True Metal-Catalyzed Hydroboration with Titanium

Dicarbonyltitanocene is an efficient and highly selective catalyst
for alkyne hydroborations by catecholborane
and dimethyltitanocene is an efficient and highly selective catalyst
for alkene hydroborations. These results contrast
the hydroboration chemistry with other early transition metal complexes
that simply lead to decomposition of
catecholborane to form diborane and parallel the hydroboration
chemistry of permethylcyclopentadienyl lanthanide
complexes. Titanocene dicarbonyl leads to exclusive
anti-Markovnikov regiochemistry and to exclusive single
additions of catecholborane across alkynes. Dimethyltitanocene
leads to predominantly anti-Markovnikov regiochemistry with alkyl-substituted olefins, and exclusive anti-Markovnikov
regiochemistry with vinylarenes. Two
titanium(III) complexes,
Cp2Ti(H2Bcat) and
Cp2Ti(Bcat2), were isolated from the
reaction mixtures. These Ti(III)
complexes, as well as [Cp2TiH]2 and
[Cp2TiMe]2, catalyze the addition of
catecholborane to olefins more slowly
than the titanium(II) and titanium(IV) compounds. These
results are rationalized by a σ bond metathsis between
catecholborane and titanocene alkene and alkyne complexes that possess
metallacyclopropane and metallacyclopropene
character as the B−C bond-forming step. This B−C bond-forming
step of the catalysis was observed directly in
model reactions.