, due to this unique expertise, we are now asked by various consortia to contribute to their proposals, leading to a substantial amount of funding which has now eclipsed the initial ERC StG budget.

The Consolidator Grant

After failing the ERC StG three times and even not getting a single interview, one could get pretty demotivated. I must admit that I was frustrated, but only for a few hours/days after I received the news. However, as time passes, you get better at putting these things in perspective. As said above, there was always a silver lining: the feedback was not bad, the panel said nice things about my career, I was able to get backup funding via a different grant scheme and the ideas were fine resulting in new cutting-edge research lines. And like I jokingly told my group: I am going to get an ERC, or die trying!

So after my one year penalty was over, I decided to write a new proposal (ERC-2018-COG) for the Consolidator scheme (making it €2 M instead of €1.5 M). The idea was to develop synthetic methods and technology for the photocatalytic modification of peptides and proteins. I thought this idea was reasonable, as my student Cecilia Bottecchia had worked on some interesting methods for peptide modification (up to 20 residues, fully unprotected)^4,5,6. She actually helped me a lot with getting the proposal ready in time!

This time, the news that came from Brussels was nicer: I got my first interview! I was really excited and I actually started with my preparations right away: studying the literature, asking colleagues for potential questions, reading books about presentation skills and even following a training from Yellow Research (an organization that specializes in preparing candidates for the interview). Everything I could control, I did and I was ready for the battle!

The day of the presentation I was excited but not nervous. I went to Brussels by train, was a bit too early and just went upstairs to the floor where the interviews were to be held. I even met a friend who was also there for an interview so we chatted and I felt really relaxed. Then it was my turn, I went in and I thought my presentation went pretty smooth: proper pace, well-timed hand movements and perfectly within time. The first questions went really well. I still remember that after 10 min I was really proud and I thought it was in the bag. But then the questions about selectivity issues started and despite my answers, the same question was returned to several times. It seemed that the panel was rushing to ask them and I felt I barely had time to answer the questions properly. The final question was about the fact that I asked for specialized equipment and thus exceeded the budget (FYI, I asked for > 2.6 million, which is allowed if you need something special to carry out the research). When giving my answer, one panel member sighed loudly and said in an annoyed tone “everybody wants more money”. I stayed firm and said I needed it to be able to do the experiments.

You might wonder, how the h*** does he still know those details. Well, on the train back, I wrote in my notebook every single question I received and how I answered it (I even wrote the time I arrived by train, when I went in, etc.). I can really recommend that you do this, it is helpful for analyzing your performance when the final verdict comes in.

And yes, the final verdict came a few weeks later … I still remember that day vividly. I was coming back by car from a seminar in Münster and my phone was not loading any emails while driving. Maybe for the better as the news was negative. I think it was one of the hardest blows in my career. I had to lie down the rest of the day and I was literally in tears. How could it be?, I had done everything right: I worked for months to prepare those 25 minutes (10 minutes speech + 15 minutes questions), even on the beach in summer I was reading papers/books in preparation…

After a day or two being completely down, I once again found my fighting spirit. I knew a person who was in the panel and I contacted him after a week. The information I received was extremely valuable, these panel members can give you a lot of insight in the decision making process and they know the exact reasons why your specific case could not be retained. He also encouraged me to try again and he was even willing to read my proposal.

So the year thereafter, I tried again (ERC-2019-COG): same proposal and all the criticisms were addressed, which was confirmed by that former panel member. And yes, I got another interview. This time, I am ready… I thought. To my surprise, I received the same questions all over again: issues with selectivity… I remember thinking: Did I not fix this, did I not detail that properly this time?

At that stage, I already knew the decision. It was confirmed a few weeks later: again a no… But yet again, I knew one of the panel members so I asked again for personal feedback. This feedback was a game-changer. Two issues were clear to him. Firstly, the selectivity remained an issue and this could only be dealt with by way of proof-of-concept results. Secondly, I am not a specialist of protein modification. There was a doubt that I would be the right candidate to do this work; this criticism was espoused by half of the referees and thus it could not be ignored by the panel even though my past research in general was regarded as strong. For me it was now crystal clear, the first criticism I could potentially be able to fix but I would have difficulties in fixing the second. The latter would take me years to address as I needed to gather both the papers and the credibility in that space. So the only conclusion I could come to was to abandon this proposal for now (currently I am repackaging it and I am trying to get it funded in a consortium with chemical biologists).

I had to write a new proposal which was closer to what we were doing in the group. Maybe it is good to reflect here again. A proposal closer to your expertise seems obvious. However, if you are too close to what you do already, referees might question why the ERC should invest so heavily in something that you already do. From my personal interactions with ERC holders, I often received the feedback that you need to push the boundaries. However, from my own experience, if you push too far, you also do not get funded as it is not seen as credible. So, it’s really a tight balance between high risk/high gain and staying realistic.

We wrote another proposal, called FlowHAT (ERC-2020-COG) aiming to develop synthetic methods and technological tools that would provide a breakthrough in the selective functionalisation of strong carbon–hydrogen (C–H) bonds present in small organic molecules and biologically active molecules. We had some proof of concept results which were published in Science after the submission of the proposal⁷. So I felt confident the work was definitely high gain-material.

And yes, for the third year in a row, I got an interview. As in the years before, I meticulously prepared every detail of the interview: gathering questions from my team and colleagues, preparing the talk and doing a number of mock defenses. On the interview day itself (which was on zoom due to the Covid pandemic), I felt the presentation went well (which was without slides this time, this felt weird but ok you can prepare for it) and also I felt the defense was pretty good. The questions I got were easy to rebut and my feeling afterwards was extremely positive. I wrote in my notebook: “Panel members were nice to me and very positive about the proposal. I have confidence we will be close to being funded.”

To my shock, it was not funded once again. The feedback was formulated as follows “The panel considers the proposal of high quality and fundable; however it is not in a sufficiently high position in the ranking order to be retained for funding.” I could not disagree more with the final decision, especially because I felt the overall feedback was the most positive I had ever received and maybe we could have been funded with a bit of good-will. To me, the only problem seemed to be that the proposal was “medium-risk/high-gain”, as mentioned by one referee.

ERC CoG – The last chance

I had one final chance to get a consolidator grant (ERC-2021-COG) and after that I would be “too old”. The feedback from the previous year was the best I had ever received and I was wondering what could I still improve? I disagreed that the proposal was medium risk, so why did referees say that? Can I sway the referee opinion in the right direction by making some minor adjustments?

So this is what I did. First, I pushed the risk a bit further. Not too much as I thought we were close to getting funded. Second, I made risk assessment tables for each work package, in those tables I provided for every task: the risk level (low-medium-high), the contingency plan, and the scientific impact. My hope was that I could steer the referee’s opinion in the direction that I had in mind, i.e. a good balance between doable research and high risk/high gain goals.

And yes, I got another interview, which was to be held online on January, 18th 2022. As usual, I wanted to take no risks and I blocked my agenda for the two weeks before the defence. However, the closer we came to the interview date, the more nervous and cranky I became. I guess it was mainly because it was my final chance. In my head, the lines “don’t f*** it up” were on repeat which, of course, did nothing to help the situation.

On the day itself, the presentation went really well; I think it is the one part which one can actually control and therefore I like it. However, this time, I remained nervous. During the questions, which I could answer pretty well, I always had the feeling I had to work hard to get my answers out. Of the four years, this time I couldn’t accurately tell how I had done. I’d had too much bad luck in the past years to feel confident afterwards. In the weeks thereafter, I tried my best not to think about it anymore. I even had a nightmare in which I woke up in the middle of the night bathing in sweat: in my dream, I had again screwed it up.

