Reactions between terminal
alkynes or aromatic
ketones and titanapinacolate complexes (
DMSC)Ti(OCAr(2)CAr(2)O) (2, Ar = Ph, and 3, Ar = p-MeC(6)H(4);
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 Ar(2)CO. 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-MeC(6)H(4))(2)C(p-MeC(6)H(4))(2)O] (3) with Bu(t)Ctbd1;CH revealed that the fragmentation reactions proceed via a preequilibrium mechanism, involving reversible dissociation of titanapinacolate complexes into (DMSC)Ti(eta(2)-OCAr(2)) species with release of a
ketone molecule, followed by rate-limiting reaction of (DMSC)Ti(eta(2)-OCAr(2)) species with an
alkyne or
ketone molecule.