Direct observation of enantiomer discrimination of epoxides by chiral salen complexes using ENDOR

Electron nuclear double resonance (ENDOR) spectroscopy was used to investigate the weak enantioselective binding between chiral salen complexes VO(1)] ((R,R)- and (S,S)-vanadyl N,N'-bis(3,5-di-tert-butylsalcylidene)-1,2-cyclohexanediamine) and chiral epoxides (e.g., (R)-/(S)-propylene epoxide, 5) in frozen (10 K) solution. Differences in epoxide binding by enatiomers of VO(1)] was evidenced by changes to the 1H epoxide derived peaks in the ENDOR spectra, such that (R,R)-VO(1)] + (R)-5 and (R,R)-VO(1)] + (S)-5 yield noticeably different spectra. These changes were assigned to the small structural differences between the diastereomeric metal-epoxide adducts. Simulation of the spectra revealed differences in the VO...1Hepoxide distances for the diastereomeric pairs, which was confirmed by a complementary set of density functional theory (DFT) calculations. While the epoxide molecule is very weakly coordinated, ENDOR measurements of the racemic complex in racemic epoxide nevertheless indicated the preferential coordination of the (R)-5 to (R,R)-VO(1)] (likewise (S)-(5) to (S,S)-VO(1)]), which is favored over the binding of (S)-5 epoxide to (R,R)-VO(1)] (and likewise (R)-5 epoxide to (S,S)-VO(1)]). This demonstrates the unique power of the ENDOR technique to resolve weak chiral interactions for which EPR spectroscopy alone lacks sufficient resolution.