Metal ion catalysis of oxygen-transfer reactions. II. Vanadium and molybdenum chelates as catalysts in the epoxidation of cycloalkenes

Abstract

The reactions of fert-butyl hydroperoxide with cyclohexene or cyclooctene, as catalyzed by a number of chelated diketonates of V02+, by (n-BuO)3VO, and by Mo02(acac)2, are highly selective and yield only tert-butyl alcohol and the corresponding epoxy cycloalkanes. Rate laws for the vanadium-catalyzed systems are consistent with reaction via rate-determining attack of olefin on a vanadium(V)-hydroperoxide complex. Association constants for the complexes and activation parameters associated with the various vanadium catalysts are different, indicating that these catalysts retain, at least in part, their ligand environments when converted to catálytically active species. Competitive inhibition by f-BuOH, reflecting the formation of catalyst-alcohol complexes, is observed in all cases; association constants for four of these alcohol complexes have been evaluated. Epoxidations of cyclohexene catalyzed by Mo02(acac)2 proceed approximately 102 as rapidly as those by the vanadium complexes under corresponding conditions. Kinetic behavior in the molybdenum system is consistent with the formation of molybdenum-hydroperoxide and molybdenum-olefin complexes, as well as a molybdenumhydroperoxide-olefin complex. Arguments are presented supporting the view that the molybdenum-catalyzed epoxidation, like those involving vanadium, proceeds by reaction of olefin with a metal-hydroperoxide complex, rather than through the ternary complex.The epoxidation of olefins using hydroperoxides or H2O2 in the presence of catalysts derived from vanadium, molybdenum, or tungsten2 \ / \ / is highly selective and, under favorable conditions, leads to quantitative yields of epoxides. Several workers2t)_d have suggested that this reaction is heterolytic, rather than free radical, in character, and an earlier study20 of the reaction in the presence of vanadyl acetylacetonate, VO(acac)2, provided evidence of the intervention of a hydroperoxide-vanadium-(V) complex and of severe competitive inhibition by alcohols, which are products when hydroperoxides are employed as oxidants. However, questions were left unanswered as to how the nature of the catalyzed reaction was affected by variation in the identity or oxidation state of the catalytic metal or by change in its ligand environment. The present report is directed toward partial resolution of these points; in addition, it presents further information concerning the changes occurring when VO(acac)2, a typical catalyst, is transformed to the principal active species during the early stages of reaction. Experimental SectionMaterials. Vanadyl acetylacetonate, VO(acac)2 (J. T. Baker), was recrystallized three times from anhydrous acetone; the purified chelate dissolved in benzene without leaving a residue. Since benzene solutions of this and other vanadyl chelates showed some deterioration on standing in air for several hours,20'3 each series of experiments employed a freshly prepared solution of metal chelate. Vanadium(III) acetylacetonate, V(acac)3 (J. T. Baker), was purified by dissolving in anhydrous...

Mechanistic insight into the cyclohexene epoxidation with VO(acac)2 and tert-butyl hydroperoxide

“…This mechanism was already proposed in the early seventies [4,[39][40][41][42][43]. In mechanism Ia the electron-deficient oxygen of the hydroperoxide ROOH is placed in the vicinity of the double bond in the transition state.…”

VAPO as catalyst for liquid phase oxidation reactions. Part II: stability of VAPO-5 during catalytic operation

“…Mechanistic investigations show that vanadium complexes are immediately converted to a vanadium peroxo complexes while in contact with the solution, resulting in comparable activities [13]. For example, the difference in reaction rate in the oxidation of cyclohexene with TBHP between the least active catalyst, VO(acac)2 and the most active catalyst (nBuO)3VO is only 3.7 [14]. It is likely that the vanadium ions leached from the VAPO-5 are present in solution as a peroxo complex because of the excess peroxide in the reaction mixture.…”

Peroxometal complexes

“…Therefore an effort has been made in the past years to deduce the structure of the species in solution from indirect evidence, mainly kinetic and spectroscopic [37][38][39][40][41][42][43][44][45][46][47]. The resulting picture may not be very accurate but it certainly offers a rationale to the different behavior observed.…”