Making Meta-Arylated Anilines Attainable with Flow – Behind the Scenes

We are happy to announce that our latest work on the modular flow design for meta-selective arylation of anilines was published earlier this week in Angewandte Chemie International Edition (DOI: 10.1002/anie.201703369). Today, we want to take you behind-the-scenes on how the project came into existence, what the major bottlenecks were along the way, and which results lead us to the breakthrough of this publication.

Bigger, better, faster?

The saga started in the late summer of 2015 when Kirsten Verstraete, a graduate student from UHasselt Belgium, arrived in our lab. Not knowing what her graduation work would entail, I presented to her the landmark paper published by Gaunt et al.[1]. The paper describes an elusive transformation: the copper-catalyzed meta-selective arylation of protected anilines, which apparently neglects all conventional rules of electrophilic aromatic substitution chemistry (Figure 1).

Figure 1: A) Conventional reactivity trends in electrophilic aromatic substitution. B) Meta-selective catalytic C-H bond arylation of anilines.[1]

Masking my doubts with enthusiasm, I explained to her that we could make this transformation better, faster and broadly applicable. I told her (still full of enthusiasm) we would tackle all the limitations in order to give way to an efficient, cost-effective and practical process which would yield us direct access to valuable meta-arylated anilines in gram scale.

From trial to flow.

Our initial hypothesis was that the copper source (i.e. copper triflate = Cu(OTf)2) used by Gaunt et al. was perhaps not the only option as a catalyst for this transformation. The reaction is believed to proceed via an electrophilic metalation on the aniline substrate.[2] Cu(I) species are oxidized to the Cu(III)-aryl intermediate, a highly electrophilic intermediate that can undergo Heck-like meta-arylation on the aniline (Figure 2). Gaunt speculated that the active Cu(I) species were formed in situ via reduction and or disproportionation of the copper triflate. However, looking at the reaction conditions, we concluded that the use of metallic Cu(0) could be an alternative choice. Apart from being inexpensive, readily available and air stable, we hypothesized that the catalytically active species (i.e. Cu(I)) could more easily be formed from Cu(0) in the presence of highly electrophilic diaryliodonium salt.

Figure 2: Our hypothesis to use a Cu(0) as a more a suitable and inexpensive catalyst source for the meta-selective C−H arylation reaction.

We started the initial investigation by conducting parallel batch reactions with either a Cu(II) or Cu(0) catalyst source. By checking the first GC results we knew we hit the jackpot. Not only was the Cu(0) (in this case inexpensive copper powder) suitable as catalyst precursor, but also a significant acceleration was observed. Unlike Cu(OTf)2, that needed 24 hours to complete conversion, copper powder resulted in reaction completion within 2 hours.

We speculated that this reaction could further benefit from using the so-called copper tube flow reactors (CTFRs)[3], which would allow for a significant breakthrough in operational simplicity for C−H activation chemistry. A homemade 20 mL CFTR was constructed from general purpose 1/8” copper tubing. Such tubing is normally used as gas lines for gas chromatography and could be readily ordered for less than 4 euro per meter. Using the flow reactor, full conversion and an improved yield (88%) was obtained within 20 minutes residence for our model substrate.

Homemade reagents.

After initial success with the copper tube flow reactor, it became clear to us that the diaryliodonium salts (that are used as the arylating reagent) were bringing in some limitations to the process. Despite being shelf-stable, nontoxic and synthetically useful, diaryliodonium salts continue to be limitedly available and very expensive. However, by trying to synthesize these reagents ourselves in the lab, we understood instantly why they are often neglected for application purposes. The preparation turned out to be cumbersome since hazardous reagents such as triflic acid (a super acid) and meta-chloroperbenzoic acid (a strong oxidant) needed to be handled in large amounts. On top of that, the synthesis was characterized by a huge release of energy, resulting in uncontrollable heating of the reactor at gram scale production. It was obvious that if we wanted to create a library of such compounds, this procedure needed some proper reevaluation and design.

