Scope and mechanism of the intermolecular addition of aromatic aldehydes to olefins catalyzed by Rh(I) olefin complexes

Rhodium (I) bis-olefin complexes Cp*Rh(VTMS)(2) and CpRh(VTMS)(2) (Cp* = C5Me5, Cp = C5Me4CF3, VTMS = vinyl trimethylsilane) were found to catalyze the addition of aromatic aldehydes to olefins to form ketones. Use of the more electron-deficient catalyst CpRh(VTMS)(2) results in faster reaction rates, better selectivity for linear ketone products from alpha-olefins, and broader reaction scope. NMR studies of the hydroacylation of vinyltrimethylsilane showed that the starting Rh(I) bis-olefin complexes and the corresponding Cp*Rh-/(CH2CH2SiMe3)(CO)(Ar) complexes were catalyst resting states, with an equilibrium established between them prior to turnover. Mechanistic studies suggested that CpRh(VTMS)(2) displayed a faster turnover frequency (relative to Cp*Rh(VTMS)(2)) because of an increase in the rate of reductive elimination, the turnover-limiting step, from the more electron-deficient metal center of CpRh(VTMS)(2). Reaction of Cp*Rh-/(CH2CH2SiMe3)(CO)(Ar) with PMe3 yields acyl complexes Cp*Rh-/[C(O)CH2CH2SiMe3](PMe3)(Ar); measured first-order rates of reductive elimination of ketone from these Rh(III) complexes established that the Cp ligand accelerates this process relative to the Cp* ligand.