Mechanistic studies of titanocene-catalyzed alkene and alkyne hydroboration: Borane complexes as catalytic intermediates

The bis(borane) complex Cp2Ti(HBcat')(2) is a highly active catalyst for th e hydroboration of vinylarenes. We provide a detailed mechanistic analysis of this hydroboration process and of the hydroboration of alkynes catalyzed by titanocene dicarbonyl. The hydroboration of alkynes showed a reaction r ate that was first order in the concentration of alkyne, inverse second ord er in the concentration of carbon monoxide, and first order in the concentr ation of borane. These data are consistent with a mechanism in which two eq uilibria involving Cp2Ti(CO)(PhCCPh) generate the alkyne borane complex Cp2 Ti(HBcat')(PhCCPh), which forms the hydroboration product. The hydroboratio n of alkenes catalyzed by the bis(borane) complex Cp2Ti(HBcat')(2) involves the similar intermediate Cp2Ti(CO)(RCH=CCH2), containing a coordinated alk ene, rather than alkyne. The catalytic process for alkene hydroboration is inverse first order in borane and first order in alkene, indicating that th e reaction occurs by reversible dissociation of borane and coordination of alkene to form an alkene borane complex that undergoes elimination of alkyl boronate ester. Reactions of this complex that compete with the production of alkylboronate ester include the formation of vinylboronate esters, presu mably by beta-hydrogen elimination of the intermediate complex formed by ad dition of alkene to coordinated borane. This beta-hydrogen elimination path way was suppressed by using excess catecholborane.