At that point (in the late spring of 2016), Gabriele Laudadio arrived from Pisa University in Italy to join our team as a Ph.D. candidate. Enthusiastic and courageous of working with these hazardous reagents, Gabriele soon developed a preliminary capillary flow reactor and gave it a go. Substrates, acid, and oxidant were all fed separately to the flow reactor via syringe pump in order to guarantee maximum safety during operation. However, tides turned when on the first run the capillary started clogging severely and the process needed to be stopped immediately. Unable to recuperate the clogged tubing, we reconstructed a second reactor. Anticipating that under sonication clogging could be prevented, Gabriele ran to the other side of the lab to ‘borrow’ the ultrasonication bath of our colleagues. The reaction coil was submerged in the bath and subjected to ultrasound. Nicely, no reactor clogging was observed and the reaction ran smoothly. Finally, after some further optimization, the reactor could readily produce grams of diaryliodonium salts for us in a time-efficient fashion (up to 3.8 g/h). A library of diaryliodonium salts was made in in no time (Figure 3).

Figure 3: Library of diaryliodonium salts obtained at gram scale using the flow reactor.

Copper removal within EU standards approval.

Having obtained the required diaryliodonium salts from our first module. Gabriele and I started to work on the scope of the meta-arylation reaction in flow and we obtained a total of 23 compounds. In particular, the complete scope could be conducted with only one self-made copper coil without any loss of reactivity. Even better, when conducting the reaction on gram scale, nearly quantitative yield of product was obtained. This was unprecedented. This fact in combination with the fast reaction conditions obtained in flow (20 minutes) highlighted the potential of these CTFRs to enable C−H activation chemistry for gram scale drug manufacturing.

However, we were expecting that some copper leaching would take place when running the meta-arylation reaction. Indeed, after proper analysis, we realized that a significant copper content (around 4720 ppm) was present in our reaction mixture. To tackle this problem we constructed a 5 mL extraction coil which was attached to the end of the CTFR. This way, the organic phase exiting the CTFR was merged with an aqueous ammonia solution capable of extracting the copper. The performance of the extraction could be easily verified by the naked eye since copper complexation with ammonia resulted in a deep blue color of the aqueous phase (Figure 4). To make the extraction step directly coupled with an inline phase separation, a commercially available Zaiput liquid-liquid membrane separator was attached to the extraction coil which resulted in a perfect separation of the two phases. Satisfyingly, analysis of the extracted organic phase revealed that 97% of copper was extracted with a single pass through the module. This corresponds to a residual 14.3 ppm of copper, confirming to EU norms on the limits for parenterally administered pharmaceutical substances.[4]

Figure 4: Left: Test setup for the in-line extraction of copper from the organic reaction phase. Right: Deep blue color of aqueous phase after copper extraction with ammonia.

Why wouldn’t it hydrolyze?!

The last problem to circumvent was the flow deprotection of the amide function in order to obtain, finally, our valuable meta-arylated anilines. This turned out to be more problematic than we first anticipated. Because of the inert nature of our protection group (pivalamide), hydrolysis proved to be extremely challenging. After trying many different conditions (feel free to explore our supporting information on that matter J) no presentable results were obtained. Weeks later and about to give up on this endeavor, our last hope was the use of a 50:50 HCl:dioxane solvent mixture to further boost the deprotection procedure. Happily, our compound could be deprotected under such conditions. Translating our procedure to flow, we were thrilled to see that the pivalic protection group could be easily cleaved within only 40 minutes using superheated flow conditions (aka using a solvent above its boiling point). Such conditions are often discouraged in batch, but in flow this environment can be readily obtained by using the so called back pressure regulators. Thus, in this fashion, a series of desired meta-arylated anilines could be readily deprotected.

Last but not least, orchestrating all modules in an integrated process allowed to prepare meta-arylated anilines within a total time frame of 1 hour with excellent yield and purity, and without the need of chromatography! (Figure 5).

Figure 5: Overview of modular flow experiment for the synthesis of meta-arylated anilines.

A final word.

Looking back at this project, I think our winning strategy was the ability to identify the four key elements in the synthesis of meta-arylated anilines, reevaluate and improve them in order to obtain a final orchestrated process. The four separate modules that we developed (i.e. diaryliodonium synthesis, meta-selective C−H arylation, inline copper extraction and aniline deprotection) allowed us to directly access meta-arylated anilines in a time-efficient manner. The success of this project is, of course, the results of a great team effort, and it was my pleasure to share the struggles and satisfactions of this work with Gabriele, Kirsten, and Timothy.

