Room temperature synthesis of α,β-unsaturated nitriles by manganese-catalysed base-free coupling of α-saturated nitriles with aldehydes

Abstract

α,β-Unsaturated nitriles have broad applications, from polymers to pharmaceuticals, but their synthesis still poses significant challenges, as it often involves multistep procedures, or the use of precious metal catalysts, or the generation of significant amounts of waste. Herein, we report a new approach to the synthesis of such nitriles through single-step coupling of α-saturated nitriles with aldehydes, catalysed by a pincer complex of earth-abundant manganese. This reaction proceeds at room temperature in the absence of base or other additives, does not require substrate pre-activation and generates water as the only byproduct. The catalytic system was shown to tolerate various functional groups, including base-sensitive ones, and afforded a wide range of unsaturated nitriles in generally high yields. α-Deuterated vinyl nitriles were also obtained by coupling CD₃CN with different aldehydes in one-step room temperature syntheses. Experimental and computational data demonstrate that the catalytic reaction involves reversible binding of the nitrile substrate across the metal–ligand framework of the pincer complex, leading to ketimido and enamido complexes as intermediates. The enamido species functions as a carbon nucleophile that forms a C–C bond with an aldehyde to generate a β-hydroxynitrile intermediate, which is subsequently dehydrated into the vinyl nitrile product.