Mechanism and Origin of Chemoselectivity of Ru-Catalyzed Cross-Coupling of Secondary Alcohols to β-Disubstituted Ketones

Ru-catalyzed cross-coupling of secondary alcohols with only byproducts H-2 and H2O provides a green synthetic strategy to prepare beta-disubstituted ketones. Density functional theory (DFT) calculations were performed with the coupling of 1-phenylethanol and cyclohexanol as a model reaction to gain deeper mechanistic insights herein. The mechanistic details of the main reaction and the key steps of possible side reactions were clarified, and the obtained results are consistent with reported selectivity. Hydrogenation of alpha,beta-unsaturated ketones and dehydrogenation of ruthenium hydride intermediate are direct chemoselectivity-determining stages. The hydrogenation via 1,4-addition generates more stable intermediates, being favored over that via 1,2-addition, and thus avoids the formation of alkene products. The conjugation and pi-pi stacking effects of phenyl and the weak electronic effect of alkyls explain the dominance of specific ketone products in the hydrogenation stage. Hydrogenation of ketone products is kinetically operative but not exergonic enough to stop the irreversible dihydrogen release in an open reaction system, and thus alcohol products are absent. Furthermore, water evaporation in aldol condensation is found to be a double-edged sword, as it can accelerate the hydrogenation stage to prevent alpha,beta-unsaturated ketones from being the main products but decrease the selectivity therein from thermodynamics overall.