ONIOM Study of the Mechanism of Olefin Hydrogenation by the Wilkinson's Catalyst: Reaction Paths and Energy Surfaces of trans- and cis-Forms

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<jats:title>Abstract</jats:title> <jats:p>The mechanism of the olefin hydrogenation by Wilkinson’s catalyst is examined by the ONIOM method taking account of the substituents of the phosphine ligands. We optimized the equilibrium and transition state structures involved in the catalytic cycles by both active species, trans-[RhCl(PPh3)2] (1Rt) and cis-[RhCl(PPh3)2] (1Rc), and determined the energy profiles of these catalytic cycles. The steric repulsion between the two phosphine ligands in the cis-form was very small throughout the reaction, and in fact 1Rc was more stable by 3.3 kcal mol−1 than 1Rt by the stacking interactions between Ph groups, indicating that not only 1Rt but also 1Rc is a possible catalytically active species. The entire energy profiles showed that the olefin insertion is a rate-determining step in the case of the trans active species 1Rt. Although the activation energy of the olefin insertion is reduced so much in the case of the cis active species 1Rc, the transition states TS2 and TS3 of olefin insertion and isomerization also lie at the highest point of the energy surface. However, the cis-form is more favorable than the trans-form, since the top of the energy surface is lower for the cis-form than for the trans-form. The calculations suggested that if we think a large-size substituent of olefin, the cis-form that provides a larger space for the reaction will have an energy advantage. The steric or electronic effect of the Ph substituents of the phosphine ligands did not affect the entire energy profile substantially in both cases of trans- and cis-forms, although the inclusion of the Ph groups is important to evaluate the stability of isomers and to determine the most favorable path.</jats:p>

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