Role of vanadium alkylperoxo complexes in epoxidation of cyclohexene and oxidation of cyclohexane by organic hydroperoxides in the presence of bis (acetylacetonato)vanadyl

Abstract

Vanadium complexes formed from VO (acac) 2 as the starting material for the catalysts have been characterized in situ with 51V and 1H NMR and EPR in the course of three types of catalytic reactions: (1) decomposition of cumene and tert-butyl hydroperoxides in C 6D 6, (2) epoxidation of cyclohexene in C 6D 6:cyclohexene=1:1 mixture and (3) oxidation of cyclohexane in C 6D 6: cyclohexane = 1:1 mixture. In the course of all three reactions VO (acac) 2 transforms initially into the alkylperoxo complex VO(acac) 2OOR and the alkoxo complex VO(acac) 2OR, which further decompose to produce VO (OR) 3 and two other vanadium (V) species: an alkylperoxo complex I and an alkoxo complex II. VO (acac) 2OOR reacts with cyclohexene to form VO (acac) 2OR and cyclohexene oxide. I reacts with cyclohexene to form II and cyclohexene oxide and with ROH to form II and VO(OR) 3. II does not react with cyclohexene but reacts with ROOH to form I. No acetylacetonate ligands have been detected in I or II with 1H NMR. I is suggested to be the key species that permits cyclohexene epoxidation with ROOH under steady-state conditions. No complexes of I with cyclohexene are observed with 51V NMR. I does not react directly with cyclohexane, but through the equilibrium V vO(OOR) ⇌ V IVO+RO 2 · produces free radical RO 2 · that initiates oxidation of cyclohexane.