화학공학소재연구정보센터
Journal of Catalysis, Vol.390, 170-177, 2020
Highly enantioselective undirected catalytic hydroxylation of benzylic CH2 groups with H2O2
Chiral bis-amine-bis-pyridine Mn complexes of the type [LMnII(OTf)(2)] (L is 2,2'-bipyrrolidine derived chiral ligand, bearing trifluoroethoxy and methyl substituents), efficiently catalyze the enantioselective hydroxylation of organic substrates at benzylic CH2 positions with H2O2. The use of beta-polyfluorinated alcohols instead of CH3CN as the reaction solvents enhances the yield of chiral secondary alcohol from 5-6% up to 50-70%. The enantiomeric purity of the alcohol can be further increased (up to 97% ee) by diluting the mixture with CH3CN at late stage, which facilitates stereoconvergent oxidative kinetic resolution of the alcohol formed. Using this one-pot sequential asymmetric hydroxylation/oxidative kinetic resolution approach, the oxidation of a series of 3,4-dihydrocoumarin derivatives and 3,4-dihydroquinolinone has been realized, affording the target 4-hydroxo compounds in 40-60% isolated yield and in up to 93% ee. Besides the 4-hydroxo derivatives, formation of 3,4-epoxidation and 3,4-desaturation byproducts has been observed in some cases, thus providing evidence for unprecedented substrate-dependent hydroxylase/desaturase/epoxidase reactivity of bis-amine-bis-pyridine Mn complexes. (C) 2020 Elsevier Inc. All rights reserved.