The final answer came on Wednesday March 9. As Figure 1 shows, nothing can be found in the email itself. You had to login on the portal. And I dared not click on the link… When I did, I had to fill in my password, which of course, I did not remember. So I had to request another one. Time was ticking and my heart was going like crazy.

Figure 1. Email received from the European Commission.

Finally, I got in. And I started reading (Figure 2), I got an A and the proposal was ranked for funding. But I really didn’t like the “if sufficient funds are available” line. What did it mean? Is it now funded or not? The panel comment seemed to be going in the direction of funded: “The panel therefore recommends the proposal to be retained for funding with a grant not exceeding 2,000,000 Euro.” But I was still not sure, so I opened the president letter (Figure 3): it starts with a lot of generic information but then I read “I am pleased to inform you that your proposal was ranked at a sufficiently high position to allow it to be funded.”  I was now sure that it was granted and I let out a scream of pure joy!

Figure 2. Screenshot from the Evaluation Letter

Figure 3. The so-called president letter with the clear message: Proposal funded.

Conclusion

We are now one week after the fantastic news and I still cannot believe it. After 7 submissions, 4 interviews and 6 heartbreaks, I had finally got an ERC proposal funded. It is a story of six downs and one final, glorious up. I am grateful to the many people who helped me in the past years, including my research group members, my colleagues and all those who were prepared to give advice.

I hope that this story might be useful to you too in obtaining a grant, whether it is ERC or something else. My main advice is: never give up and keep trying, you always have a chance to get it. And even if you do not get it first time, or the second, or the xth time… you always learn from the experience and sooner or later you will nail it. Trust me, you will get it!

Good luck!

Timothy Noel, March 17, 2022

References
(1) Cambié, D.; Zhao, F.; Hessel, V.; Debije, M. G.; Noël, T. A leaf-inspired luminescent solar concentrator for energy-efficient continuous-flow photochemistry. Angewandte Chemie International Edition 2017, 56 (4), 1050-1054 DOI: 10.1002/anie.201611101
(2) Cambie, D.; Dobbelaar, J.; Riente Paiva, P.; Vanderspikken, J.; Shen, C.; Seeberger, P.; Gilmore, K.; Debije, M. and Noël, T. Energy-Efficient Solar Photochemistry with Luminescent Solar Concentrator-Based Photomicroreactors. Angewandte Chemie International Edition 2019, 58 (40), 14374-14378 DOI: 10.1002/anie.201908553
(3) Masson, T. M.; Zondag, S. D. A.: Kuijpers, K. P. L.; Cambié, D.; G. Debije, M. and Noël, T. Development of an off-grid solar-powered autonomous chemical mini-plant for producing fine chemicals. ChemSusChem 2021, 14 (24), 5417-5423 DOI: 10.1002/cssc.202102011
(4) Bottecchia, C.; Rubens, M.; Gunnoo, S.; Hessel, V.; Madder, A.; Noël, T. Visible Light-Mediated Selective Arylation of Cysteine in Batch and Flow. Angewandte Chemie International Edition 2017, 56 (41), 12701-12707, DOI: 10.1002/anie.201706700
(5) Bottecchia, C.; Erdmann, N.; Tijssen, P. M. A.; Milroy, L-G.; Brunsveld, L.; Hessel, V.; Noël, T. Batch and flow synthesis of disulfides by visible light induced TiO2­ photocatalysis. ChemSusChem 2016, 9 (14), 1781-1785 DOI: 10.1002/cssc.201600602
(6) Bottecchia, C.; Wei, X-J.; Kuijpers, K. P. L.; Hessel, V.; Noël, T. Visible light-induced trifluoromethylation and perfluoroalkylation of cysteine residues in batch and continuous flow. Journal of Organic Chemistry 2016, 81 (16), 7301-7307 DOI: 10.1021/acs.joc.6b01031
(7) Laudadio, G.; Deng, Y.; van der Wal, K.; Ravelli, D.; Nuño, M.; Fagnoni, M.; Guthrie, D.; Sun, Y. and Noël, T. C(sp3)–H Functionalizations of Light Hydrocarbons Using Decatungstate Photocatalysis in Flow. Science 2020, 369 (6499), 92-96 DOI: 10.1126/science.abb4688

I have some very exciting news to share with you: The Noel Research Group will move up North and join the Van ‘t Hoff Institute for Molecular Sciences (HIMS) at the University of Amsterdam, where I will be promoted to Full Professor and be the chair of Flow Chemistry. The move is planned for September 1^st of this year, so quite rapidly.

What can you expect from us at UvA? Our mission has always been to extend the available chemical space by embracing technology to the fullest extent. For sure, we will keep doing that using our key technology, flow chemistry! But we would not move if there were no exciting new opportunities. HIMS has a strong hub of homogeneous catalysis with a.o. the groups of Profs. Joost Reek, Bas de Bruin, Tati Fernández-Ibáñez and Francesco Mutti. We foresee some strong interactions with their teams in the years to come. So keep an eye on our website and social media channels.

Via this way, I also would like to thank the colleagues at Eindhoven University of Technology. They gave me the chance to kick start my academic career and supported me all the way, even now with the transfer to Amsterdam. Needless to say that I will miss the colleagues and the great atmosphere in the Chemical Engineering and Chemistry department.

Finally, thank you to everyone for the support and faith that has brought us to this moment.

Tim

July 30, 2020

Amsterdam, The Netherlands

Hot off the press! Our latest work on photochemical activation of light alkanes just appeared in Science (https://doi.org/10.1126/science.abb4688). We are stoked to present our group’s first Science paper to the community. In this blog article, we would like to discuss the human story behind this amazing project.

New project, new challenge

After our first project on the C(sp³)–H oxidation using decatungstate-photocatalysis, published almost two years ago in Angewandte Chemie (https://doi.org/10.1002/anie.201800818), we were looking for a new challenge where we could exploit the potential of this potent hydrogen atom transfer (HAT) catalyst. The NRG-TBADTeam (nickname of the subgroup dedicated to TBADT chemistry) focused its attention to a valuable -yet absent in organic synthesis- class of gasses which could benefit from continuous-flow microreactor technology: the volatile alkanes, such as methane, ethane, propane and butane. These compounds are mostly burned to release its energy for heating applications or propulsion, while in the chemical industry they are used as inexpensive starting materials for the synthesis of haloalkanes or polymer monomers (e.g. ethylene). Hence, we wondered if we could activate these inert and insoluble gasses with TBADT and immediately engage them in synthetically-useful transformations of interest to the organic chemistry community. Such a strategy would allow us to bypass the current toxic halogenation industry.

We commenced our investigations with propane and borrowed a bottle of high purity propane from our TU/e colleague Prof. F. Gallucci. To our delight, we could immediately spot some interesting reactivity. We were particularly delighted by the excellent selectivity provided by decatungstate for the most substituted carbon of propane (86:14 ratio for the instalment of a secondary propyl versus primary propyl moiety). At that point, master student Klaas van der Wal joined the group and together with him the real optimization commenced. Klaas did an amazing job, especially in the design of the flow setup, which was crucial to bring the gaseous reactants into contact with the soluble catalyst and substrates, and the initial reactivity screening. He received a great score for his MSc defense (Figure 1). However, we were not done yet!

Figure 1. TU/e M.Sc. student Klaas in action: left) at the Master thesis presentation and defence; right) graduation ceremony.

Shine brighter

Even if an extensive optimization was already conducted at that point, we managed to run our reactions only in stop-flow mode to reach full conversion. We realized that a more powerful light source was needed to reduce the reaction time. More photons were going to be crucial in order to move to a continuous operation of the flow reactor. For this reason, we decided to employ a Vapourtec setup, which was provided to us by Vapourtec’s CEO Duncan Guthrie and Dr. Manuel Nuño. Thanks to this photochemical flow system equipped with a powerful 365 nm LED set (60 W or 150 W, the latter high power 365 nm light source was even a prototype which proved crucial in our reaction discovery), we could easily move to a continuous-flow mode and complete our last screening.

