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 Bu¹C≡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.