Regio- and stereo-selectivity of acetoxymercuration of acetylenes

Abstract

Acetoxymercuration of alkylphenylacetylenes (PhCCR; R = Me, Et, Prⁿ, or Buⁿ)(1) in acetic acid at 20–60 °C proceeds completely in a trans-fashion to afford a regioisomeric mixture of two vinylmercury(II) compounds, Ph(OAc)CC(HgCl)R (2) and Ph(HgCl)CC(OAc)R (3), the extent of attack of Hg at the carbon a to alkyl group to form (2) being increased upon increasing the carbon chain length ofthe alkyl group [(2)/(3)= 3.0–16.5]. In the case of R = Prⁱ(2) is the sole product, while no acetoxymercuration occurs when R = Bu^t. A bridged mercurinium ion intermediate in which phenyl and alkyl groups are bent toward the attacking side of acetate anion is proposed for explaining a marked dependence of the isomer ratio (2) : (3) on the kind of alkyl groups. Similar treatment of diphenylacetylene at 20–95 °C gives only the cis-adduct as reported previously, while that of diethylacetylene affords the trans-adduct and a mixture of trans- and cis-adduct at 20 °C and at 65 °C, respectively. The NaBH₄ reduction of (2; R = Me) in neutral condition produces a mixture of (Z)- and (E)-1-acetoxy-1-phenylpropene (ca. 3–5 : 1) in contrast to the reduction of thallium analogue of (2) which gives only the Z-isomer.