Stepwise alkyne insertion in Au(i) acetylides: influence of the nuclearity

Abstract

The reaction between NHC-supported (NHC = N-heterocyclic carbene) gold(I) trimethylsilylacetylide complexes with NHC gold(I) hydroxide species renders different symmetrical homobimetallic Au complexes. These compounds readily undergo migratory insertion of DMAD (dimethyl acetylenedicarboxylate) at 25 °C to give the corresponding bimetallic enyne products. On the contrary, monometallic analogues require much more forcing conditions (excess of DMAD and temperature ≥110 °C) to give the same transformation. Experimental and computational studies reveal that the second metal fragment is responsible for the enhanced nucleophilicity of the reactive carbon atom of the acetylide CC bond, which initially leads to a more favorable interaction with DMAD in the rate-determining step of an unprecedented, stepwise mechanism where the lability of the Au–C bonds plays an instrumental role. The enhanced reactivity displayed by the bimetallic species was leveraged in the insertion of other substrates such as heterocumulenes.