Theoretical Investigation of Some Evidences of the Captodative Effect

Abstract

AbstractIn this paper, the stability and reactivity of some gem‐disubstituted closed‐shell and open‐shell systems are investigated using the unifying concept of thermodynamic stabilization energy. We deduce this quantity either from available experimental data or from enthalpies of formation obtained at various theoretical levels. The energies of open‐shell species, medium‐size closed‐shell systems and large diamagnetic compounds are respectively calculated at the CISD/6‐31 G(d,p), MP2 = FULL/6‐31 G(d,p) and HF/6‐31 G(d,p) levels.It is found that captodative (cd) substitution significantly stabilizes carbon‐centred radicals and ethylenic compounds but generally destabilizes methane derivatives. Didative (dd) substitution most often stabilizes both the methyl radicals and their parent molecules. Finally most cc‐substituted species are destabilized. On the whole, the cdCH2 compounds do actually have the smallest C‐H bond strengths due to the joint action of the captodative and stereoelectronic effects. For this reason, they deserve to be named proradical. Consequently hydrogen transfer reactions are strongly favoured by captodative substitution which explains the selectivity induced by this substitution in dehydrodimerization reactions. Radical additions to alkenes are facilitated by dicaptor and captodative substitutions while donor groups have a relatively small influence on the thermochemistry of these reactions. In the absence of steric effects, didative and captodative olefines could probably give rise to radical polymerization.