Enantioselective Benzylic Hydroxylation of Arylalkanes with H2O2 in Fluorinated Alcohols in the Presence of Chiral Mn Aminopyridine Complexes

A series of chiral bioinspired Mn-aminopyridine complexes of the type [L*Mn-II(OTf)(2)] (where L* is 2,2 '-bipyrrolidine derived ligand, bearing trifluoroalkoxy and alkyl substituents) have been tested as catalysts in benzylic C-H hydroxylation of arylalkanes with H2O2 in fluorinated ethanols media. In 2,2,2-trifuoroethanol, the yield of the target ethylbenzene oxidation product, chiral 1-phenylethanol, reaches 45 %, which is much better than in the common solvent CH3CN (5-6 %). The selectivity for 1-phenylethanol formation increases in the following order: CH3CN<2-fluoroethanol<2,2-difluoroethanol<2,2,2-trifuoroethanol, while 2,2-difluoroethanol ensures the highest asymmetric induction in this series, affording chiral benzylic alcohols with up to 89 % ee. In trifluoroethanol, the observed primary k(H)/k(D) value of 2.3 has been measured for the oxidation of 1-phenylethanol/alpha-D-1-phenylethanol, which is similar to that in CH3CN (2.2). At the same time, depending on the solvent, CH3CN or 2,2,2-trifuoroethanol, the oxidations of 1-phenylethanol demonstrates drastically different linear free-energy relationships; possible effect of the hydrogen-bond donor (HBD) nature of CF3CH2OH is discussed in this context. Noticeably, it has been shown that by switching the absolute chirality ((S,S)- or (R,R)-) of the catalyst, the oxidation of complex substrate of natural origin, estrone acetate, can be diverted to predominant formation of either the tertiary C9-alcohol or of the C6-ketone, respectively.