OXIDATIVE COUPLING OF PHENOLS USING VANADIUM TETRACHLORIDE AND VANADIUM OXYTRICHLORIDE

The oxidation of most simple phenols and some aniline deivatives with vanadium tetrachloride or vanadium oxytrichloride affords dimeric products which are coupled predominantly at the para position. For example, admixture of equimolar quantities of phenol and vanadium tetrachloride in an inert solvent followed by hydrolysis led to the isolation of 4,4ʹ-diphenyl, 2,4ʹ-diphenol, and 2,2ʹ-diphenol in an approximate ratio of 8:4:1 and an over-all yield of 55-65%. The remainder of the product mixture was composed of unreacted phenol and <5% of chlorinated diphenols. Phenol itself was not oxidized by vanadium oxytrichloride, under the condition of vanadium tetrachloride oxidation; however, those phenol derivatives with lower oxidation potentials were coupled by this reagent. Thus, 1- and 2-naphthol reacted smoothly with vanadium oxytrichloride to produce 4,4ʹ-dihydroxybinaphthyl and 2,2ʹ-dihydroxybinaphthyl in 56 and 65% yields, respectively. The oxidative coupling reaction is believed to occur by a rearrangement of electrons in a complex containing at least two phenoxide (or phenol) residues and at least one metal center. Evidence in support of the existence of vanadium phenoxides has been obtained. The selectivity of the coupling reaction may be interpreted in terms of a polar or ionic transition state in which charges are developed in the aromatic ring. © 1969, American Chemical Society. All rights reserved.