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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.

The Wittig reaction of the unsubstituted triphenylphosphonium ylide (Ph3PCH2) on an aldehyde with two stereogenic centres (2S,3R)-2,4-dimethyl-3-pyrrol-1-yl-pentanal) has been investigated with a density functional (DF) approach at the B3P86/6-31G* level in vacuo and in tetrahydrofuran (THF) to shed some light on the mechanistic details when bulky chiral substrates are employed in the absence of E/Z selectivity issues. The interest is focused on the reaction intermediates, primarily on those that cannot be observed spectroscopically. Both the concerted mechanism (with a four-centre transition state (TS)) and the stepwise one (occurring via a betaine-type zwitterionic intermediate) have been explored, examining the potential energy profiles as well as the free energy ones. The solvent effect in the integral equation formalism-polarizable continuum model (IEF-PCM) framework has been also considered for THF. Interestingly, the early TS found in vacuo for both mechanisms becomes vanishingly small in THF, while the reactant adducts turn out to be somewhat destabilized, with an overall similarity in the relevant profiles.