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

Abstract

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 –.

The preparation of new palladium (II) complexes with Schiff base type ligands and its impregnated Al2O3 materials: As the catalysts for degradation/reduction of organic dyes

“…Additionally, the C H O proton of benzaldehyde was known to monitored at 11.07 ppm in the literature. [ 35 ] In the 1 H‐NMR spectra of the Schiff base ligands ( HL1 – 3 ), the C H N protons were observed as a singlet at 8.69, [ 32 ] 8.53, and 8.67 ppm, respectively. After complexation with Pd (OAc) 2 , the C H N protons of ligands shifted upfield and were monitored at 7.54, 7.65, and 8.45 ppm as singlet for C1 – 3 , respectively.…”

“…The HL1 and HL2 were prepared from the literature procedure. [ 32,33 ] A mixture of the compound 3 (1 mmol) and 2‐hydroxyaniline (1 mmol) in ethanol (EtOH) (25 ml) was stirred at reflux conditions for overnight. The HL3 was obtained from the slow evaporation of the above mixture.…”

Deoxygenative Transition-Metal-Promoted Reductive Coupling and Cross-Coupling of Alcohols and Epoxides

“…A mechanistic study of the oxo-vanadium-catalyzed oxidation-reductive coupling of activated alcohols was conducted employing experimental and computational methods. 31 The vanadium-catalyzed reaction of 1:1 benzhydrol/fluorene produced diphenylmethane, the homodimers tetraphenylethane and bifluorenyl, and the mixed dimer diphenylmethyl-fluorenyl, suggestive of carbon free-radical intermediates. Electronic substituent effects on the relative reaction rates of X-BnOH and (Y-salimin)VO 2 are indicative of developing radical and positive charge on the benzyl unit in the oxidation step.…”

Oxo-Rhenium-Catalyzed Radical Addition of Benzylic Alcohols to Olefins

“…19 In contrast, oxovanadium complexes were found to catalyze redox disproportionation of benzylic and allylic alcohols (Scheme 1B), resulting in coproduction of carbonyl products and the reductively coupled hydrocarbon dimers. 20 Experimental and computational studies of the rhenium- 19,21 and vanadium-catalyzed 22 reductive coupling reactions point to the intermediacy of carbon radicals that are generated by the facile homolytic C−O cleavage of reduced metal−alkoxide intermediates. This novel transformation is a rare example of C−C bond formation from alcohol substrates.…”