The deconjugation effect in allyle ions under influence of an approaching reagent

Bimolecular addition reactions are modelled and studied between an allyle cation and nucleophile, as well as between an allyle anion and electrophile. Charge- and bond order matrices of the reacting allyle are derived and analyzed. Specific terms are revealed in these matrices that are responsible for emergence of asymmetry of charge- and bond order distributions in allyle under influence of the attack of the reagent upon a single carbon atom. Two effects follow from these terms, viz. an induced lengthwise polarization of the ion and a partial switch of bond order from one C-C bond to another. As a result, a trend is observed towards formation of a lone electron pair and of vacancy at the carbon atom under attack for systems anion+ electrophile and cation+ nucleophile, respectively, along with weakening of the nearest C-C bond and strengthening of the remaining bond. A relation is established between the above-described deconjugation effect and particular indirect interactions between molecular orbitals of the allyle ion by means of orbitals of the reagent. Moreover, the effect is predicted to start at the very early stage of the addition process. Applications of the results obtained to specific systems also are discussed.