Theoretical study of the nucleophilic 5-endo-trigonal cyclization of 1,1-difluoro-1-alkenes

The nucleophilic 5-endo-trigonal cyclization of 1,1-difluoro-1-alkenes has been studied at the B3LYP/6-31+G(d) level in an Onsager continuum model for DMF. The reaction takes an addition-elimination path. Both the transition-state structures and the IRC analyses suggest the delocalization of the negative charge to highly electronegative two fluorine atoms during the addition reaction is the origin of the high reactivity of 1,1-difluoro-1-alkenes. Judging from the activation energies, both dichloro and dibromo counterparts are much less reactive for 5-endo-trigonal cyclization. In these substrates, the cyclization reaction is promoted by chlorine or bromine atom with their good leaving-group ability, and the addition of oxyanion to the pi-bond occurs along with the simultaneous elimination of halogen atom. The study on the cyclizations of beta-monofluoro-o-hydroxystyrenes and beta-bromo-beta-fluoro counterparts shows that one fluorine atom is not enough to delocalize the negative charge in the addition step.