Franck Le Vaillant
Aliphatic alkynes and nitriles are privileged motifs in organic chemistry. Therefore, alkynes and nitriles have played a central role for the exploration and development of novel strategies to forge C-C bonds in efficient manner. They are broadly used as versatile building blocks for applications in every fields of chemistry, from pharmaceuticals to material sciences. Alkynes are also of paramount importance in chemical biology for labelling experiments due to their bio orthogonality.
In this context, radical chemistry provides endless alternatives to nucleophilic and electrophilic alkynylation and cyanation reactions. For decades, organic chemists have designed and improved the structure of alkynylating reagents in order to enhance the efficiency of group transfer process. Methods enabling alkynylation and cyanation of alkyl radicals have emerged and while providing useful tools, they set the basis for further improvements. Notably, the formation of alkyl radicals is limited to harsh conditions using peroxides, elevated temperature and radical initiators. To overcome these major limitations, photoredox catalysis has emerged as a powerful tool for the generation of alkyl radicals under ambient conditions. The combination of photocatalysts and alkynylating reagents allowed the photoredox-catalyzed alkynylation of organoboron trifluoroborates salt and N-phtalimide esters.
In this regard, the goal of my PhD was first to extend the photoredox catalyzed alkynylation to carboxylic acids. Indeed, carboxylic acids are broadly available from biomass, and widely occurring in drugs and fine chemicals, thus making them attractive starting materials. Their conversion to alkynes would provide efficient disconnections for the construction of Csp3-Csp, and find useful applications in chemical biology. Ethynyl Benziodoxolone (EBX) reagents, CsOBz and iridium photocatalysts were identified as a promising combination for the development of this novel transformation, in which silyl, aryl and alkyl substituted alkynes were transferred. ï¡ïamino, ï¡ïoxy and non-heteroatom stabilized radicals could be alkynylated in good to excellent yields upon visible light irradiation.
This strategy was next applied to the decarboxylative cyanation of carboxylic acids. In that regard, Cyano Benziodoxolone CBX was introduced for the first time in photoredox-catalyzed cyanation. The combination of CBX, CsOBz and the same iridium photocatalyst allowed the smooth conversion of amino and oxy acids to their corresponding nitriles, and this novel method was applied to the synthesis of important intermediates in the synthesis of APIs. During scope investigations, hydantoins were isolated as side products. This background reaction was further optimized as a novel and efficient synthesis of chiral hydantoins starting from amino acids and CBX. Finally, investigations of the reaction mechanisms of the photoredox-catalyzed decarboxylations revealed that the cyanation followed a divergent mechanism compared to alkynylation. A additional redox step between CBX and ï¡-amino radical allows the generation of cyanide and iminium, which upon recombination affords the nitrile product.
Then, the second goal of my PhD was to implement the photoredox mediated alkynylation and cyanation to other starting materials. In this regard, the use of carboxylic acids as starting materials to initiate photoredox catalyzed radical fragmentations was an appealing strategy.EPFL