Hammett and Broensted-Type Relationships in Reactions of 9-Substituted Fluorenyl Anions with Benzyl Halides

Article abstract of DOI:10.1021/jo01304a036

The rates of reaction of 9-substituted fluorenyl anions, 9-G-Fl^- (G=CN, Ph, Me), with substituted benzyl chlorides (p-MeO, p-Me, m-Me, p-F, p-Cl, m-Cl, m-CF3, m-NO2) in Me2SO solution have been measured.A "U-shaped" Hammett plot was obtained for 9-CN-Fl^- (ρ_meta <*> 1.3), but a "normal" Hammett plot was obtained with 9-Ph-Fl^- (ρ = 1.8), which is 9.6 pK units more basic.Broensted-type plots of log k vs. pK_a obtained for three 9-G-Fl^- families (9-PhSO2, 9-PhS, and 9-Ph) reacting with m-CF3C6H4CH2Cl gave slopes (β_Nu) near 0.4.The β_Nu values for the 9-Ph-Fl^- family reacting with ArCH2Cl were increased by the presence of electron accepting substituents, p-MeO (0.30), H (0.365), m-CF3 (0.45), m-NO2 (0.485).The small increases in Hammett ρ values and β_Nu values caused by electron-withdrawing substituents in ArCH2Cl are interpreted to mean that there is a small increase in the negative charge density in the benzylic carbon atom and/or chlorine atom in the transition state.Rates for the reaction of PhCH2Br with eight 9-G-Fl^- anions were measured.For 9-CN-Fl^- the k^Br/k^Cl rate ratio was found to be 4 times larger in Me2SO than in EtOH.The reactivity-selectivity principle is discussed and the conclusion drawn that it should be abandoned.