Jf. Hartwig et Cn. Muhoro, Mechanistic studies of titanocene-catalyzed alkene and alkyne hydroboration: Borane complexes as catalytic intermediates, ORGANOMETAL, 19(1), 2000, pp. 30-38
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.