Mechanism of the Platinum/Tin-Catalyzed Asymmetric Hydroformylation of Styrene: A Detailed Computational Investigation of the Chiral Discrimination

The Pt/Sn-catalyzed asymmetric hydroformylation of styrene in the presence of the model complex PtH­(SnCl₃)­(chiraphosH) (chiraphosH = (2S,3S)-2,3-bis­(diphosphino)­butane) has been investigated by means of DFT calculations. All elementary steps of the catalytic cycle have been considered: i.e., the olefin coordination, its insertion into the Pt–H bond, CO activation and its subsequent insertion into the Pt–alkyl bond, dihydrogen activation, and reductive elimination resulting in aldehydes. The olefin insertion has been found as the rate-determining and selectivity-determining steps for both the linear and branched pathways. At the PBEPBE level the regio- and enantioselectivities were predicted to be 86% and 51%, respectively, when solvation corrections were employed, providing acceptable to excellent agreement with the experimental results. MP4­(SDQ) calculations predicted 96% for the branched regioselectivity and an ee of 47% with slightly higher barriers in comparison to the DFT results. On inspection of the olefin insertion transition states there is an energetic preference at both levels of theory for the SnCl₃ ligand occupying the equatorial position. The calculations have also been in line with the somewhat reduced activity of the chiraphos-containing catalytic system, which may be attributed to the restricted ability of chiraphos to balance the charge distribution during the rate-determining step.