OXIDATIVE ADDITION OF C–X BOND IN HCCX (X = H, CH3, ANDSiH3) TO RHODIUM(I) COMPLEX AND SUBSEQUENT 1,3-MIGRATION OF X: A DFT LEVEL STUDY

Abstract

Oxidative addition of the C-X bond in HCCX (X = H, CH3, SiH3) to RhCl(PH3)(2), as well as the subsequent 1,3-migration of the substituent X from Rh to the carbon of the ethynyl group leading to vinylidine-Rh complex, is studied using the localized molecular orbital (LMO) analysis at the density functional theory level. The highest Delta C double dagger values of 20.2, 16.3, and 18.5 kcal/mol are obtained respectively for the endothermic C-H bond oxidative additions of HCCH, HCCCH3, and HCCSiH3, which correspond to the slippage of the eta(2) alkyne complex RhCI(eta(2)-H-CCX)(PH3)(2). The 1,3-hydrogen migration, a one step process from Rh(H)(CCX)(Cl)(PH3)(2) proceeds via a three center transition state involving H center dot center dot center dot (CC) interactions with Delta G double dagger values of 18.7, 16.3, and 18.1kcal/mol for X = H, CH3, and SiH3, respectively. The overall nature of the reaction profiles and the LMO features suggest nearly identical behavior for all the C-H bond oxidative additions as well as the 1,3-hydrogen migrations. However, the energy profiles and LMO features of the C-CH3 and the C-SiH3 oxidative additions as well as the 1,3-shift of methyl and silyl groups show significant differences. The C-CH3 oxidative addition is highly endothermic (21.7kcal/mol) and thermodynamically disfavored as it require a Delta G double dagger of 42.2 kcal/mol. On the other hand, the C-SiH3 oxidative addition is the least endothermic (6.4kcal/mol) and it requires a moderate Delta G double dagger of 19.6kcal/mol. Further, the 1,3-SiH3 migration is the most favored reaction among all the reactions studied here as it proceed with a Delta C double dagger of 13.3kcal/mol. The small Delta C double dagger for C-SiH3 oxidative addition and the 1,3-SiH3 migration axe well explained based on the hypervalent character of silicon atom in the corresponding transition states as evidenced from their geometric and LMO features.