ic8b02968_si_002.xyz (38.17 kB)
Mechanistic Features of the Oxidation–Reductive Coupling of Alcohols Catalyzed by Oxo-Vanadium Complexes
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posted on 2018-12-07, 21:22 authored by Eric Steffensmeier, Matthew T. Swann, Kenneth M. NicholasThe 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–.