NMR-STUDY OF STEREOELECTRONIC ANOMERIC AND HOMOANOMERIC EFFECTS ON THE AXIAL AND EQUATORIAL CH BONDS IN 1,3-DIAZACYCLOHEXANES AND 1,5-DIAZABICYCLO[3.2.1]OCTANES

The H-1 and C-13 NMR spectra of a series of 1,3-di-tert-butyl-1,3-diaz acyclohexanes 6, and of 1,5-diazabicyclo[3.2.1]octanes 7, have been in vestigated in an attempt to find evidence regarding a stereoelectronic interaction of the non-bonding electron pairs on nitrogen or of the C -N bonding electron pairs in the ring, with the axial and equatorial C H bonds of the alpha- or beta-methylene groups. In 6, the orbital carr ying the non-bonding electron pair lies antiperiplanar to the axial al pha-CH bond; a significant n(N)-->sigma(CH) 3-orbital interaction (th e anomeric effect) is sterically possible, and this shows up in the va lues of delta(H) and of (1)J(CH). In 7, the doubly-occupied orbital is orientated gauche to both the axial and equatorial alpha-CH bonds, an d no anomeric effect is apparent. In 7 however, in contrast to 6, the stereochemistry is appropriate for a W-plan n(N)-->sigma(CH) 4-orbita l interaction with the equatorial beta-CH bond, and comparison of the NMR data of 6 and 7 suggests that such a homoanomeric effect may opera te, though it is much weaker than in the analogous 1,3-dioxanes. No ev idence could be found for a sigma(CN)-->sigma(CH) 3-orbital interactio n.