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 the 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 rate that was first order in the concentration of alkyne, inverse second order in the concentration of carbon monoxide, and first order in the concentration of borane. These data are consistent with a mechanism in which two equilibria involving Cp2Ti(CO)(PhCCPh) generate the alkyne borane complex Cp2Ti(HBcat')(PhCCPh), which forms the hydroboration product. The hydroboration of alkenes catalyzed by the bis(borane) complex Cp2Ti(HBcat')2 involves the similar intermediate Cp2Ti(CO)(RCHCCH2), containing a coordinated alkene, rather than alkyne. The catalytic process for alkene hydroboration is inverse first order in borane and first order in alkene, indicating that the reaction occurs by reversible dissociation of borane and coordination of alkene to form an alkene borane complex that undergoes elimination of alkylboronate ester. Reactions of this complex that compete with the production of alkylboronate ester include the formation of vinylboronate esters, presumably by β-hydrogen elimination of the intermediate complex formed by addition of alkene to coordinated borane. This β-hydrogen elimination pathway was suppressed by using excess catecholborane.