Investigation of alkyl metal intermediates formation in the rhodium-catalyzed hydroformylation: experimental and theoretical approaches

This review describes experimental results obtained for regioselectivities and diastereoselectivities in rhodium-catalyzed hydroformylations and deuterio-formylations of a variety of unsaturated substrates with unmodified Rh catalysts and compares them with values computed in the density functional theory (DFT) framework. Deuterioformylation experiments pointed out that under mild reaction conditions isomeric alkyl metal intermediate formation is non-reversible; hence the selectivity for alkyls reflects the selectivity for the aldehydes. The stability of the relevant alkyl rhodium transition states (TS) has been determined with computational methods at the B3P86/6-31G* level (employing effective core potentials for Rh in the LanL2DZ valence basis set). Theoretical results turn out to be in good agreement with the experimental ones obtained under mild reaction conditions. Significant differences between theory and experiment are conversely obtained for hydroformylations of vinylidenic olefins, such as 1,1-diphenylethene, carried out at high temperature, where beta-hydride elimination takes place. To clarify the reaction mechanism under those reaction conditions, it was necessary to compute the whole reaction mechanism, including zero point and thermal corrections also. The calculations, performed on the hydroformylation of 1,1-diphenylethene (yielding almost exclusively linear aldehydes), allowed us to put forward a novel explanation for that behavior: the addition of the fourth CO group to the tricarbonyl intermediate to give the tetracarbonyl one is prevented in the branched isomer, but not in the linear isomer.