Hydroboration of substituted alkynes using a solid polymeric carboxylic acid catalyst

Abstract

A polymer-supported benzoic acid catalyst for the hydroboration of substituted alkynes with pinacolborane is reported. The robustness of the heterogenous catalyst is demonstrated through catalyst recycle, with comparable reactivity to the homogeneous catalyst. A mechanistic investigation that includes determination of a reaction rate law, 1H and 11B NMR studies, as well elucidation of the H/D kinetic isotope effect is used to identify the pathway through which product formation occurs. The polymer support provided a recoverable handle, with the active sites in close proximity to promote a two-site reaction pathway with the identification of key boryl species through 11B NMR. The activated alkynyl species is converted through a concerted hydroboration mechanism, supported by inverse KIE values, releasing product and regenerating the active site. Kinetic studies, where initial rates are proportional to the concentration of both the pinacolborane and the catalyst, but become inhibited by the alkyne at higher concentrations, suggest that the catalytic cycle can be described by a two-site rate expression with alkyne-derived surface species present during catalysis.