Metal-Ion Catalysis in Nucleophilic Displacement Reactions at Carbon, Phosphorus, and Sulfur Centers. 5. Alkali-Metal Ion Catalysis and Inhibition in the Reaction of p-(Trifluoromethyl)phenyl Methanesulfonate with Ethoxide Ion

Article abstract of DOI:10.1021/jo00019a022

The reactions of alkali-metal ethoxides with p-(trifluoromethyl)phenyl methanesulfonate (1) in anhydrous ethanol at 25 deg C, yielding p-(trifluoromethyl)phenolate ion and ethyl methanesulfonate, have been investigated in order to reveal the effects of alkali-metal ions on reaction rates.Kinetic spectrophotometric studies of the nucleophilic displacement reaction of 1 with alkali-metal ethoxides in the absence and presence of complexing agents showed that the observed rate constants increase in the order LiOEt < EtO^- < NaOET < CsOEt < KOEt.Thus, Li⁺ inhibits the reaction of ethoxide ion, while the other alkali-metal ions all act as catalysts.The kinetic data are analyzed in terms of parallel reactions of free ethoxide ion and alkali-metal ethoxide ion pairs, and rate constants for the reactions of these species are calculated.Association constants governing the interaction of the various metal ions with the transition state for the reaction of ethoxide ion with 1 are derived from the kinetic data and compared to association constants for interaction of metal ions with ethoxide ion in the ground state.The trend in the sizes of the association constants for the methanesulfonate transition state, Li⁺ < Na⁺ < K⁺ < Cs⁺, is believed to arise from ion pairing of the transition state with solvated metal ions.A similar ordering is observed for the transition state in the reaction of p-nitrophenyl benzenesulfonate (2) with alkali metal ethoxides, while an inverted ordering is seen for the transition state for the reaction of ethoxides with p-nitrophenyl diphenylphosphinate (3).These results are interpreted in terms of the extent of charge delocalization in the transition states and its effect on interactions with bare or solvated metal ions.