Highly Enantioselective C−H Oxidation of Arylalkanes with H 2 O 2 in the Presence of Chiral Mn‐Aminopyridine Complexes

Bioinspired chiral Mn‐aminopyridine complexes [( S , S )‐LMn II (OTf) 2 ] and [( R , R )‐LMn II (OTf) 2 ] (where ( S , S )‐L=(2 S ,2′ S )‐1,1′‐bis((3‐methyl‐4‐(2,2,2‐trifluoroethoxy)pyridin‐2‐yl)methyl)‐2,2′‐bipyrrolidine, and ( R , R )‐L=(2 R ,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 N ‐Boc‐( L )‐proline, in combination with [( R , R )‐LMn II (OTf) 2 ] complex, affords 1‐arylalkanols in up to 86 % ee , which is the highest reported enantioselectivity for direct benzylic hydroxylations with H 2 O 2 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.