Enantioselective binding of structural epoxide isomers by a chiral vanadyl salen complex: a pulsed EPR, cw-ENDOR and DFT investigation

Abstract

The mode of chiral interaction between a series of asymmetric epoxides (propylene oxide, butylene oxide, epifluorohydrin and epichlorohydrin) and a chiral vanadyl salen complex, N, N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino-vanadium (IV) oxide, [VO(1)], was investigated by a range of electron magnetic resonance techniques (EPR, ENDOR, HYSCORE) and DFT. Enantiomer discrimination of the weakly bound epoxides by the vanadyl complex was evident by cw-ENDOR. The origin of this discrimination was attributed to a number of factors including H-bonds, steric properties and electrostatic contributions, which collectively control the outcome of the chiral interaction. DFT revealed the role of a key H-bond, formed between the epoxide oxygen atom (O_epoxide) and the methine proton (H_exo) attached to the asymmetric carbon atom of the chiral vanadyl salen complex, thereby providing a direct pathway for stereochemical communication between complex and substrate. These findings reveal the potential importance of weak outer sphere interactions in stereoselectivities of enantioselective homogeneous catalysis.