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.