Hydrochlorination of acetylene using a supported gold catalyst: A study of the reaction mechanism

A detailed study of the hydrochlorination of acetylene and higher alkynes using a supported gold catalyst is described and discussed. A series of reactions using sequential exposure of the catalysts to C2H2 and HCl demonstrate that exposure to HCl before reaction of C2H2/HCl leads to enhanced activity, whereas exposure to C2H2 leads to deactivation. The reaction of higher alkynes is affected by steric factors, with the following trend in activity: acetylene (ca. 40% conversion) ≫ hex-1-yne (10%) > phenyl acetylene (7%) > hex-2-yne (2%). Using 1H NMR spectroscopy, we found that for hex-1-yne and phenyl acetylene, the anti-Markovnikov product is formed by anti-addition of HCl, but the Markovnikov products are equivalent for syn- and anti-addition of HCl. Thus, we investigated the reaction using deuterated substrates and confirmed that the products are formed by the anti-addition of HCl. The reaction mechanism is discussed in detail.