Optically active amines. 30. Application of the salicylidenimino chirality rule to aliphatic and alicyclic amines

The salicylidenamino chirality rule: a method for the establishment of the absolute configurations of chiral primary amines by circular dichroism

“…A similar analysis predicts the observed, positive CEs for the IV-salicylidene derivatives of (S)-50b,c. 32 In both, conformer 51c is the principal contributor to the CD spectrum, contributions from the C(4)-C( 5) and other C-C bonds being unimportant compared to that of the C(3)-C(4) bond.…”

“…General Considerations Application of the SA chirality rule to amines without unsaturated groups is not as easy as when an unsaturated group is present, and although the CEs are less intense, application of the rule to amines without unsaturated groups is also based on the coupled oscillator mechanism.32 For these IV-salicylidene derivatives, the CEs associated with bands I and II originate from coupling of the transition moments of the SA chromophore with transition moments in the rest of the molecule. 32 The effect due to the polarizability of a C-H bond is assumed to be negligible,73,74 and C-C and C-0 bond transition moments vicinal and homovicinal to the SA chromophore are the dominant contributors to the CEs. Since the polarizability of a C-0 bond is smaller than that of a C-C bond,73,74 the contribution of a vicinal or homovicinal C-0 bond is less than that of a corresponding C-C bond.…”

“…For the SA chromophore, the transition moment directions of bands I and II, although slightly different in direction, are approximately parallel to the attachment bond of the SA group (Figure 4), and the sign of the contribution to the CEs made by a particular vicinal or homovicinal bond can usually be determined from the chirality that the bond has with the attachment bond of the SA group, a positive contribution for positive chirality (right-handed screw) and a negative contribution for negative chirality (left-handed screw). 32 In cases where the bond and the SA group attachment bond are coplanar, the chirality that the bond has with the transition moments of the SA chromophore (Figure 4) is used.…”

“…The IV-salicylidene derivatives of 11a-(68a) and ll/3-amino-5/3-pregnane-3a,20/S-diol (68b) show almost H b, R = H; R' = NHj enantiomeric CD spectra (Table V) that are unusual in that the sign of band I is different from that of band II or the long-wavelength component of band II. 32 The sign of the latter band, however, is the same as the chirality that the vicinal C(9)-C(10) bond has with the SA group attachment bond, negative for the IVsalicylidene derivative of 68a and positive for that of 68b.…”