Estimated distribution between one- and two-equivalent reaction paths in vanadium(V)-induced electron transfer

Abstract

The reactions of V^V with [Co(NH₃)₅L]²⁺ complexes of α-hydroxy-acids, mandelic (α-hydroxybenzene-acetic), lactic, and glycolic acids yield Co^II and a carbonyl compound to the extent of 50% of [Co^III]_initial; [V^V]_consumed is 1.5–1.7 times [Co^III]_initial. The other product of the reaction, phenylglyoxylatocobalt(III) complex, obtained in 50% yield by a two-electron oxidation, is responsible for the 50% change in absorbance at 502 nm. The rate of the reaction is proportional to the concentration of a V^V–Co^IIIcomplex, observed spectrophotometrically. This is consistent with the one-electron route preferred by V^V, producing a radical which, by nearly synchronous C–C fission and one-electron transfer to Co^III, produces Co^II. Stoicheiometric studies made with V^V and mandelic acid (Co^III-free system) reveal that the yields of PhCHO and phenylglyoxylic acid are nearly equal. When coupled with the results of the induced electron transfer using V^V, this suggests that the one- and two-equivalent paths prevalent in the oxidation of α-hydroxy-acids by V^V are very nearly of equal importance.