Anthracene-induced turnover enhancement in the manganese porphyrin-catalyzed epoxidation of olefins

Anthracene and related compounds function as lifetime-extending cofactors in the (meso-tetraphenylporphine)Mn(III) chloride-catalyzed epoxidation of olefins. An experiment with a chiral porphyrin catalyst shows that enantioselectivity is preserved in the presence of the cofactor. Additional experiments show that (a) turnover number enhancement is greatest for the least reactive substrates, (b) derivatization of anthracene at the 9 and 10 positions largely eliminates the enhancement effect, and (c) anthracene is ultimately converted to anthraquinone. The origin of the observed enhancements is in the reaction of anthracene with the normally unreactive dimeric oxo-bridged form of the catalyst. This reaction, which produces anthraquinone, regenerates the catalytically active monomeric form of the manganese porphyrin.