Dinaphthoazepines a Vibrational Circular Dichroism Study

Chiral dinaphthoazepines are important chirality inducers in enantioselective organometallic catalysis and organocatalysis. We have prepared and characterized four dinaphthoazepines in pure optical form, hereafter referred as: dinaphthoazepine (1), N-hydroxy-dinaphthoazepine (2), 2-cyano-dinaphthoazepine (3) and 2-cyano-N-hydroxy-dinaphthoazepine (4). The chirality of the binaphthyl moiety was P in all compounds 1-4. The main scientific question we wish to address in this contribution is the determination of the absolute configuration (AC) of the asymmetric carbon in azepines 3 and 4 by means of chiroptical methods. Electronic circular dichroism (ECD)[1] is unable to provide useful information, being ECD spectra of compound 1-4 almost identical, as the binaphthyl electronic chromophore is the same in all molecules and the chiral carbon in 3 and 4 seems not influential. Vibrational circular dichroism (VCD)[2] spectra of 1-4 have been measured in CDCl3 solutions. They show a number of different features which revealed useful for the AC assignment. Theoretical predictions have been worked out at DFT level of theory (B3LYP/TZ2P and B97-D/TZVP) for both the R and S configuration of the asymmetric carbon in 3 and 4. The comparison between experimental and theoretical (conformationally averaged) VCD spectra resulted particularly good for the R configuration of the asymmetric carbon, leading to a confident assignment.