Mechanistic Insight into Fragmentation Reactions of Titanapinacolate Complexes

Reactions between terminal alkynes or aromatic ketones and titanapinacolate complexes (DMSC)Ti(OCAr2CAr2O) (2, Ar = Ph, and 3, Ar = p-MeC6H4; DMSC = 1,2-alternate dimethylsilyl-bridged p-tert-butylcalix[4]arene dianion) occur via rupture of the C−C bond of the titanacycle. Thus, reactions of 2 and 3 with terminal alkynes produce 2-oxatitanacyclopent-4-ene or 2-oxatitanacycloheptadiene complexes along with free Ar2CO. These compounds have been characterized spectroscopically and by X-ray crystallography. Because metallapinacolate intermediates have been implicated in important C−C bond-forming reactions, such as pinacol coupling and McMurry chemistry, the mechanism of the fragmentation reactions was studied. Analysis of the kinetics of the reaction of (DMSC)Ti{OC(p-MeC6H4)2C(p-MeC6H4)2O} (3) with ButC⋮CH revealed that the fragmentation reactions proceed via a preequilibrium mechanism, involving reversible dissociation of titanapinacolate complexes into (DMSC)Ti(η2-OCAr2) species with release of a ketone molecule, followed by rate-limiting reaction of (DMSC)Ti(η2-OCAr2) species with an alkyne or ketone molecule.