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Homoanomeric Effects in Six-Membered Heterocycles Igor V. Alabugin,* Mariappan Manoharan, and Tarek A. Zeidan
 

Summary: Homoanomeric Effects in Six-Membered Heterocycles
Igor V. Alabugin,* Mariappan Manoharan, and Tarek A. Zeidan
Contribution from the Department of Chemistry and Biochemistry, Florida State UniVersity,
Tallahassee, Florida 32306-4390
Received July 15, 2003; E-mail: alabugin@chem.fsu.edu
Abstract: Structural and energetic consequences of homoanomeric n(X) f -*(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 -fashion, orbital interactions responsible for homoanomeric effects can follow different patterns imposed
by the geometric restraints of the respective cyclic moieties. For the equatorial -C-Y bonds in oxa-, thia-
and selena-cyclohexanes, only the homoanomeric n(X)ax f *(C-Y)eq interaction (the Plough effect) with
the axial lone pair of X is important, whereas the n(X)eq f *(C-Y)eq interaction (the W-effect) is negligible.
On the other hand, the W-effect is noticeably larger than the n(X)ax f *(C-Y)eq interaction in
azacyclohexanes. Hyperconjugation is a controlling factor which determines relative trends in the equatorial
-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)axfR-*(C-Y)ax interactions, their importance increases
significantly when the acceptor ability of *orbitals increases as a result of bond stretching and/or polarization.

  

Source: Alabugin, Igor - Department of Chemistry and Biochemistry, Florida State University

 

Collections: Chemistry; Biology and Medicine