Highly regioselective hydride transfer, oxidative dehydrogenation, and hydrogen-atom abstraction in the thermal gas-phase chemistry of [Zn(OH)]+/C3H8

Abstract

The thermal reactions of [Zn(OH)]⁺ with C₃H₈ have been studied by means of gas-phase experiments and computational investigation. Two types of C–H bond activation are observed in the experiment, and pertinent mechanistic features include inter alia: (i) the metal center of [Zn(OH)]⁺ serves as active site in the hydride transfer to generate [i-C₃H₇]⁺ as major product, (ii) generally, a high regioselectivity is accompanied by remarkable chemoselectivity: for example, the activation of a methyl C–H bond results mainly in the formation of water and [Zn(C₃,H₇)]⁺. According to computational work, this ionic product corresponds to [HZn(CH₃CHCH₂)]⁺. Attack of the zinc center at a secondary C–H bond leads preferentially to hydride transfer, thus giving rise to the generation of [i-C₃H₇]⁺; (iii) upon oxidative dehydrogenation (ODH), liberation of CH₃CH₂CH₂ occurs to produce [HZn(H₂O)]⁺. Both, ODH as well as H₂O loss proceed through the same intermediate which is characterized by the fact that a methylene hydrogen atom from the substrate is transferred to the zinc and one hydrogen atom from the methyl group to the OH group of [Zn(OH)]⁺. The combined experimental/computational gas-phase study of C–H bond activation by zinc hydroxide provides mechanistic insight into related zinc-catalyzed large-scale processes and identifies the crucial role that the Lewis-acid character of zinc plays.