At that point, Tim asked Yuchao Deng, who was a visiting Chinese PhD student in our lab, to join the TBADTeam. With her hard work and dedication, we were able to rapidly carry out the propane optimization and to identify the substrate scope. During the scope exploration, the input from Profs. Maurizio Fagnoni and Davide Ravelli was of great value to make rapid progress. They have been working with this catalyst for many years and TBADT has essentially no secrets for them anymore.

Next, we evaluated a variety of different alkane gases, starting with isobutane. Activation of isobutane led to a new methodology which allowed to introduce tert-butyl groups in a straightforward and selective fashion (96:4, tert butyl versus isobutyl).

Figure 2. Gabriele and Yuchao working with the Vapourtec setup.

Methane? Yes, we can!

We must acknowledge that at point we were quite satisfied with the results but we were still anxious as two formidable challenges laid ahead of us: the activation of ethane and methane. These alkanes possess the strongest C(sp³)–H bonds known in Nature and we were not sure whether decatungstate would be able to cleave those selectively… Methane for example has been a daunting challenge for many decades, occupying many researchers without much success so far. Cleaving methane’s C(sp³)–H bonds requires extremely high temperatures (> 500 C) and is only industrially done for a few processes.

For ethane, we needed to crank up the pressure a bit to get it into a liquid state. However, overall the activation went pretty smooth and only minor optimization was required to obtain a satisfactory scope.

Finally, we were ready to take on the challenge of methane. If this one would work, we knew it would be a big deal. And … we had probably a good shot to get it into a true top journal. In our first attempts, we could spot the right product only in traces. We realized that the major products obtained were propylated adducts or compounds derived from the activation of acetonitrile. The presence of propylated compounds puzzled us for a while. However, we realized that the gas line might still contain some propane from our previous experiments. Indeed, by applying vacuum on the gas lines, we could remove traces of condensed propane fairly quickly. With propylation being resolved, we saw that the HAT on acetonitrile was still prominent, and only a low yield was observed for the desired methylated product. We then realized that activation of acetonitrile was favoured over the activation of methane. Even if this result could be expected based on BDE, we originally thought that the polarity mismatch of the C-H bonds of acetonitrile would prevent this side reaction. This was indeed the case for butane, propane and ethane but not for the strongest C–H bonds present in methane. To circumvent this, we employed d₃-acetonitrile. Once we did that, we finally obtained the desired products in synthetically useful yields. After isolating our first methylated molecule, Yuchao and I were so happy that I cannot even describe it in words. We repeated at least a thousand times “WE DID IT!” jumping all over the lab. Definitely, that was not a quiet day for our labmates.

Hard times, high hopes

When we were wrapping up our experiments, the pandemic crisis was advancing very fast. As an Italian, I was extremely aware of the potential threat of COVID-19. Yuchao was even more scared than I was, especially because her time in the NRG group was almost over and she would not be able to go back to China as her flights got systematically cancelled. Despite the COVID stress, we completed all the necessary experiments. Tim wrote the paper and submitted it to Science on February 26, a couple of weeks before the inevitable lockdown of our group (March 13, 2020). Then we waited, isolated at home.

On April 7, we got news! We received a request to submit a revision, so not rejected. What a relief! After reading the feedback from the referees, we were cautiously optimistic. Yet we were also scared! We felt we could smell our first paper in Science and we did not want to mess it up. After some Zoom calls with all co-authors, we submitted a carefully revised version and waited again, all of us still sheltered in place. On May 6, 2020, we finally got the acceptance letter (Figure 3, Top). As the email came in quite late in Europe, Tim was already sleeping. On May 7 at 6.20 AM, he send me via WhatsApp some very exciting messages (Figure 3, Bottom).

Figure 3. Top: Email with the long awaited final decision. Our first paper in Science is a fact. Bottom: WhatsApp conversation between Tim and myself (Gabriele Laudadio).

We are now July 2^nd, almost two months later. The paper just got published in the latest issue of Science. But I still need to pinch myself regularly to know I am not dreaming. I will cherish these memories forever.

Ciao,

Gabriele

The paper discussed in this blog was published as “C(sp³)–H functionalizations of light hydrocarbons using decatungstate photocatalysis in flow”. by Gabriele Laudadio, Yuchao Deng, Klaas van der Wal, Davide Ravelli, Manuel Nuño, Maurizio Fagnoni, Duncan Guthrie, Yuhan Sun, and Timothy Noël, Science 2020, DOI: 10.1126/science.abb4688.

During the Flow Chemistry Europe 2020 (Cambridge), Tim has used his speaker slot to commemorate our colleague and friend Prof. Jun-ichi Yoshida, who passed away on Saturday, September 14, 2019.  He was one of our regional editors at Journal of Flow Chemistry. He also served as the president of the National Institute of Technology Suzuka College and was emeritus professor of Kyoto University. Most of all we will remember Prof. Yoshida for his excellent work in Flow Chemistry. He coined the term “Flash Chemistry” for ultrafast reactions which can be done selectively in microreactors by taking advantage of the fast mixing and the excellent heat control. He also pioneered the so-called “cation pool” method, which allowed to accumulate anodically radical cations at low temperature. These could be subsequently converted in the presence of nucleophiles in a very controlled and reliable fashion.

Generally, one cannot overstate the achievements of Prof. Yoshida in Flow Chemistry. He can rightfully be called one of the pioneers and legends of the field. Many of us, including myself, have found inspiration in his early work and have tried to follow his lead at the outset of our career.

On a personnel level, I always enjoyed talking to Jun-ichi. He was very supportive to young flow chemists. He was very social and loved drinking a good glass of wine or sake.

Prof. Yoshida’s death is a great loss for the scientific community and we will miss him dearly.

The lecture can be downloaded as pdf using the following link: Talk_YoshidaMemorial

Timothy Noël

Flow Chemistry Europe 2020, Cambridge (UK); March 3, 2020

Our most recent paper dealing with the electrochemical synthesis of sulfonamides was just published in Journal of American Chemical Society (DOI: 10.1021/jacs.9b02266). For our group, it was the first paper in this prestigious journal and we are particularly proud of the work itself. It has been an interesting project which required a lot of hard work by many co-authors and we are happy to take you today behind the scenes and providing you with some insights in the discovery process.

Figure 1. Graphical abstract of our “Sulfonamide synthesis through electrochemical oxidative coupling of amines and thiols”.

In search of inspiration

Our group is mostly known for its work on flow photochemistry but about three years ago, we started getting interested in electrochemistry. For us, electrochemistry seemed like a complementary activation mode compared to photoredox catalysis, both allowing to generate synthetically useful radicals. In addition, electrochemical setups are often affected by process-related problems, like mass- and heat-transfer limitations, which was for us the proof that a technological solution would of great added value.

After our first paper on the electrochemical oxidation of thioethers and thiols,¹ we decided to design a new electrochemical flow microreactor to carry out our chemistry in a more reliable and scalable way.² In the meantime, we started thinking about new electrochemical transformations that we could develop. The group’s aim is to develop new, useful synthetic methodology and utilize cutting-edge technology to give a further boost to the chemistry. At that time, our experience on electrochemistry was still very rudimentary and we were reading a lot of different publications to get a bit an image of the field. Ultimately, we found an amazing review entitled “Nonaromatic Aminium Radicals”.³ After reading this review carefully, we had an idea: what if we try to make sulfenamides electrochemically and then further oxidize these species to make sulfonamides? We had already some expertise with the oxidation of thioethers and if we could combine both the S–N coupling with the subsequent oxidation of the sulfur, we had a great method that allowed us to activate commodity chemicals, such as thiols and amines. Moreover, sulfonamides are interesting moieties which are quite common in pharmaceuticals and agrochemicals. Recently, a couple of interesting methods to prepare or functionalize sulfonamides appeared in the literature, incl. the work of Willis⁴ and MacMillan.⁵ Taken all of this information together, we had the feeling that this project would be both challenging and of high interest to the synthetic community.

