A quantum chemical study of the interaction of carbenes with 2-alkenylfurans and thiophenes

The study of the interaction mechanism of monoalkenyl‐substituted furans and thiophenes with carbenes was performed by AM1 and MNDO methods. The energy profiles of two pathways were calculated, one leading to ylide formation and the second to cyclopropanation species. The insertion of methyl groups into the vinyl fragment does not change the energetic parameters of path 1. The reactivity of the exocyclic double bond was shown to depend on the degree of substitution. The calculations prove the presence of the intermediate ylide formation, based on experimental data. The stability of the ylides is determined by the heights of the cycloaddition barrier and the one of the reversed reaction 1 as shown by calculations.