A DENSITY-FUNCTIONAL THEORY STUDY ON THE REACTIVITY OF DISUBSTITUTED CYCLOIMMONIUM YLIDES

The aim of this study is to develop from a static point of view the re activity of disubstituted cycloimmonium ylides, based on ab initio cal culations, using the density functional theory approach, together with experimental results. First, we elucidate some electronic and structu ral properties of these compounds and we demonstrate for the first tim e that the coulomb attractive strength is one of the principal factors responsible for the stability of cycloimmonium ylides. Otherwise, usi ng the frontier molecular orbital theory, we could estimate the reacti vity of some virtual reaction centres by their absolute values of atom ic orbital coefficients corresponding to the frontier HOMO-LUMO orbita ls, The planar disubstituted cycloimmonium ylides are ideal compounds which undergo a 3 + 2 dipolar cycloaddition reaction. However, in the case of non-planar disubstituted cycloimmonium ylides, the rotational barriers for the ylidic bond are higher than those of planar disubstit uted cycloimmonium ylides, Also, the absolute values of atomic orbital coefficients, for the ylidic carbon atoms and the alpha aromatic carb on atoms, corresponding to the frontier HOMO-LUMO orbitals are smaller than those of planar disubstituted cycloimmonium ylides. In this way we show a significant difference between the planar and non-planar cyc loimmonium ylides, (C) 1998 Elsevier Science B,V. All rights reserved.