Highly Enantioselective C-H Oxidation of Arylalkanes with H2O2 in the Presence of Chiral Mn-Aminopyridine Complexes

Bioinspired chiral Mn-aminopyridine complexes [(S,S)LMnII( OTf)(2)] and [(R,R)-LMnII(OTf)(2)] (where (S,S)-L=(2S, 2'S)-1,1'bis(( 3-methyl-4-(2,2,2-trifluoroethoxy) pyridin-2-yl) methyl)-2,2'bipyrrolidine, and (R,R)-L=(2R,2'R)-1,1'-bis((3-methyl-4-(2,2,2-trifluoroethoxy) pyridin-2-yl) methyl)-2,2'-bipyrrolidine) have been shown to efficiently catalyze the benzylic C-H oxidation of arylalkanes with hydrogen peroxide in the presence of carboxylic acid additives, affording enantiomerically enriched 1-arylalkanols and the corresponding ketones. Optically pure additive NBoc-(L)-proline, in combination with [(R,R)-LMnII(OTf)(2)] complex, affords 1-arylalkanols in up to 86% ee, which is the highest reported enantioselectivity for direct benzylic hydroxylations with H2O2 in the presence of transition-metal catalysts. Oxidative kinetic resolution only slightly contributes to the increase of the observed enantiomeric excess over the reaction course. The observed k(H)/k(D) values (3.5-3.6 for the oxidation of ethylbenzene/d(10)-ethylbenzene) and competitive oxidation data are consistent with either a hydrogen-atom transfer/oxygen rebound or hydride transfer/oxygen rebound asymmetric hydroxylation mechanism.