Dynamics of the reactions of [meso-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato]-manganese(III) hydrate with various alkyl hydroperoxides in aqueous solution. Product studies and comparison of kinetic parameters

Article abstract of DOI:10.1021/ja00033a030

The second-order rate constants (k_ly) for reactions of [meso-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato]manganese(III) hydrate [(1)Mn^III(X)₂, X = H₂O or HO^-] with t-BuOOH and (Ph)(Me)₂COOH have been determined in aqueous solution in the pH range 7.3-12.6. The pH dependencies of k_ly were fitted to a kinetic expression (eq 2) that was similar to that shown previously to describe the pH dependence of the reaction of (1)Mn^III(X)₂ with (Ph)₂(MeOCO)COOH. Comparison of the very similar pH-rate profiles for t-BuOOH, (Ph)(Me)₂COOH, and (Ph)₂(MeOCO)COOH (ROOH) showed that the log of the second-order rate constants exhibits only a modest dependency on the acidity of the ROH leaving group (-0.32 for the pH 7.3-10.0 range) as would be expected of a homolytic reaction. Product analysis on the reactions with t-BuOOH in the absence of the ABTS trapping agent provided (Me)₂CO (60-70%) as the major product with the remainder of the oxidant recovered as t-BuOH (12%), t-BuOOMe, (t-BuO)₂, MeOH, and HCHO. The product distributions showed no significant dependence on the pH of the reaction solutions. In the presence of ABTS (Me)₂CO is formed in 5% yield, and the main product is t-BuOH (89%). These findings are consistent with a mechanism involving the homolytic (but not heterolytic) cleavage of the O-O bond of manganese(III)-coordinated alkyl hydroperoxide. Addition of imidazole to the reaction of (1)Mn^III(X)₂ with t-BuOOH resulted in a ～4-10-fold enhancement in the rate of reaction. The pH dependence of log k_lm for the reaction in the presence of imidazole, from pH 5.3 to 12.6, was found to be in accord with that determined previously for (Ph)_2- (MeOCO)COOH. The product distribution for the reactions in the presence of imidazole showed significant dependence on the pH of the reaction mixtures. At pH 7.8 and 10.0 the product profiles were only consistent with a homolytic mechanism for the O-O bond cleavage where the major product was (Me)₂CO (63-67%), with the remainder being t-BuOH (19%), t-BuOOMe (13-16%), (t-BuO)₂, MeOH, and HCHO. At pH 12.6, the yield of t-BuOH (63%) increased dramatically with concomitant decreases in the yields of (Me)₂CO (34%), t-BuOOMe (4%), (t-BuO)₂, MeOH, and HCHO. The latter product distribution finds explanation in a change in mechanism of the O-O bond cleavage from homolysis to heterolysis as a result of the proton dissociation of the manganese(III)-coordinated ImH (i.e., (1)Mn^III(OOR)(ImH) → [(1)Mn^III(OOR)(Im)]^-). The acidity dependences of the 1e^- oxidation and reduction potentials of (1)Mn^III(X)(ImH) have been used to determine the acid ionization constants for the mono-imidazole-ligated (1)Mn^II(H₂O)(ImH), (1)Mn^III(H₂O)(ImH), and (1)Mn^III(H₂O)(ImH) species. The change in 1e^- oxidation potentials with pH has also been compared to the change in rate constants with pH for reactions occurring in the presence and absence of imidazole.