Figure 2. Original idea for the synthesis of sulfonamides.

A working reaction = A wedding present for Gabriele

At that time, Lisa Struik joined our group for her master thesis. Initially, she worked on the experimental validation of our electrochemical flow reactor.² When that project was finalized, we suggested her to try this idea we had on the synthesis of sulfonamides. Eager to work on this new project, she started with the experiments. One of the first things she said was the following: “We have a huge problem: the reaction is forming a solid and crashes out in pure acetonitrile. I can NOT carry this out in flow!” Our answer to this was “It is probably just the acid-base adduct, just add a bit of water and it will be fine”. Indeed that solved the issue pretty well and not only that, it really boosted the reactivity significantly. To our delight, the results were almost immediately perfect. You cannot imagine how excited we were when we saw that the major product was not the sulfenamide, but the sulfonamide directly!!! This gave us the confidence to increase our research efforts: We started right away with the further optimization and got some help from Christiane Schotten. Christiane was a visiting PhD student coming from the Browne group at Cardiff University and she helped us to test different reaction conditions, residence times, mediators and acids.

Once we had the optimal reaction conditions in hand, Christiane and Lisa started working on the substrate scope. Gabriele had to take a short break at that point as he got married and went on honeymoon on a cruise. Gabriele still remembers that every time they went on land he was anxious to check his emails to know if there was any progress. Upon his return, we had to say goodbye to Christiane and Lisa and for a while, Gabriele had to work alone on the scope. Luckily, Efstathios Barmpoutsis (AKA Stathis), another master student, joined the project, rolled up his sleeves and got immediately to work. It was at that point that we realized how crucial it was to clean the electrodes thoroughly in between reactions. If you don’t do that, yields drop gradually and the results are difficult to reproduce. However, with the proper cleaning, as we described in detail in the Supporting Information of our manuscript, this issue is completely overcome and highly reproducible results are obtained. A little bit later another issue arose: heterocyclic arenes were completely unreactive under our reaction conditions (Figure 3). After some optimization of the reaction conditions, we found that the reaction could be carried out when 1 equiv. of pyridine was added to the reaction mixture. We surmise that pyridine functions mainly as an electron mediator. To be sure, we also evaluated whether its presence was required in the other reactions we already screened but there it did not prove to be essential.

Figure 3. Optimization of heteroaromatic compounds

While Stathis and Gabriele were working on the reaction scope, an Erasmus student Sebastian Govaerts joined the team. (Basically, he returned to our group as he previously worked with Gabriele on the sp³ C–H oxidation project.⁶) He focused mainly on the batch scope as we wanted a thorough comparison with the results obtained in flow. In most cases, the yields were lower in batch and the reaction required 24 h and 100 mol% of supporting electrolyte to reach full conversion. In flow, the reaction was done within 5 min and needed only 10 mol% of supporting electrolyte. However, batch does have its advantages. For those reactions that were very slow or resulted in precipitate formation, we preferred the batch procedure.

Figure 4. (Left) Electrochemical batch reactor; (Right) Electrochemical flow reactor

With the finish line in sight – What is the mechanism?

Within sight of the finish line, it was time to gain some mechanistic understanding. Kinetic experiments revealed that the thiol dimerizes within 20 seconds to the corresponding disulfide. This observation indicates that some of the most odorous thiols can be replaced by their corresponding disulfide, a feature most chemists will be happy about. Next, Gabriele started with some quenching experiments and he was able to rapidly trap the aminium radical. With that preliminary result in hand, we were almost sure that the sulfenamide was one of the intermediates in our transformation. Indeed, we were able to isolate some sulfenamide and when subjected to our reaction conditions, the targeted sulfonamide was obtained in quantitative yield.

Finally, we had to do some cyclic voltammetry. Using a potentiostat, Gabriele recorded spectra of all relevant starting materials, including amines, thiols and disulfides. Despite some hints, it proved to be not enough evidence to pinpoint the entire mechanism. Then he decided to “titrate” thiophenol with cyclohexylamine and recorded different spectra. We firmly believe in the famous Latin maxim “Fortuna audaces iuvat”, which means literally “fortune favours the bold”. Indeed, at a certain moment, we found some white precipitate in the cell. It was a Friday evening, around 8 pm. Gabriele filtered the precipitate and ran to the NMR: indeed, it was the cyclohexylammonium thiophenolate (Figure 5), as was also observed by Lisa. And, when we tried to record the cyclic voltammogram of that compound, we knew that we had finally found the missing piece (Figure 6). We still carried out some additional control experiments to be 100% sure (you can find them in the supporting information of the paper) but from that moment on we knew the work in the lab was basically done.

Figure 5. Picture of the white solid and its 1H NMR spectrum

Figure 6. CV of cyclohexylammonium thiophenolate

A final word

The possibility to obtain complex moieties, such as sulfonamides, simply by stitching two commodity chemicals together using electrons is, in our opinion, a key feature of our electrochemical methodology. Moreover, we saw that microflow technology really made a difference in this transformation providing the experimentalist with practical reaction conditions and operational flexibility to rapidly investigate the reaction scope. This can be attributed to the small interelectrode gap (250 μm), the high mass transfer to the electrodes and the large electrode surface to volume ratio.

Currently, we are working hard to finish some more electrochemical methods of which we are really excited. So stay tuned and keep an eye on our group website and twitter accounts.

Ciao,

Gabriele and Tim

The paper discussed in this blog was published as “Sulfonamide synthesis through electrochemical oxidative coupling of amines and thiols”. by Gabriele Laudadio, Efstathios Barmpoutsis, Christiane Schotten, Lisa Struik, Sebastian Govaerts, Duncan L. Browne, and Timothy Noël, Journal of American Chemical Society 2019, DOI: 10.1021/jacs.9b02266

References:

[1] Laudadio, G.; Straathof, N. J. W.; Lanting, M. D.; Knoops, B.; Hessel, V.; Noël, T. Green Chem. 2017, 19 (17), 4061–4066.

[2] Laudadio, G.; de Smet, W.; Struik, L.; Cao, Y.; Noël, T. J. Flow Chem. 2018, 8 (3–4), 157–165.

[3] Chow, Y. L.; Danen, W. C.; Nelsen, S. F.; Rosenblatt, D. H. Chem. Rev. 1978, 78 (3), 243–274.

[4] Chen, Y.; Murray, P. R. D.; Davies, A. T.; Willis, M. C. J. Am. Chem. Soc. 2018, 140, 8781-8787.

[5] Kim, T.; McCarver, S. J.; Lee, C.; MacMillan, D. W. C. Angew. Chem. Int. Ed. 2018, 57, 3488-3492.

[6] Laudadio, G.; Govaerts, S.; Wang, Y.; Ravelli, D.; Koolman, H. F.; Fagnoni, M.; Djuric, S. W.; Noël, T. Angew. Chem. Int. Ed. 2018, 57, 4078-4082.

Frustration as a source of inspiration

I am pretty sure any PI can name a few experiments which their students detest. Not because they are useless (despite the often-used bluff feedback that it is “not absolutely necessary” for the paper), but because these experiments are tedious, time-consuming and frustrating. Amongst the most hated experiments, one could easily place measuring IR spectra (Such an old technique, who needs that anyway?), distilling reagents and solvents (Can’t we just buy a new bottle?), titrations (Do you not believe the concentration written on the bottle?), … In our lab, and probably in many other ones, a special place on that ranking is occupied by Stern-Volmer analysis.

