Dissymmetric gold(i) N-heterocyclic carbene complexes: a key unexpected structural parameter for highly efficient catalysts in the addition of alcohols to internal alkynes

Gold(I) N-heterocyclic carbene complexes (Au-NHC) with symmetric (bis-benzyl, -propyl and -mesityl substituents) and dissymmetric (mesityl and alkyl--benzyl or propyl--substituents) NHC ligands were synthesized and tested as catalysts for the addition of methanol to 3-hexyne, as a representative internal alkyne. While symmetric ones--bis-alkyl (propyl or benzyl) or bis-mesityl systems--displayed low activity, dissymmetric Au-NHC systems with one alkyl (benzyl or propyl) and one mesityl groups on the NHC unit were unexpectedly highly active with rates and turnover numbers up to 294,000 h(-1) and 800,000 mol.mol(Au)(-1).