Electron Delocalization in Electron-Deficient Alkenes and Push-Pull Alkenes

Abstract

We have studied π electron delocalization in electron-deficient alkenes and push-pull alkenes by means of natural bond orbital analysis at the B3LYP/6-311+G(d,p) level. The study revealed that the rarely mentioned π electron donation from an electron-accepting group (Acc) toward the C=C double bond in push-pull alkenes and electron-deficient alkenes can provide up to 10 % of total π electron stabilizing energy of a push-pull system and as much as 45 % in a strongly electron-deficient tetracyanoethene. The Acc → C=C bond π electron donation is more intense in s-trans than in s-cis conformational arrangement, but is less dependent on Z/E isomerism in push-pull alkenes, being slightly more pronounced in Z isomers. Among different Acc substituents, CN and COO- groups contribute the largest percent of stabilizing energy and NO2 the smallest. Increase in the number of Acc groups increases percentage contribution of Acc → C=C bond π electron delocalization to a system stabilization. A difference in π*C=C orbital occupancy between isomers can be related with their chemical reactivity.