Those of you working on photoredox catalysis probably know too well what I am talking about. For those who don’t, here’s a brief explanation: once in their excited state, photocatalysts tend to return to their ground state by dissipating the energy in the form of emitted light (also called fluorescence). However, in the presence of appropriate organic molecules (often referred to as quenchers), the excited photocatalyst can transfer that excited state energy to a suitable quencher via a single electron or energy transfer. As a result of this interaction, a reduction in the fluorescence of the photocatalyst can be observed (Figure 1). The kinetics of this phenomenon can be explained by the Stern-Volmer relationship, thus making this experiment crucial to elucidate which molecule(s) can interact with the photocatalyst and at which rate they can do so. In other words, the Stern-Volmer relationship provides valuable information in the elucidation of any photocatalytic reaction mechanism.

However, as for all kinetic studies, Stern-Volmer kinetics is not that easy (Figure 1). The process is labor-intensive and sensitive to oxygen, thus often making it difficult to obtain quantitative data. In our own experience, we noticed that, depending on the students’ experience (BSc, MSc, Ph.D. or postdoc level), we usually obtained good qualitative results but the reproducibility and accuracy of the data were often problematic. Since we strive to publish the best achievable data, this eventually resulted in the necessity to repeat the experiment several times, sometimes even by several different students. Talking with other colleagues in the field, we realized this was somewhat a general issue in many groups. In some labs, they even attempted to fix this problem by appointing one student as the Stern-Volmer expert, in order to avoid wasting too much time.

As frustration is typically a good reason to initiate change, we felt that an automated tool to conduct Stern-Vomer analysis would be extremely useful (Figure 1). It would not only greatly reduce valuable experimental time but it would also make sure that the process can be carried out with high precision and reproducibility by basically any chemist, regardless of their experience. Automated platforms have become increasingly important and have often been combined with flow chemistry to assist chemists in their synthetic efforts. Seminal works by the groups of Jensen (MIT), Bourne (Leeds), Ley (Cambridge), Rueping (RWTH) and many others have paved the path for a future in which a machine-assisted approach towards organic synthesis allows chemists to disregard the routine aspects of their laboratory work.^[1] Indeed, we felt that Stern-Volmer and other quenching studies were requiring too much of our precious research time.

The kick-off of our automation activities

At that point, we had little to no experience with the automation of chemical processes. However, at TU Eindhoven, we are lucky to have every year bright Chemical Engineering MSc students, who are motivated to take on important challenges within their graduation thesis. Under Koen Kuijpers and Dario Cambie’s supervision, Koen Drummen (having too many Koens at the same time in the lab was also confusing for us, so we started calling them Koen² for brevity J) started his thesis in September 2016 working on the project. He took our UV-VIS spectrophotometer (Avaspec-2048) and equipped it with a 100 μl quartz flow cuvette which allows measuring the fluorescence of the photocatalyst (Figure 2). Inspired by the Glorius mechanism-based luminescence screening method,^[2] we also envisioned to carry out automated screening experiments of various quenchers. For this reason, Koen Drummen also installed an autosampler which allowed us to inject different quenchers sequentially into the continuous-flow stream.

Installing and connecting all the different units together was relatively straightforward. The real confrontation, in this case, was to write a suitable code to make sure all the different units can talk to each other. Hereto, we had to learn Python. I still remember entering the office and seeing Koen² watching YouTube videos to solve all their issues with the code. Who doesn’t love a nice YouTube tutorial? We partly have to thank some YouTubers: in the end, Koen² fixed the code and started to run the first automated fluorescence quenching screenings.

Next, we also developed a graphical user interface which allows you to rapidly fill in the compounds to be screened (Figure 3). This makes the entire automated setup really intuitive, basically making it possible for anyone to perform Stern-Volmer quenching experiments, regardless of their ability to code. A cool feature of our software is that, once the analysis is finished, the results are automatically sent by email as a pdf file. Easy enough, right?

At that point (we are now early 2017), another MSc student, Niels Koenig, joined our group eager to further improve the software. He added a new feature to the software, namely the possibility to automatically color-code the results obtained in the quenching experiments. If a compound was found to be a strong quencher (> 50% quenching efficiency), a green color was assigned to its fluorescence peak while a weak quencher (< 25% efficiency) was assigned a red color. This feature gives in one blink of an eye the confirmation on which quenchers are interesting to test in further reactions and which ones can be discarded.

Time to test the automated platform.

Once our engineers had a working platform ready, it was time to put it to the test. At this point, I asked our photocatalysis expert Cecilia Bottecchia to collaborate with Koen Kuijpers to see how we could take advantage of the technology. The first thing we did was a Stern-Volmer analysis to determine the rate at which TMEDA can quench the excited state of 9-mesityl-10-methyl acridinium perchlorate (Mes-Acr⁺) (Figure 4). By adding in flow increasing concentrations of quencher, a progressive decrease of the fluorescence emission intensity can be observed. Next, the software converts the obtained data into the classical linear Stern-Volmer profile. From the slope of the line, one can now easily obtain the quenching kinetic constant for a certain quencher. Notably, ten consecutive experiments demonstrated that the data produced by our automated platform was consistent and reproducible, with exactly the same slope (and thus quenching kinetic constant) in each experiment! What a difference compared to our laborious manual procedure!

With the optimized system, we could now run a single Stern-Volmer experiment within only 15 minutes. The only thing you need to do is to prepare the photocatalyst and the quencher solutions. After that, it’s all about hitting the start button, sit back, relax (or work on something else :)) and enjoy your results.

Next, we investigated several case studies to validate the screening option of our platform. One specific example that we carried out was the mechanistic investigation of the Ir(ppy)₃-catalyzed photocatalytic decarboxylation of cinnamic acids developed by Xiao-Jing Wei in our group.^[3] Cecilia prepared the stock solutions and found rapidly that the software and hardware could rapidly generate the data which was required to elucidate the mechanism (Figure 5). It should be noted that in all cases, we obtained high-quality data with R² >0.99. Now, in one single hour, we obtained both qualitative and quantitative data, while previously we had to struggle sometimes days to get the quantitative data. It might still be true that, as one referee pointed out, “a good photochemist can run a Stern-Volmer analysis in 30 minutes” but we still believe that you will have to work really hard to get everything done in this timeframe. We did some other screening procedures with our system as well but we invite you to have a look at our paper for all the experiments we did. The bottom-line is that such experiments have become a piece of cake making it actually fun to do.

Outlook

Automating luminescence quenching studies has been one of those projects that really changed the daily life in our group. Before we had access to this platform, students hated me when I asked them to do this experiment. One even said (not a joke): “I’d rather add ten more substrates to the scope instead of doing the Stern-Volmer studies!”. While I could not be bribed (I asked for both the substrates and the SV analysis :)), I still felt that it was crucial to at least alleviate the unnecessary issues that these experiments bring along. We also like the story behind this project because it clearly shows how certain ideas take shape within our group: we wanted to develop some novel synthetic methods via visible light photoredox catalysis, but we ended up with a creative solution to overcome technical hurdles along the way. Simply put, we found a mean to our end. One anonymous referee during the peer-review process really made us proud:  “This article presents a valuable and practical advancement for the field of photoredox catalysis. […] Overall, the development of an automated flow platform that significantly accelerates the collection and processing of fluorescence quenching data in a time and labor-efficient manner will be immediately impactful to the organic community.” Our system was and still is impactful in our group and we all hope it will be for your work as well! Don’t hesitate to contact us if you want to give it a shot!

See you,

Tim, Cecilia and Koen

The paper discussed in this blog was published as: A fully automated continuous-flow platform for fluorescence quenching studies and Stern-Volmer analysis. K. P. L. Kuijpers, C. Bottecchia, D. Cambie, K. Drummen, N. Koenig, T. Noël, Angew. Chem. Int. Ed. DOI: 10.1002/anie.201805632.

