Mechanistic and Synthetic Studies of the Addition of Alkyl Aldehydes to Vinylsilanes Catalyzed by Co(I) Complexes

Abstract

The mechanistic details of the cobalt-catalyzed intermolecular hydroacylation 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 alkyl aldehydes (2a−l) to vinylsilanes to give the corresponding ketones with 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−l which are in equilibrium. Kinetic investigations along with low-temperature NMR spectroscopy establish a sensitive balance between resting states during catalysis which is strongly dependent on substrate structure. The turnover-limiting step was established as the reductive elimination of ketone from intermediate 4. Using ferrocenecarboxaldehyde (Fc-C(O)H), 2l, as substrate, the intermediate 4l [C5Me5Co(CO)(Fc)(CH2CH2SiMe3)] was isolated at low temperatures and characterized by X-ray crystallography. Complex 4l was used to study the carbon−carbon bond-forming step directly by thermolysis in the presence of a trapping ligand L (P(OMe)3, PMe3). Kinetic analysis showed competitive ligand dependent and ligand independent pathways for ketone formation. Deuterium scrambling, isomerization of aldehydes prior to ketone formation, and production of isomeric ketones in certain cases establish that complex isomerization processes occur prior to productive ketone elimination from 4. A detailed mechanism accounting for all observations is proposed. Catalyst deactivation was shown to involve primarily decarbonylation to yield [C5Me5Co(CO)]2 and [C5Me5Co(CO)(C2H3SiMe3)]. When excess aldehyde is present, catalytic aldehyde dimerization occurs to give esters.