TY - JOUR

T1 - Enantioselective Benzylic Hydroxylation of Arylalkanes with H2O2 in Fluorinated Alcohols in the Presence of Chiral Mn Aminopyridine Complexes

AU - Ottenbacher, Roman V.

AU - Talsi, Evgenii P.

AU - Rybalova, Tatyana V.

AU - Bryliakov, Konstantin P.

PY - 2018/11/22

Y1 - 2018/11/22

N2 - A series of chiral bioinspired Mn-aminopyridine complexes of the type [L*MnII(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 kH/kD value of 2.3 has been measured for the oxidation of 1-phenylethanol/α-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.

AB - A series of chiral bioinspired Mn-aminopyridine complexes of the type [L*MnII(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 kH/kD value of 2.3 has been measured for the oxidation of 1-phenylethanol/α-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.

KW - Asymmetric catalysis

KW - C−H hydroxylation

KW - enzyme models

KW - hydrogen peroxide

KW - hydrogen-bond donor

KW - manganese

KW - NONHEME IRON

KW - ACTIVATION

KW - PORPHYRIN-CATALYZED EPOXIDATION

KW - MECHANISM

KW - HYDROGEN-ATOM TRANSFER

KW - ASYMMETRIC HYDROXYLATION

KW - OXIDATION REACTION

KW - C-H BONDS

KW - SELECTIVITY

KW - C-H hydroxylation

KW - PEROXIDE

UR - http://www.scopus.com/inward/record.url?scp=85056107536&partnerID=8YFLogxK

UR - https://www.mendeley.com/catalogue/2025c3a2-17fa-3777-955e-70c6ad6d144b/

U2 - 10.1002/cctc.201801476

DO - 10.1002/cctc.201801476

M3 - Article

AN - SCOPUS:85056107536

VL - 10

SP - 5323

EP - 5330

JO - ChemCatChem

JF - ChemCatChem

SN - 1867-3880

IS - 22

ER -