MECHANISTIC AND SYNTHETIC STUDIES OF THE ADDITION OF ALKYL ALDEHYDES TO VINYLSILANES CATALYZED BY CO(I) COMPLEXES

The mechanistic details of the cobalt-catalyzed intermolecular hydroac ylation reaction have been investigated using kinetic, spectroscopic, and crystallographic methods. The Co(I) bisolefin complex 1, [C5Me5Co( C2H3SiMe3)(2)], was shown to catalyze the addition of a series of alky l aldehydes (2a-1) to vinylsilanes to give the corresponding ketones w ith exclusive anti-Markovnikov selectivity under mild conditions. The catalytic cycle exhibits two resting states, complex 1 and a bisalkyl carbonyl complex, [C5Me5Co(CO)(R)-(R')], 4a-1 which are in equilibrium . Kinetic investigations along with low-temperature NMR spectroscopy e stablish a sensitive balance between resting states during catalysis w hich is strongly dependent on substrate structure. The turnover-limiti ng step was established as the reductive elimination of ketone from in termediate 4. Using ferrocenecarboxaldehyde (Fc-C(O)H), 21, as substra te, the intermediate 41 [C5Me5Co(CO)(Fc)(CH2CH2SiMe3)] was isolated at low temperatures and characterized by X-ray crystallography. Complex 41 was used to study the carbon-carbon bond-forming step directly by t hermolysis in the presence of a trapping ligand L (P(OMe)(3), PMe3). K inetic analysis showed competitive ligand dependent and ligand indepen dent pathways for ketone formation. Deuterium scrambling, isomerizatio n of aldehydes prior to ketone formation, and production of isomeric k etones in certain cases establish that complex isomerization processes occur prior to productive ketone elimination from 4. A detailed mecha nism accounting for all observations is proposed. Catalyst deactivatio n was shown to involve primarily decarbonylation to yield [C5Me5Co(CO) ](2) and [C5Me5Co(CO)(C2H3SiMe3)]. When excess aldehyde is present, ca talytic aldehyde dimerization occurs to give esters.