Computational Results Provide a Synthetically Unprecedented Explanation for the beta-Regioselectivity in the Rh-catalyzed Hydroformylation of Vinylidenic Substrates

The approaching path of CO to the alkyl-Rh(CO)3 intermediate, considered for 1,1-diphenylethene as a test case to investigate the reason of its failure to produce quaternary aldehydes in hydroformylation reactions, shows a remarkable activation barrier in DFT calculations for the branched alkyl-Rh intermediate, but not for the linear one. Consideration of the free energy associated to this step strengthens this result, whereas the other steps remain fairly comparable. The path leading to the branched aldehyde is thus abandoned early, and this species then returns to the catalyst and the olefin, with subsequent reformation of the branched and linear alkyl-Rh intermediates until consumption of the olefin. The branched backward pathway is more favourable indeed than the forward one, as can be inferred from its free energy reaction profile. The heuristic value of theoretical/computational methods in exploring experimentally inaccessible species and in rationalizing the results, together with the importance of the organic part structure in organometallic complexes, is also put forward.