Electron reorganization in allowed and forbidden pericyclic reactions: multicenter bond indices as a measure of aromaticity and/or anti-aromaticity in transition states of pericyclic electrocyclizations

摘要

The multicenter bond indices (MCI), recently proposed as quantitative measures of cyclic delocalization in aromaticsystems, have been applied to characterize the differences in the nature of the electron reorganization in a series ofallowed and forbidden electrocyclic reactions of linear neutral polyenes of general formula C.1-1,)+2 and relatedcharged systems of formula CH±2(+) and CH±2(—) for n ranging from 4 to 7. The proposed methodology, which isbased on the monitoring of the variation of the extent of cyclic delocalization along the concerted reaction paths, isshown to be completely consistent with the empirical Evans/Dewar classification anticipating aromatic transitionstates for allowed and anti-aromatic transition states for forbidden electrocyclic reactions. Although the study reportsthe results of the analysis of electron reorganization just for the above-mentioned particular class of electrocyclicreactions, the proposed approach is completely general and its conclusions remain valid for any pericyclic reaction.