Mechanistic Features of the Oxidation–Reductive Coupling of Alcohols Catalyzed by Oxo-Vanadium Complexes

The oxo-vanadium-catalyzed redox disproportionation of activated alcohols (oxidation–reductive coupling, Ox–RC) produces carbonyl compounds and hydrocarbon dimers. A mechanistic study of this novel reaction is reported herein. Following our initial disclosure, new findings include the following: (1) The [(salimin)­VO₂]⁻-catalyzed Ox–RC of Ph₂CHOH in the presence of fluorene affords the products of H-atom abstraction and all possible hydrocarbon dimers. (2) Electronic substituent effects on the relative rates of Ox–RC with respect to 4-X-BnOH reactants and Bu₄N­[(Y-salimin)­VO₂] catalysts (1a–c) reveal (a) a correlation of the oxidation rate of X-BnOH reactants with the radical σ parameter and (b) correlation of the oxidation rate for (Y-salimin)­VO₂^– with the standard Hammett σ parameter. (3) The ease of electrochemical reduction of 1a–c is Y = NO₂ > OMe > H. (4) Ambient ¹H NMR studies of the interaction of 1 with alcohols suggest only a weak equilibrium association. (5) Density functional theory computational modeling of the Ox–RC reaction supports a ping-pong-type catalytic pathway, beginning with alcohol oxidation by (salimin)­VO₂^–, preferably by stepwise-H-atom transfer from the alcohol to 1, affording the carbonyl product and the reduced (salimin)­V­(III)­(OH)₂^–. The reduction half-reaction likely begins with condensation of the latter species with R₂CHOH to give the alkoxide complex (salimin)­V­(OR)­OH^–; homolysis of the R···OV­(III)­(salimin) bond affords (salimin)­V­(IV)­OH­(O)⁻ and the R-radical; the latter dimerizes and the former can disproportionate via H-transfer to reform catalyst (salimin)­VO₂^– (1) and (salimin)­V­(OH)₂^–.