Metal-free thioesterification of α,β-unsaturated aldehydes with thiols

For the first time, the synthesis of thioesters starting from enals and thiols has been performed in the presence of a bulky N-heterocyclic carbene (NHC) as a catalyst. This new method has been proved to be effective with a wide substrate scope giving selective thioesters in yields above 85% under mild and metal-free conditions. This green protocol does not require elevated temperatures, or addition of oxidants or other additives. The steric bulk of the carbene was found to markedly influence the reaction chemoselectivity. Bulky NHC carbene ligands, in contrast to those with less sterically developed ligands, completely change the chemoselectivity of the reaction leading to 1,2-addition products and not sulfa-Michael addition (SMA) adducts.