We hope that each of the developed modules will be of high value and find widespread use in the synthesis of drugs, natural products and other molecules of relevance for the chemical industry.

As a group, and thanks to incentive funding from the Netherlands Organization for Scientific Research (NWO), we believe in the importance of open access scientific publications, which is why this manuscript is available free of charge for everybody who is interested!

Enjoy the reading!

Ciao,

Hannes Gemoets.

 

The paper discussed in this blog was published as: A Modular Flow Design for the Meta-selective Arylation of Anilines. H. P. L. Gemoets, G. Laudadio, K. Verstraete, V. Hessel, T. Noël, Angew. Chem. Int. Ed. DOI: 10.1002/anie.201703369.

References :

[1]          R. J. Phipps, M. J. Gaunt, Science 2009, 323, 1593-1597.

[2]          B. Chen, X. L. Hou, Y. X. Li, Y. D. Wu, J. Am. Chem. Soc. 2011, 133, 7668-7671.

[3]          Y. Zhang, T. F. Jamison, S. Patel, N. Mainolfi, Org. Lett. 2011, 13, 280-283.

[4]          European Medicines Agency (EMEA), “Guidelines on the specification limits for residues of metal catalysts or metal reagents”, can be found under http://www.ema.europa.eu/ema/, 2008.

An Artificial Leaf for Organic Synthesis – The making-of version

Today our paper on the development of a novel photomicroreactor for solar photochemistry was published in the ASAP section of Angewandte Chemie International Edition. We are very excited about this work and we would like to take you behind the scenes on how we conceived this idea, got the first results and finally made the required breakthrough that led to this publication.

Since the start of my independent academic career in 2012, my group has been intrigued by visible light photoredox catalysis. Photoredox catalysis provides neat solutions for previously elusive organic transformations (broad scope, high functional group tolerability, mild reaction conditions). However, one of the biggest hurdles of this chemistry was its scalability and we have worked on continuous-flow microreactor solutions to overcome these challenges.[1]

One of the major selling points of photoredox catalysis is that – at least theoretically – sunlight can be used to drive these reactions forward.[2] However, when scrolling through the photoredox literature (given its popularity, we can assure you this is a tremendous effort), sunlight was almost never used. And, when used, the reaction times became unrealistically long. We realized that continuous-flow microreactors could be beneficial here to ensure that the entire reaction mixture was irradiated equally. However, such flow reactors can only be useful when solar energy is abundant and focused towards the reactor (there is a recent review on this topic if you are interested in it).[3] We are living in The Netherlands, not really the most sun-rich region on this planet, so forget it – this approach will only work on sunny, cloudless days but they are far too scarce! There is even more: typical photoredox catalysts can only absorb light in a narrow wavelength-window (typically around their absorption maximum), which means that only a small amount of the solar energy is harvested to enable the chemical conversion. So photoredox catalysis on its own can never be energy efficient.

To address these issues, we looked at Nature’s tree leaf. It uses various antenna pigment molecules which allow harvesting solar energy. This harvested energy is subsequently transferred to the reaction center where CO2 and H2O are converted into sugars. So the question is, can we come up with a solution to mimic the behavior in the tree leaf?

A potential solution came when I was discussing my idea with Michael Debije. He is one of the leading experts in so-called luminescent solar concentrators (LSC).[4] When he showed me the material, we both had immediately the Eureka-feeling. Luminescent solar concentrators have been used in the past to improve the efficiency of photovoltaics. These polymeric plates contain fluorescent dyes which absorb the light and, due to internal reflection, the light is guided towards the edges. What is more, by changing the dye, you can also change the color of the polymeric material.

So I believe you can already see where we are heading? The idea is to carve channels into this luminescent solar concentrator AND to match the emission of the embedded dye with the absorption maximum of the photocatalytic system flowing in the microchannels (Fig. 1). As such, at least in theory, we would be able to mimic the principle of the tree leaf.

Fig.1. Comparison and analogy of solar harvesting and photon transfer in a leaf and in a LSC-based photomicroreactor. Note that the LSC Photomicroreactor leaf is a real device!