References:

[1] (a) D. E. Fitzpatrick, C. Battilocchio, S. V. Ley, Enabling Technologies for the Future of Chemical Synthesis. ACS Cent. Sci. 2016, 2, 131-138. (b) B. J. Reizman, K. F. Jensen, Feedback in Flow for Accelerated Reaction Development. Acc. Chem. Res. 2016, 49, 1786-1796.

[2] M. N. Hopkinson, A. Gomez-Suarez, M. Tederes B. Sahoo, F. Glorius, Accelerated Discovery in Photocatalysis using a Mechanism‐Based Screening Method. Angew. Chem. Int. Ed. 2016, 55, 4361-4366.

[3] X.-J. Wei, W. Boon, V. Hessel, T. Noel, Photocatalytic Decarboxylation of α,β-Unsaturated Carboxylic Acids: Facile access to Stereoselective Difluoromethylated Styrenes in Batch and Flow. ACS Catalysis 2017, 7, 7136–7140.

Our latest paper on a real-time reaction control system for the solar production of chemicals under fluctuating irradiance was published online today in Green Chemistry (DOI: 10.1039/c8gc00613j). Our solar-photochemistry research has always been characterized by the desire to reduce the gap between the lab and the rooftop, in other words we want to enable the use of sunlight as a free light source for many photochemical reactions in a simple and efficient way. With the Luminescent Solar Concentrator PhotoMicroreactor (LSC-PM) design we introduced a novel way to convert the polychromatic solar spectrum into a narrow band, while now we addressed the issue of light intensity fluctuations (e.g. due to passing clouds).¹ Here, we will give you some inside information on our solution for this important issue within solar photochemistry.

The problem with sunlight variable intensity
To the chemist inclined to perform photochemical reactions with solar light, one of the main limitations is represented by the uncontrollable fluctuations in solar irradiance. In particular, while the daily and seasonal variation can be predicted, short fluctuations introduced by passing clouds are the most difficult to address. In the past, we experienced this first-hand when we tested outdoors our reactions (e.g. see Figure2).

The idea

To use the words of an anonymous reviewer, “It is common knowledge that the kinetics of photoreactions are often governed by the supplied photon flux.” Using a light sensor for adjusting the irradiation time to compensate the different irradiance might, therefore, seem trivial. Alas, reality is unfortunately far from that!

For example, the spectral distribution of solar light can change significantly throughout the day. Other than the obvious red color of sunlight at sunset and sunrise, also cloud coverage, atmospheric humidity, ground albedo and other parameters can affect the wavelength distribution of solar radiation at the ground. The only reason why we could keep our system simple is that we used a Luminescent Solar Concentrator PhotoMicroreactor (LSC-PM). In an LSC-PM, the incoming solar radiation is absorbed by a fluorescent dye that re-emits the absorbed photons in a polymeric lightguide as a down converted and narrowband beam corresponding to the dye fluorescence spectrum (see Figure 3). As a consequence of the adoption of the LSC-PM design, we could neglect the variation in sunlight spectral distribution! Furthermore, by placing a light sensor in direct contact with the device edge, the light intensity measured is proportional to the photon-flux experienced by the reaction mixture flowing in the reactor channels.²

Figure 3 LSC-PM working principle. The solar photons reaching the device are either reflected (1), transmitted (2)  or absorbed within the device. Most of the photons are absorbed by the dispersed dye molecules (red dot) that re-emit fluorescent photons that are waveguided to the reaction channels (5a) or to the device edge (4).Eventually, our reaction control system is simple and elegant. As in the quote often attributed to Leonardo da Vinci, we could say that simplicity, in this case, “is the ultimate sophistication”. 🙂

Merging chemistry and electronics

Being aware of the relationship between the light intensity emitted at the device edge and the photon flux in the reaction channel, we had to find a way to measure the former as an estimation for the latter. Thanks to the advancements in electronics, that is not a difficult task. After all, most of our smartphones have a little sensor to adjust the screen backlight to the ambient light conditions.

After a brief screening of different light sensors (photoresistors, phototransistor, integrated circuits based on PV cells) we decided to use a phototransistor thanks to its simplicity. By including it in a voltage divider circuit, the variation in the light intensity can be measured as a variation in the voltage drop across a resistor connected in series (see circuit in Figure 4).

Kinetic data

With a working sensor in our hands, we needed to perform the calibration between the different light intensity levels, and the corresponding conversion at different residence time. This can be a rather laborious work but, as you might have noticed at this point, in our group we are not afraid of merging chemistry and technology. Therefore, we have developed a simple python script to control pump flow rate and LED light intensity (with a MOSFET) while measuring the reaction conversion was measured in line with a UV-VIS spectrometer. As such, this data can be obtained in fast, automated and reliable fashion.

Timing is everything

After gathering all the required data, finding the voltage/residence time relationship to keep the conversion stable was relatively easy. The first tests of the system with an artificial set of light variation were both encouraging and alarming. While the system was actually capable of adjusting the residence time to match the variation of light intensity, its reactiveness was too slow.

For a moment we feared that this might be an intrinsic limitation of our reactor (e.g. due to its elastic nature or modest volume). It turned out that the major source of variability was the delay between the light intensity reading and the adjustment of the pump flow rate. While the phototransistor responsiveness to light variation was almost instantaneous³, most of the delay was in the computer controlling the pump and the pump actually changing the flow rate. By reducing the loop time in the reaction control script to the pump response time, an update rate of less than a second was achieved. After this correction, a good stability in the reaction conversion was obtained even for large and fast variation to the external irradiance.

Arduino

Up to this point, we had been using a PC to interface with a voltmeter to read the voltage drop and to change the pump flow rate accordingly. The system was fine for lab development, but clearly, we could not propose to use it as such to control multiple reactors in a hypothetic solar mini-plant. The advantages of switching from a reaction analysis approach to monitor the reaction progress to a physical measurement that directly addresses the source of variability and that can predict the reaction outcome represented a significant conceptual difference.

We went back to the drawing board and decided to simplify the overall system with the adoption of an Arduino microcontroller. Once programmed with the kinetic investigation data, the controller would automatically send commands to the syringe pump to correct the flow rate based on the voltage measured in the light sensor. In this way, both the multimeter and the PC are replaced by an inexpensive integrated circuit. Tim particularly liked the inexpensive nature of this solution, whose total cost was about 50€. Actually, by using a generic Arduino instead of a genuine one or by including all the components in a single printed circuit board, the price of the system could be decreased to less than 10€ per piece (and even further in case of mass production).  We believe that this system has potential to be applied for the reaction control of every solar photoreactor tested outdoors. To describe it with the words of another anonymous reviewer: “The potential of this to thus be completely automated will bring legitimacy to the idea of actually using solar to perform synthetic organic chemistry in practice”. And that is exactly the goal we are working towards!

Outdoor experiment

Finally, as usual in our LSC-PM papers, we tested the reaction control system outdoors on a cloudy day, luckily those cloudy days are quite common here in Eindhoven… The system worked as planned and could handle challenging weather conditions (read: frequent and steep variations due to passing clouds). Once we got these results, we went back to the office with a big smile and finalized the paper writing process.

Outlook

It is undeniable that the dilute nature of solar energy will never make it suitable for the production of bulk chemicals. The volumes and margins of fine and specialty chemicals, on the other hand, constitute a real possibility for solar photochemistry. One should also realize that the use of solar energy is basically for free and abundantly available. Especially when energy prices will rise (it is just a matter of time), these technological solutions might make the difference! We believe that our paper is yet another step in our lab’s effort towards simple and more efficient solar photoreactors. Ultimately, we hope to see our technology implemented in the solar synthesis of some life-saving drugs! 🙂 🙂

Bye!

