Nucleophilic Attacks on Carbon-Carbon Double Bonds. 34. Intramolecular Element Effect in Competitive Expulsion of Two Halide Nucleofuges as a Tool for Investigating the Rapid Step of Nucleophilic Vinylic Substitution

Article abstract of DOI:10.1021/ja00256a023

The substitution of 9-(bromochloromethylene)fluorene (8) and β,β-bis(p-nitrophenyl)-α-bromo-α-chloroethylene (9) by p-toluenethiolate and p-cresolate ions gives the monobromo, the monochloro, and the disubstitution products.The / substitution product ratios were determined in CD3CN, DMSO-d6, and DMSO-d6-CD3OD under conditions where the disubstitution was negligible.The ratios were 2.0-3.2, were slightly higher for 8 than for 9, and showed no discernible solvent dependence.The ratios did not change in the presence of radical traps although an ESR spectrum was observed with 8 and p-MeC6H4S^-.The "intermolecular element effects" k_Br/k_Cl derived from competitive substitution of 8 or 9 with their dibromo or dichloro analogues were 1.2-1.76.The results were interpreted in terms of a multistep nucleophilic vinylic substitution proceeding via an intermediate carbanion, which may be formed either directly or by an initial single-electron transfer followed by combination of the anion radical and the radical.The ratios of the products were hence identified as the ratios of the rate constants for expulsion of Br^- and Cl^- (k_el(Br)/k_el(Cl)-the "intramolecular element effect") from the carbanion.The low ratios and their relative insensitivity to the solvent and to the delocalizing ability of negative charge of the β-substituents were ascribed to an early transition state for the expulsion of halide ions from the carbanion.Generalizations concerning the expulsion of poor and good nucleofuges from carbanions substituted by poor and good electron-withdrawing groups are discussed.