β-Elimination of 9-halogeno-9,9′-bifluorenyls. An E1cB dehydrohalogenation

Abstract

Unusually small leaving-group effects (e.g. k_Cl/k_F 0.7, k_Br/k_Cl 1.9) have been measured for the methoxidepromoted β-elimination of 9-acetoxy- and 9-halogeno-bifluorenyls to form bifluorenylidene in methanol. It is suggested that reaction occurs via an irreversibly formed carbanion intermediate and that formation of the carbanion is unassisted by halogen hyperconjugation. For β-eliminations showing larger halogen leaving-group effects a concerted mechanism is implied. Elimination of 9-bromo- and 9-chloro-bifluorenyls is accompanied by solvolysis leading to substitution product. For the bromide a non-linear kinetic dependence on base concentration was observed, formally consistent with formation of a carbonium ion intermediate common to substitution and elimination paths. Separation of substitution and elimination rate constants and measurements of solvent isotope effects, however, indicated competition between bimolecular elimination and a unimolecular solvolysis with the carbonium ion confined to the latter. The non-linear base dependence arose from a solvent isotope independent salt effect on the solvolysis reaction.