Proton-Transfer Reactions between 9-Alkylfluorene and (9-Alkylfluorenyl)lithium in Ether

Article abstract of DOI:10.1021/ja00366a038

The rates of proton-transfer reactions between 9-substituted fluorenes and 9-substituted fluorenyllithium have been examined in ether at 25 and 71 deg C.A high primary isotope effect (k^H/k^D = 9.5) and substantial secondary kinetic (1.11 +/- 0.04) and equilibrium (1.19 +/- 0.04) isotope effects are observed for fluorene.Surprisingly, intermolecular steric effects seem to play only a minor role in spite of the fact that the alkyl groups are located directly at the carbon involved in the proton transfer.The barriers for the endergonic cross reactions (i.e., those involving different alkyl groups in the anion and hydrocarbon) are half of the sum of the barriers for the two corresponding identity reactions (i.e., those involving the same alkyl groups in the anion and hydrocarbon).This leads to Broensted slopes which vary from 0.7 for reactions of fluorenyl anion to 1.8 for reactions of 9-(tert-butyl)fluorenyl anion.The rates of the identity and cross reactions give approximate linear correlations with each other and with ΔpK and are dominated by an effect which correlates with ?^*.The substituent effect correlated by ?^* is inconsistent with a classical field or repulsive steric effect and may originate from solvatation effects.The thermodynamic and kinetic relationships between the identity and cross reactions show that the transition states for the cross reac tions are only responding to half of the substituent effect on the identity reactions and that the substituent effect on the equilibria appears absent from the cross-reaction transition states.The results can be consistent with Marcus' theory only if the substituent effect on the equilibria appears in steps separate from proton transfer.The results suggest that changes in solvation and proton transfer occur as discrete kinetic steps.