Theoretical insight into the Au(i)-catalyzed hydration of halo-substituted propargyl acetate: dynamic water-assisted mechanism

Abstract

The hydration mechanism of halo-substituted propargyl acetate, catalyzed by a homogenous Au(I) complex, has been investigated with the aid of the density functional theory (DFT) method. Our results reveal that the hydration is initiated by the favoured 1,5-exo-dig cycloaddition in the anti manner, affording a desired regioselective Markovnikov product. We also verify that neither the pathway towards the anti-Markovnikov product triggered by 1,6-endo-dig cycloaddition, nor direct nucleophilic attack by water, would happen without the help of neighbouring carbonyl groups. The favoured pathway mainly includes three processes: nucleophilic attack after 1,5-exo-dig cycloaddition, protodeauration, and enol–keto tautomerization. It turns out that the third process (enol–keto tautomerization) is the rate-determining step. Additionally, different halo-substituents cannot change the reaction trend, but slightly affect the relative energies. Particularly, cluster-continuum solvent models were established for some proton-transfer steps to rationally simulate reaction processes and evaluate energy barriers. Our study suggests that the presence of an explicit water-bridge is crucial to promote the hydration reaction. Computational results provide theoretical support for experimental observations, and insight into the hydration.