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

Abstract

Bioinspired chiral Mn-aminopyridine complexes [(S,S)-LMnII(OTf)2] and [(R,R)-LMnII(OTf)2] (where (S,S)-L=(2S,2S)-1,1-bis((3-methyl-4-(2,2,2-trifluoroethoxy)pyridin-2-yl)methyl)-2,2-bipyrrolidine, and (R,R)-L=(2R,2R)-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 N-Boc-(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 kH/kD values (3.5–3.6 for the oxidation of ethylbenzene/d10-ethylbenzene) and competitive oxidation data are consistent with either a hydrogen-atom transfer/oxygen rebound or hydride transfer/oxygen rebound asymmetric hydroxylation mechanism.