Simple enough? Well theoretically yes, but then you need to find funding and that is nowadays not a walk in the park, especially for conceptually new ideas. About a year later, I was able to secure Marie Curie funding (Photo4Future) and I got a personal award (VIDI), which gave me sufficient cash to start the hiring of a Ph.D. student and a postdoc to do the experiments.

In 2014, I had an interview with Dario Cambié and I was immediately convinced that he was the right person for the challenge. He had a pharmacist background but he was very broadly educated and was genuinely interested. So Dario came to Eindhoven and started to work on the project LSC-based photomicroreactors. While the project is fairly complicated (chemistry, material science, and engineering), we felt that we had assembled the right competencies to tackle this problem.

The team selected rapidly Lumogen F Red 305 (LR 305) as the dye for the LSC and methylene blue as the photocatalyst. As you can see from Figure 2, LR 305 can absorb a broad range of wavelengths and re-emits light perfectly at the absorption of methylene blue.

Fig. 2 Wavelength conversion scheme of the LR305/MB based LSC-photomicroreactor. The LR305 wide absorption (red) is responsible for the good light-harvesting properties of the device with respect to the solar spectrum (gray). The spectral overlap between the emitted photons (green) and methylene blue absorption maximum (blue) is crucial to allow effective coupling of the luminescent photons with the reaction system

Dario set out to work and he found he could embed LR 305 into PDMS. This polymer material has been used quite a lot in microfluidics since it can be easily shaped with soft lithography and print-and-peel techniques. Already with the naked eye, we could see that the wave guiding was prominent at high dye-loadings. Just check the picture of one of our earliest prototypes, as soon as you shine light on it from a cell phone you see the red halo at the edges of the device (Fig. 3).

Fig. 3. (A) One of the first prototypes of the LSC-photomicroreactor. Note the shining edges demonstrating the waveguiding potential of the device. (B) Using cell phone light results in a strong wavelength conversion as is evident by the red halo.

However, when we tried these prototypes to the singlet oxygen-mediated [4+2] cycloaddition of 9,10-diphenylanthracene, the results were initially not that great. So we set out to do some Monte Carlo ray-tracing simulations to understand the fundamental phenomena which were operative in our LSC photomicroreactor (More details on this model will soon be reported in a separate manuscript). We rapidly found that the height of the channels was crucial. The higher the channel, the higher the chance that the waveguided photons were captured within the reaction channel (see figure). Indeed, we observed for the first time a significant rate enhancement compared to a normal photomicroreactor.

At that point, Dr. Fang Zhao (a highly skilled chemical engineer) joined the team and we could increase our efforts to prove the efficacy of our device. In a series of fundamental experiments, we proved the light converting and the wave guiding ability of the device. Most of these experiments were carried out in the laboratory with LEDs or solar simulators. However, the real test for our device was an experiment outdoors on a normal day with quite some cloud coverage (see Figure 4a). Dario and Fang developed an ingenious experimental setup which allowed us to measure the conversion in both the LSC reactor and the normal non-doped version in real time. While we had some high expectations, the real data was even better than anticipated! The LSC-based system consistently showed higher conversion than the non-doped version, e.g. 40% higher yield for a residence time of 10s (see Figure 4b). However, and more importantly, the LSC photomicroreactor showed a very stable performance while the non-doped version was showing ups and downs in conversion. This can be very easy explained by the cloudy sky conditions. The LSC photomicroreactor allows harvesting both direct and diffuse light. The harvesting area of this device is quite large and once a photon falls on the device it will be converted and guided to the reaction channels. For a non-doped version, only light that falls directly on the channels will be used.  Just to give you an idea, our LSC device harvests up to 7x more photons than the non-doped version. And we believe that with some further modifications we can go up to 10x better performance. The fact that our device also works under cloudy sky conditions indicates that it can be used in those countries which are not blessed with a sunny climate.

Fig. 4. (A) A common cloudy summer day in The Netherlands. (B) Stable performance for the LSC photomicroreactor. Note that the dip in conversion is attributed to a large cloud, resulting in a drop in the light intensity.

