Homoanomeric Effects in Six-Membered Heterocycles

Structural and energetic consequences of homoanomeric n(X) --> beta-sigma(C-Y) interactions in saturated six-membered heterocycles where X = O, N, S, Se and Y = H, Cl were studied computationally using a combination of density functional theory (B3LYP) and Natural Bond Orbital (NBO) analysis. Unlike the classic anomeric effect where the interacting donor and acceptor orbitals are parallel and overlap sidewise in a pi-fashion, orbital interactions responsible for homoanomeric effects can follow different patterns imposed by the geometric restraints of the respective cyclic moieties. For the equatorial beta-C-Y bonds in oxa-, thia- and selena-cyclohexanes, only the homoanomeric n(X)(ax) --> sigma(C-Y)(eq) interaction (the Plough effect) with the axial lone pair of X is important, whereas the n(X)(eq) --> sigma(C-Y)(eq) interaction (the W-effect) is negligible. On the other hand, the W-effect is noticeably larger than the n(X)(ax) --> sigma(C-Y)(eq) interaction in azacyclohexanes. Hyperconjugation is a controlling factor which determines relative trends in the equatorial beta-C-H bonds in heterocycloxanes. In contrast, all homoanomeric interactions are weak for the respective axial bonds where relative lengths are determined by intramolecular electron transfer through exchange interactions and polarization-induced rehybridization. Although the homoanomeric effects are considerably weaker than the classic vicinal anomeric n(X)(ax)-->alpha-sigma(C-Y)(ax) interactions, their importance increases significantly when the acceptor ability of sigmaorbitals increases as a result of bond stretching and/or polarization. Depending on the number of electrons and the topology of interactions, homoconjugation interactions can be cooperative (enhance each other) or anticooperative (compete with each other). Such effects reflect symmetry of the wave function and can be considered as weak manifestations of sigma homoaromaticity or homoantiaromaticity.