Nucleofugality index in α-elimination reactions

We propose that a specific property of a chemical fragment, its regional electrophilicity, be taken as defining its nucleofugality index ν(PG) in α-elimination reactions. We tabulate this quantity for a series of molecular fragments present as leaving groups in a series of 17 α-elimination reactions that follow the E1cB or B AC 2 mechanisms. The difference in the nucleofugality index assesses well the type of mechanism which is operative: a large positive or negative difference in nucleofugality discriminates well between the E1cB and B AC 2 channels, respectively, whereas the region for which this difference is small correctly describes the borderline zone.