MECHANISM OF MANGANESE PORPHYRIN-CATALYZED OXIDATION OF ALKENES - ROLE OF MANGANESE(IV) OXO SPECIES

The mechanism for the bimolecular reaction of meso-tetrakis(2,6-dichlorophenyl)porphinato-oxo-manganese(IV), [(Cl8TPP)MnIV(O)], with alkenes has been investigated by kinetics and product identification. Kinetic studies were carried out with 11 alkenes (trans-4-methoxystilbene, cis-4-methoxystilbene, 1,4-diphenyl-1,3-butadiene, 4-methoxystyrene, 1,1-diphenylethylene, 4-methylstyrene, 2,3-dimethyl-2-butene, trans-stilbene, cis-stilbene, styrene, 4-acetoxystyrene) in methylene chloride solution (30-degrees-C) in air. The reactivities of the alkenes show that the trans alkenes are slightly more reactive than their cis isomers and that electron releasing substituents slightly favor the reaction. The second-order rate constant values (k2) correlate well with the potentials for the 1e- oxidation (E1/2) of the alkenes. The slope of the linear plot of log k2 vs E1/2 for the series of alkenes (slope = -0.89 V-1) indicate that a mechanism of epoxidation involving rate-determining formation of an alkene derived pi-cation-radical is unlikely. For the reaction with substituted styrenes, the linear free-energy relationship of log k2 vs sigma+ (rho+ = -0.99) supports a transition state with very little charge separation. Product yields determined for the reactions with cis-stilbene, trans-stilbene, 2,3-dimethyl-2-butene, cis-4-methoxystilbene, and trans-4-methoxystilbene are in accord with a mechanism involving the formation of a (porph)Mn(III)OCC. radical intermediate. Thus, the products of cis-stilbene oxidation under aerobic conditions are cis-stilbene oxide (7%), trans-stilbene oxide (5%), and benzaldehyde (3%). Comparison with the reactions carried out under conditions favoring the transiently stable manganese(V)-oxo species showed more efficient epoxidation with a greater degree of stereospecificity. In a search for radical intermediates the cis olefinic substrate (Z)-1,2-bis(trans-2,trans-3-diphenylcyclopropyl)ethene was used as a radical trap. While no epoxide products were found, a polar oxygen-containing product resulting from the opening of one trans-2,trans-3-diphenylcyclopropyl ring by a cyclopropylcarbinyl to homoallylcarbinyl radical rearrangement (CPCRR) was detected supporting the formation of a neutral carbon radical species.