Mechanism of Oxidation of the 2-Hydroxycyclohexyl Radical to Cyclopentanecarbaldehyde by Copper Ions in Aqueous Solutions

Copper ions are known to catalyze the oxidation of cyclohexene by persulfate to yield cyclopentanecarbaldehyde. The mechanism suggested in the literature for this process involves the reaction of 2-hydroxycyclohexyl free radicals with Cu²⁺(aq). In the present study the kinetics of reaction of these free radicals with copper ions were studied by the pulse-radiolysis technique. The results indicate that the mechanism of the catalytic process involves the following reactions: The reaction of the free radical with Cu⁺(aq), and not with Cu²⁺(aq), yields the transient complex 1-cuprio-2-hydroxycyclohexane(1+). This transient complex rearranges in a reaction that involves the ring contraction step to form 6-hydroxy-7λ²-cuprabicyclo[3.2.0]heptane. The latter intermediate decomposes via heterolysis of one of the copper(II)-carbon σ bonds to form (1-cupriocyclopentyl)methanol(1+), which reacts with Cu²⁺(aq) to form the final product, cyclopentanecarbaldehyde. The rates of the first three reactions in this sequence and the spectra of the first two intermediates are reported. The results thus point out the possibility to use the pulse-radiolysis technique to elucidate the mechanisms of complex catalytic processes.