A Kinetic Study of the Epoxidation 2,3-Dimethyl-2-butene by tert-Butyl Hydroperoxide Catalyzed by Imidazole Ligated (meso-Tetraphenylporphinato)manganese(III)

Article abstract of DOI:10.1021/ja00239a027

A kinetic study of the epoxidation of 2,3-dimethyl-2-butene (TME) by t-BuOOH in the presence of (mesotetraphenylporphinato)manganese(III) chloride (TPP)Mn(III)Cl) and imidazole (ImH) has been carried out (30 deg C, CH2Cl2 solvent).The rates of decrease in and increase in have been determined as a function of the initial concentrations of , , ImH>, and <(TPP)Mn(III)Cl>.As found previously, ImH ligation is required for the reaction of the manganese(III) porphyrin with t-BuOOH and thus for the epxidation of TME.Under the condition of _i, _i, and _i >> <(TPP)Mn(III)Cl>_i, it is found that the rate for disappearance of t-BuOOH is a;ways more than twofold greater than is the rate of epoxide formation regardlessof the ratios of , , and .This requires that in addition to an ImH ligated higher valent manganese oxo porphyrin there is the formation of an additional intermediate species capable of transferring oxygen.This must be so, because the total concentrations of (TPP)Mn(III)Cl is not sufficient to store the oxygen equivalents.Other pertinent observations are as follows: (i) Disappearance of t-BuOOH follows the first-order rate law in the absence of TME while in the presence of TME its disappearance follows two sequential first-order processes; (ii) formation of epoxide is always first-order; (iii) the maximum yield of epoxide is but 60percent; (iv) the percent yield of epoxide increases and then decreases with increase in _i; and (v) there is formation of a small percentage of di-tert-butyl peroxide ((t-BuO)2).A proposed reaction sequence which competent in accounting for these observations is presented in Scheme I. the equilibrium constants for ligation of ImH with manganese(III) porphyrin and the rate constants for oxygen transfer from t-BuOOH to both (TPP)Mn(III)(ImH)Cl and (TPP)Mn(III)(ImH)2 were determined individually while the other constants of Scheme I were obtained as the best minimal values by computer simulation of the time dependence for the disappearance of t-BuOOH and the appearence of epoxide and (t-BuO)2.