Anchoring of (tetraphenylporphmato)manganese(lll) acetate to a rigid polymer support considerably enhances the rate of cyclohexene epoxidation by this catalyst.The development of efficient oxidation catalysts after the received much attention.1 Recently, cytochrome P-450 models example of the mono-oxygenase enzymes in nature has have been reported that epoxidize olefins under relatively mild
SummaryOxidation of cyclohexene by (tetraphenylporphyrinato)m anganese(III) acetate (Mn(TPP)OAc) and sodium hypochlorite as oxidant has been studied in a two-phase w ater-dichlorom ethane system in the presence of a phase transfer reagent. A kinetic study with this system reveals the following features: (1 ) the main product of the reaction (yield > 80%) is 1 ,2-epoxycyclohexane; (2 ) in the presence of excess oxidant the reaction is zero order in cyclohexene; (3) the reaction order in Mn(III) concentration decreases from 1 to -0 with increasing concentration of this catalyst; (4) the reaction order in hypochlorite decreases from 1 to 0 with increasing concentration of this reagent; (5) pyridine and substituted pyridines enhance the reaction rate. A H am m ett treatm ent of the rate data for various substituted pyridines gives a p-value of -1.00; (6 ) anchoring of Mn(TPP)OAc onto poly(vinylpyridine) or a polym er of an isocyanide, (R-N = C < Jn, increases the reaction rate by a factor of 1.5 -6.0.Based on these findings and on evidence from the literature, a m ech anism for the epoxidation of cyclohexene is proposed. The key intermediate is an oxo-manganese(V) complex, which is formed from Mn(III) and hypo chlorite in a pyridine-catalyzed step. The Mn(V) species may react in 2 ways: either with substrate to give epoxide or with Mn(III) to form a i±-oxomanganese(IV) dimer. The latter route is suppressed when the catalyst is anchored to the polymeric support.
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