Ring size and nothing else matters: unusual regioselectivity of alkyne hydration by NHC gold() complexes

We have investigated the role of ring sizes and substituents in NHC ligands in some (NHC)Au( i ) complexes in the hydration of internal alkynes. Despite the fact that using (NHC)Au( i ) complexes in the hydration of diarylacetylenes leads to Markovnikov-type products, the precise tuning of ligands allows changing the regioselectivity in arylalkylacetylene hydration to the anti-Markovnikov-type. We have found that expanded-ring NHC gold( i ) complexes efficiently and regioselectively catalyze the hydration of alkylarylacetylenes in an anti-Markovnikov fashion due to steric repulsion build-up in the transition state, giving benzylalkyl ketones.