Despite being discovered about 70 years ago, Rhodium (Rh)-catalyzed hydroformylation still presents a challenge of understanding, especially in terms of the factors governing regioselectivity of product aldehydes. For example, a clear understanding of the factors impacting the linear:branched (l:b) ratio of aldehyde is still lacking. We have performed DFT calculations to reinvestigate the issue by studying the regioselectivity determining step of the Wilkinson catalytic cycle. A large variety of DFT functionals (e.g., B3LYP, M06-L), a range of substrates, including simple terminal (e.g. hexene, octene), hetero-atom-containing (e.g. vinyl acetate) or aromatic-substituted (e.g. styrene) alkenes, and different ligand structures (e.g., DIOP, DIPHOS, Xantphos) were selected for performing the quantum mechanical investigations. The calculations indicate that the M06-L functional reproduces the experimental trends with a reasonable degree of accuracy while the famous B3LYP functional fails to do so. The analysis of the transition states involved in the reaction explains the different behavior of two functionals - the transition states leading to the isomeric aldehyde are stabilized by the dispersion containing aromatic-aromatic (ligand-ligand) and aromatic-CH (ligand-substrate) interactions that cannot be described by B3LYP functional due to its inability of correctly predicting long-range interactions. In contrast, the M06-L functional provides a genuine description of such non-covalent effects and thus faithfully captures the experimental regioselectivity patterns. The calculations also suggest that the transition states leading to the linear aldehyde in the case of simple terminal olefins are selectively stabilized by these non-covalent interactions that accounts towards their high l:b regioselectivity ratios. These computations predict that it is the steric crowding-induced ligand-ligand and ligand-substrate interactions, but not intraligand interactions that determine the regioselectivity in Rh-catalyzed hydroformylation.

Noncovalent interactions determine regiochemistry of rhodium-catalyzed hydroformylation of unsaturated substrates: Another case of B3LYP failure

246th National Meeting (2013)

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