Now critical scientists might say that it is easier to use a photovoltaic and then use the current to power an LED strip of the right color. True this is an option but it requires a couple of energy transformations which means that you will lose a lot (Second Law of Thermodynamics). For the photovoltaic/LED option, we calculated a 2.5 % overall efficiency (from solar energy to chemical conversion). However, with our device, you can skip a few steps and we come to an overall efficiency of 10%.

Before we forget, we can make this device essentially in any shape you want at almost no cost (material cost of our devices are below 1 euro). Our leaf design works as well as the normal square design. Moreover, the color makes it esthetically appealing and indeed such materials have been used in architecture, e.g. the Musac museum in Léon (Spain), the new biochemistry building at the University of Oxford (UK), and the Palais des congrès de Montréal (Canada). It would be cool to see the boring-greyish piping of chemical plants of today being replaced with our colorful, solar driven photochemical photoreactors. Both the nice color and the use of a sustainable energy source would help to alter the “negative” public image of the chemical industry.

We hope that this design will find its way into solar-driven photochemical transformations. Currently, we are working on exploring the potential of this LSC photomicroreactor in wide series of different photocatalytic transformations powered by solar light. These results will be published in due course! So keep an eye on our website.

Tim

The paper discussed in this blog was published as: A leaf-inspired luminescent solar concentrator for energy efficient continuous-flow photochemistry. D. Cambie, F. Zhao, V. Hessel, M. G. Debije, T. Noël, Angew. Chem. Int. Ed. DOI: 10.1002/anie.201611101 (VIP publication).

References:

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

[2] D. M. Schultz, T. P. Yoon, Science 2014, 343, 1239176.

[3] M. Oelgemoeller, Chem. Rev. 2016, 116, 9664-9682.

[4] M. G. Debije, P. P. C. Verbunt, Adv. Energy Mater. 2012, 2, 12-35.

Photochemical Processes in Continuous-Flow Reactors available for preorders!

A new textbook on photochemical processes in continuous-flow reactors, edited by Tim, is available for preorders on publisher’s website.

Photochemical Processes in Continuous-Flow Reactors front coverThis book gives an overview of both technological and chemical aspects associated with photochemical processes in microreactors. It provides analysis, the first of its kind, of these new technologies developed within the field of photochemical processes, with a description and case studies of practical implementation. It specifically looks at:

  • Design considerations of continuous-flow photoreactors;
  • Detailed descriptions of photon and mass-transfer phenomena;
  • Modeling approaches for photochemical transformations;
  • Scale-up strategies for photochemical transformations;
  • Examples of continuous-flow photochemistry in organic synthetic chemistry and material science;
  • Industrial examples of photochemical transformations.

By providing a deeper understanding of underlying concepts, coupled with numerous examples, this book is an essential reference for chemistry students, researchers and professionals working on photochemistry, photoredox catalysis, flow chemistry, process chemistry, and reactor engineering.

Cecilia’s blog

Cecilia BottecchiaOne year has passed since I first moved to Eindhoven, to start my Ph.D. in the group of Dr. Timothy Noël.  One year is maybe a good timeframe to try to do an analysis of this new life…

How’s the work? Highly demanding but rewarding, just like a Ph.D. should be! Moving the first steps in the world of scientific publications wasn’t easy, but the excitement of receiving the first positive comments from a reviewer makes the effort all worth.

How’s the Boss? Timothy for sure represents the most driven person I have ever met, and the determination and enthusiasm he shows in his job is contagious for all of us in the group. Thankfully, he also has a human side, which makes it easier for us to deal with his high standards!

What about Eindhoven?  Definitely not my favorite place on earth, its grey buildings and the almost constant pouring rain make it really hard for me to call this place “Home”. But there’s always a silver lining: cycling all around town is super easy, and the nightlife in Stratum sometimes is just what you need at the end of a tough week…

How are the colleagues? Well, it’s nice to say that I consider good friends most of my colleagues, and working with them is always a pleasure!

TU/e and EU flagsThe cherry on the pie? The project I work on is part of the Marie Curie Photo4Future ITN (Innovative training network), which means that every six months all the students and professors involved in the network get to meet  in a different location to update each other on the latest progress. So far, I had such a great time in the first two meetings, and I genuinely appreciated the time spent with the other students in the network.

Therefore, it’s right to say “So far, so good’!

Cecilia