Macrocyclic Mn(III) salen complexes as recyclable catalyst for oxidative kinetic resolution of secondary alcohols
•Chiral macrocyclic Mn(III) salen complexes with variable linkers were synthesized.•Benzylic and non benzylic alcohols were resolved effectively (up to 99% ee).•A plausible mechanism was proposed based on the experimental studies.•Catalyst shows high stability in oxidative condition.•Catalyst was recycled up to 7 cycles without any loss in catalytic performance.
New macrocyclic chiral Mn(III) salen complexes (C1–C4) were synthesized and were used as catalysts for oxidative kinetic resolution (OKR) of secondary alcohols with diacetoxyiodobenzene (PhI(OAc)2) and N-bromosuccinimide (NBS), in biphasic dichloromethane: water solvent mixture. Good to excellent enantioselectivities were achieved with catalyst C2 for several secondary alcohols having different steric environment. In general with catalyst C2, NBS as a co-oxidant showed better enantioselectivity than PhI(OAc)2 in OKR. The catalyst C2 was easily retrieved from the reaction mixture by the addition of hexane and recycled seven times both with NBS and PhI(OAc)2 as co-oxidants without losing its performance. Based on the experimental results a mechanism for OKR of racemic 1-phenylethanol has been proposed where (R,R)-Mn-salen preferably binds with (S)-1-phenylethanol to give (R)-1-phenylethanol in excess at the end of the reaction.
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Journal: Applied Catalysis A: General - Volume 467, 2 October 2013, Pages 542–551