Kinetics of hydroxylation of cholesterol in the presence of the ethane-bridged Mn-Cu-bis(etioporphyrinate) heterodimer

The catalytic activity of the ethane-bridged Mn-Cu-bis(etioporphyrinate) he terodimer in the hydroxylation of cholesterol in the presence of boron sodi um hydride was studied. It was shown that the effective rate constant k(eff ) of cholesterol hydroxylation in the presence of the asymmetric dimer is b y an order of magnitude higher than that for its symmetric analogue [ethane -bridged Mn-Mn-bis(etioporphyrinate)] and two orders of magnitude higher th an for the monomer. It was suggested that this effect is associated with an increase (for heterodimer) in the rate constant of the formation of an int ermediate ternary complex composed of the reduced central Mn2+ ion, molecul ar oxygen in a singletlike state, and substrate, a limiting stage of the pr ocess. It was established that the key factor responsible for this increase is an asymmetric internal electric field of the heterodimer, which promote s the polarization of the reagents in the intermediate complex.