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Highly Enantioselective C−H Oxidation of Arylalkanes with H 2 O 2 in the Presence of Chiral Mn‐Aminopyridine Complexes
Author(s) -
Talsi Evgenii P.,
Samsonenko Denis G.,
Ottenbacher Roman V.,
Bryliakov Konstantin P.
Publication year - 2017
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201701169
Subject(s) - chemistry , medicinal chemistry , hydrogen peroxide , ethylbenzene , catalysis , hydride , enantioselective synthesis , hydroxylation , stereochemistry , metal , organic chemistry , enzyme
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.