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

Wu, Xiao-Nan; Zhao, Hai-Tao; Li, Jilai; Schlangen, Maria; Schwarz, Helmut

Inst. Chemie

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(CH3CH[double bond, length as m-dash]CH2)]+. 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 CH3CH2[double bond, length as m-dash]CH2 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.