Fang, Dario, and Tim

¹ To this is particularly relevant the Whitesides’ recent perspective on the future of organic chemistry: “Another possible direction for the future evolution of Organic Synthesis is, in effect, to fuse (at least in part) with Chemical Engineering, and use the techniques of that area to simplify processes […]”

² More details on this in the Monte Carlo ray-tracing simulations that we have published last year.

³ 10 µs according to manufacturer specifics

We are delighted to announce that our latest publication on the decatungstate photocatalytic sp³ C–H aerobic oxidation in flow is now available on the website of Angewandte Chemie (DOI: 10.1002/anie.201800818 ). Here, we would like to share the “logbook” of this scientific journey. Fasten your seatbelts and let’s go!

Digging literature

When Tim proposed to start a new, challenging project to develop an aerobic photochemical C–H oxidation method via hydrogen atom transfer catalysis, I did not have to think twice and I accepted enthusiastically. At that point, I was still working together with Hannes to wrap up our meta arylation project and I thought it was the right timing to start thinking about a next high profile target. Furthermore, as a chemist, I think the selective activation of C–H bonds is one of the most interesting topics to work on nowadays. So, in order to develop a new and broadly applicable methodology, every researcher knows what to do first: digging into the literature and see if there are any hints available to start off fast. I spent a couple of weeks trying to understand what would be the best direction. First of all, I noticed that direct C–H oxidation methodologies have been developed either using electrochemistry or using transition metal catalysts (e.g. Fe, Pd, Ir or Mn-based catalysts) that were often not commercially available. Second, I learned that the oligomer decatungstate (particularly tetrabutylammonium decatungstate (TBADT)) was recently used for many cross-coupling reactions (many of them carried out by Prof. Fagnoni’s group), for fluorination chemistry and for the oxidation of alcohols. To Tim and myself, it seemed that this relatively underrepresented catalyst could be effective for the targeted transformation. Indeed, during the past 30 years, a few reports appeared on the oxidation of C–H bonds using TBADT, in most cases hydrogen peroxide was used as the oxidant. However, it became very clear to me that this transformation has never been explored in great detail. At that point, I summarized all of this information and prepared a powerpoint presentation to outline my ideas during a Science Meeting presentation in the Noel Research Group. Fortunately, Tim and the other members of our group liked my proposed strategy and we set out to do the first experiments.

Since we had little to no experience with TBADT, we asked Prof. Maurizio Fagnoni and Dr. Davide Ravelli to collaborate with us, to teach us some tricks on the synthesis of TBADT and to discuss the obtained results. Moreover, Tim presented the idea to Steve Djuric from Abbvie and they were also very interested to join this collaborative effort.

Roll with batch

Together with my new MSc student Sebastian Govaerts, we kicked off the work carrying out some batch optimizations. From the very beginning, we noticed that the oxidation of cyclohexane (our benchmark at that stage) was very slow: only traces of product were observed after 4 hours of irradiation. It was immediately clear that we had to change some key parameters. We thought that the main reasons for failure were the limited amount of oxygen available in the reaction mixture and the low light intensity. Inspired by Prof. Fagnoni’s work, we subsequently carried out the oxidation in so-called window ledge glassware using a solar simulator as the light source (Figure 2). This allowed us to have a higher interfacial area (and thus higher oxygen availability) and it provided a greater exposure to light. Luckily, this time the reaction worked better and we saw some conversion. During the subsequent optimization studies, we found that the addition of acid was crucial to accelerating the oxidation process, while basic reaction conditions shut down the reaction. However, mass transfer of oxygen from the gas to the liquid phase remained challenging and this was for us the signal to move to flow.

Go with the flow

From our previous work, we were sure that the combination of a segmented flow regime with the enhanced irradiation provided by microreactors should be able to dramatically boost the reaction performance. And indeed, the first results were immediately better than the best batch experiments. After a meticulous optimization, we were able to obtain full conversion for our benchmark in only 45 minutes of residence time in a 5 mL capillary reactor (Figure 3b and c). We used 365 nm LEDs as it perfectly overlaps with the absorption maximum of TBADT. Other LEDs, such as blue and purple coloured ones, proved ineffective.

Interestingly, during our optimizations, we noticed that higher oxygen pressures did not affect the reaction rate, indicating that the liquid phase was saturated with oxygen even at atmospheric pressure. Furthermore, we observed that when the reaction went to full conversion, a deep blue colour appeared towards the end of the reactor coil. This points towards an inactive catalyst and provides a visual indication that the reaction was successful! All these small (but useful) pieces of information helped us a lot during the first part of the work.

Work Hard, Play Hard

Once we were satisfied with the reaction conditions, we decided to start investigating its robustness. Initially, we were screening a whole variety of different toluene substrates. However, none appeared to give any product. All these unsuccessful results could be actually visually predicted as the liquid slugs turned deep blue over the entire length of the reactor (Figure 4a). Again, this shows that the catalyst is inactive. Other compounds like amino acids and amines were overoxidized under our reaction conditions giving very complex mixtures (Figure 4c). However, despite these failures, we got a lot of insight into the mechanism and the reactivity profile of the method. E.g. while toluenes could not be oxidized to their corresponding benzaldehydes, we were able to target other positions selectively.

To make a long story short, we were able to oxidize a wide variety of different substrates containing both activated and unactivated C–H bonds, even including some biologically active molecules products such as pregnenolone acetate, sclareolide, and artemisinin. The purification of the products was often challenging as they were in some cases quite volatile. Hanging the flask a bit too long on the vacuum line was enough to have to start all over. Since we used continuous flow for the synthesis of our entire scope, the reaction conditions proved readily scalable. As an example, we were able to obtain about 870 mg of artemisitone-9, which corresponds to an isolated yield of 59%.

A Final Word

Regarding this project, I think that our photochemical C−H oxidation can be a valid alternative to the strategies reported so far. The employment of a commercially available (or even better – synthesize it yourself, it is very easy) catalyst, together with a mild and inexpensive oxidant like molecular oxygen, are the winning features of this process. On top of that, it was very pleasant to develop a procedure which really benefits from continuous-flow processing. In line with the philosophy of our group, continuous-flow microreactor technology is only a useful alternative to batch if it really improves the process significantly. I firmly believe that this work is a strong example of the benefits of microreactor technology.

Finally, I would like to thank everyone who helped and contributed to the success of this scientific journey including my co-authors and the people in our group. A special thanks to Sebastian who worked side-by-side with me in the lab, helping and pushing me with his constructive criticism when a result was simply not good enough. It made the paper in the end much better! Finally, my gratitude goes to Tim, who gave me the chance to work on this very intriguing, challenging and pleasant project and supported me during all the difficult moments (e.g. low reactivity, compound disappeared due to volatility, etc.).

Ciao,

Gabriele

The paper discussed in this blog was published as Selective sp³ C–H Aerobic Oxidation enabled by Decatungstate Photocatalysis in Flow. Gabriele Laudadio, Sebastian Govaerts, Ying Wang, Davide Ravelli, Hannes F. Koolman, Maurizio Fagnoni, Stevan W. Djuric, T. Noël, Angew. Chem. Int. Ed. DOI: 10.1002/anie.201800818.

With great joy we’d like to share the news that our latest work on the Visible Light-Mediated Arylation of Cysteine was just published yesterday in Angewandte Chemie International Edition (DOI: 10.1002/anie.201706700). Recently, it became somewhat of a tradition in our group to write a short blog on the behind-the-scenes of our papers. So here’s all about the “visible light-mediated arylation of cysteine” ☺.

