Decisive effects of solvent and substituent on the reactivity of Ru-catalyzed hydrogenation of ethyl benzoate to benzyl alcohol and ethanol: A DFT study

Abstract

Density functional theory (DFT) calculations have been implemented to clarify the hydrogenation mechanism of a non-active ester: ethyl benzoate (PhCOOC2H5) to ethanol (C2H5OH) and benzyl alcohol (PhCH2OH) catalyzed by RuIIPNN. The calculated catalytic cycle involves two similar hydrogenation processes with the persistent participation of the catalyst: hydrogenation of PhCOOC2H5 to C2H5OH and intermediate PhCHO, and further hydrogenation to PhCH2OH. Three potential mechanisms, named as carbonyl insertion mechanism, stepwise double-hydrogen-transfer mechanism, and direct reduction mechanism, have been investigated in details in different solvents. It is found that the solvent has decisive influence on the hydrogenation mechanism. In a less polar solvent, the reaction prefers a stepwise double-hydrogen-transfer hydrogenation mechanism consisting of dihydrogen activation, stepwise double-hydrogen-transfer, and hydrogen abstraction rather than the carbonyl insertion mechanism proposed in the experimental work. However, in a more polar solvent, the reaction proceeds via the direct reduction mechanism involving the separated ions (PhCH(OC2H5)O− and monocation) instead of the experimentally proposed hemiacetal intermediate PhCH(OH)OC2H5. The dihydrogen activation as common starting point of the reaction in all three potential mechanisms can be facilitated by the products (C2H5OH and PhCH2OH). The substituent group on an ester has little influence on the reaction mechanism while it greatly affects the reactivity: the electron withdrawing group favors the hydrogenation, while the electron donating group makes the reaction more difficult. These theoretical results are expected to provide valuable guidance for the experimental study of the hydrogenation of non-active esters.