PHOTORCA · Photoregulated organocatalysis - From caged catalysts to photoswitchable catalytic systems

Photoregulated organocatalysis - From caged catalysts to photoswitchable catalytic systems

The purpose of our project was the development of a controllable catalytic system with influence of light. We based our work on the amino acid organocatalytic system that allows for the formation of the reactive enamine species with enolisable aldehydes or ketones. Mainly, two approaches were proposed. The first one is based on the introduction of orthogonal photolabile protecting groups (deprotection by irradiation at different wavelengths) to provide for selective activation of a bifunctional catalyst (figure 1) and its corresponding different reactivity. The second approach deals with a photoswitch and concerns the inactivation of the free catalyst by complexation with the “open” merocyanine (MC) that can be obtain after irradiation of its closed spiropyran form (SP). In order to simplify and validate our proposed system, we first developed a system based on known protecting groups that can be removed chemically. We also simplified the secondary amine catalyst for the intended enamine catalysis. Thus, we chose TEOC protected pyrrolidine that can be readily deprotected in the presence of fluoride as a trigger to provide the free amine prone to form enamines with aldehydes or ketones. This system proved to be an applicable test system as the reaction upon addition of acetone to nitrobenzaldehyde could be followed by 1H NMR (figure 2). This system can be considered as a good fluoride sensing test as the amplification by the initiation of catalysis by the fluoride allows for a high sensitivity. To improve our method, an easier method of visualization of the reaction evolution was needed. For this purpose we developed and tested a number of different fluorochromic substrates for the reaction described above. However, we observed that maleimide based fluorochromic molecules themselves reacted with fluoride and hence the use of these fluorochromes allows for a direct fluoride detection and quantification. We proved that fluoride induces a polymerization reaction of the maleimide by a Baylis-Hillman type reaction mechanism. This reaction constitutes a new qualitative and quantitative test for fluoride (figure 3). These results were already published in New Journal of Chemistry in early 2011. The article was among the three most accessed articles in March, April and May 2011. We also proved that it was possible to activate a pyrrolidine-based catalyst bearing a 2-nitrobenzyloxy-carbonyl group by a simple light irradiation. The free catalyst undergoes the addition of acetone onto the already described fluorogenic maleimide. In order to get closer from our purpose and being able to switch the selectivity of the catalyzed reactions, the desired type 1 catalysts were synthesized in good yields. However, a detailed study on the irradiation of the molecule in solution with different wavelengths and different light sources showed that it was not possible to get an efficient and selective general method for the needed deprotection leading to the desired active catalyst.

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