The beginning

The beginning of this project coincided with the start of my PhD in the Noёl research group, back in 2015. Right from the start, Tim and I agreed we wanted to apply the expertise of his group on photoredox catalysis to new methodologies for biomolecule modifications. But how to do it? Following on the recent findings from Tim and my former colleague Natan Straathof, we knew thiols are efficient moieties in trapping radicals generated via photoredox catalysis, thus enabling C-S bond formation.^[1] This knowledge, combined with the fact that post-translational modification of amino acids in peptides and proteins are of pivotal importance in chemical biology, encouraged us to attempt the visible light-induced modification of cysteine residues. Firstly, we investigated the reactivity of cysteine towards trifluoromethyl radicals.^[2] After few positive results, it became clear that the idea to use cysteine to trap other radicals was worth investigating. Starting from highly electrophilic diazonium salts, we hypothesized it would be possible to generate an aryl radical through oxidative quenching of the excited state of a photocatalyst, such as Ru(bpy)₃^2+*. To our delight, upon exposure to visible light of a mixture of diazonium salt, Ru photocatalyst and cysteine, a significant amount of the desired arylated cysteine derivative was observed.

All hands on deck

By summer 2015, our promising arylation strategy started to take shape. However, around the same period, my colleague Dario and I started to fully dedicate our time to the preparation of a review article.^[3] Luckily for me, an enthusiastic and smart master student, Maarten Rubens, joined our lab and committed to the challenge of optimizing our methodology. We wondered whether the in-situ formation of diazonium salts would be compatible with our conditions. By employing t-BuONO as nitrite source and catalytic amounts of p-toluenesulfonic acid, the in situ formation of the diazonium salt, its conversion into an aryl radical, and its trapping by the thiol moiety of cysteine, were achieved in a one-pot system. Avoiding the preparation and isolation of diazonium salts proved convenient both from a time-saving perspective and, most importantly, from a safety point of view. Moreover, with Maarten’s help, we demonstrated the possibility to employ an organic dye, Eosin Y, as the photocatalyst for our transformation, therefore rendering our arylation strategy metal-free.

With the optimized conditions in hand, we started to explore the scope of different diazonium salts as coupling partners. We observed a very broad tolerance of functional groups: almost all anilines in our DUPA closet reacted in moderate to excellent yields to give the corresponding arylated cysteine derivatives! (Figure 2).

After some weeks of batch experiments and with a good scope of isolated compounds, we started to feel the urge to… go with the flow! After all, the Noёl research group is a flow group and we were curious to see what advantages microflow technology would bring to this methodology.

Maarten quickly assembled a capillary microreactor (Figure 3) consisting of PFA tubing and coiled around a homemade 3D-printed holder (if you’re interested in “microcapillary reactors for dummies” ☺ please see DOI:10.1038/nprot.2015.113 ). The efficient irradiation of the reaction mixture and the optimal mixing achievable in a microreactor resulted in a remarkable acceleration of the reaction kinetics and in increased yields.  Therefore, within few weeks of hard work, we could expand our scope and report several examples of substrates showing an increased yield in flow.

Starting materials wanted!

The batch and flow results made us undoubtedly happy, but we knew that to strengthen our methodology it was time to start working on more complex model substrates. Being mainly a flow group, we didn’t have access to a nice library of frozen peptides, nor to a peptide synthesizer, so we started looking into the fastest way to obtain relevant cysteine containing dipeptides to test the applicability and the selectivity of our methodology towards other amino acidic residues. In order to reduce the synthetic steps to the minimum, while avoiding protection and deprotection steps of the amino acids side chains, we decided to prepare our dipeptides through native chemical ligation (NCL).^[4]

NCL is a powerful strategy used by chemical biologists to construct large peptides or small proteins starting from two or more unprotected peptide fragments. In our case, NCL represented a straightforward methodology to easily access in two steps a small library of relevant peptides.^[5] NCL proceeds through the formation of a thioester intermediate (see Figure 4: step 1) and its rearrangement (through the so-called S,N-acyl shift) to yield a native amide bond (see Figure 4: step 2).

All prepared dipeptides were subjected to our batch methodology and successfully converted to the desired arylated derivative in moderate to good yields. In our eyes, these looked like satisfying results: not only did we prove the applicability of the methodology to simple model peptides, but we also demonstrated the selectivity of our protocol in presence of other amino acids that could interfere (i.e. C-2 or C-3 arylation of the indole in tryptophan).

The next level.

One of the pieces of advice that Tim gives to us PhD students is to be always critical to our own work and to put ourselves in the reviewers’ shoes. This advice sure led me to the conclusion that, despite all the nice results obtained so far, our methodology still wasn’t ready for publication. We needed more convincing results on more complex substrates, we simply needed more.

It was also clear to us that our resources and expertise in the field of peptide modifications were limited. Luckily, we knew on which door to knock to ask for some help! Annemieke Madder’s group at Ghent University had just the expertise we needed. Both Tim and I were familiar with Annemieke’s group (Tim obtained his Ph.D. at Ghent University and I did my master thesis project in the very same group!) and were enthusiast about the opportunity to collaborate with them. After a first meeting, Annemieke and one of her Postdocs, Smita Gunnoo, decided to give us a hand and to kindly host me for a short period in their lab to perform some experiments.

Smita invested some time to get to know the project and discussed with me the aspects that needed improvements in order to facilitate the application of our methodology to peptides. She prepared for me a small library of interesting peptides and together we started to look for the optimal reaction conditions. After screening several solvent systems, reaction times and sequences we finally obtained good results on the modification of a hexapeptide containing, among others, L-serine and L-lysine residues. This model peptide allowed us to demonstrate the compatibility of our cysteine-modification strategy with other commonly used post-translational modifications involving these same residues. The simplest reaction conditions turned out to be the best ones as well: a PBS buffered solution as solvent, pre-made diazonium salts and Eosin Y were sufficient to fully convert the model peptide to its arylated derivative within 30 minutes of irradiation time. With these results in hand, we finally felt confident that the project was ready for publication.

A final word.

Writing this short blog gave me the opportunity to re-think about the story of this project. As with every challenging project, the achievement of positive results was only possible after many other negative ones. This sure was a demanding work to complete, and the visible light-mediated arylation of cysteine wouldn’t have been possible without the help of all other people involved.

My gratitude goes to Maarten for his great help with the flow scope, to Smita for showing me how to make this work on peptides and to Tim for always believing in the project (and in me).

This work was funded by the Marie Curie ITN grant “Photo4Future” (check out our website: http://www.photo4future.com)

In our group, we believe in the importance of open access scientific publications, which is why this manuscript is available free of charge for all those interested!

I hope you will enjoy the reading,

Ciao!

Cecilia Bottecchia

The paper discussed in this blog was published as: Visible Light-Mediated Arylation of Cysteine in Batch and Flow. C. Bottecchia, M. Rubens, S. B. Gunnoo, V. Hessel, A. Madder and T. Noël, Angew. Chem. Int. Ed. DOI: 10.1002/anie.201706700.

References :

[1]          (a) X. Wang, G. D. Cuny, T. Noël, Angew. Chem., Int. Ed. 2013, 52, 7860-7864; (b) N. J. W. Straathof, B. J. P. Tegelbeckers, V. Hessel, X. Wang, T. Noël, Chem. Sci. 2014, 5, 4768-4773.

[2]          C. Bottecchia, X. J. Wei, K. P. Kuijpers, V. Hessel, T. Noel, J Org Chem 2016, 81, 7301-7307.

[3]          D. Cambié, C. Bottecchia, N. J. W. Straathof, V. Hessel, T. Noël, Chem. Rev. 2016, 116, 10276-10341.

[4]          E. C. B. Johnson, S. B. H. Kent, J. Am. Chem. Soc. 2006, 128, 6640-6646.

[5]          L. Markey, S. Giordani, E. M. Scanlan, J. Org. Chem. 2013, 78, 4270-4277.