Nucleophilic displacement on 4-nitrophenyl dimethyl phosphinate by ethoxide ion: alkali metal ion catalysis and mechanism

Abstract

We report on a spectrophotometric kinetic study of the effect of Li⁺ and K⁺ cations on the ethanolysis of 4-nitrophenyl dimethylphosphinate (4a) in ethanol at 25 °C. The nucleophilic displacement reaction of 4a with LiOEt and KOEt in the absence and presence of 18-crown-6 ether (18-C-6) furnished observed first-order rate constants which increase in the order EtO⁻ < KOEt < LiOEt. The kinetic data are analyzed in terms of a scheme which assigns concurrent kinetic activity to free ethoxide and metal alkoxide, to obtain the second-order rate coefficients for reaction of the metal ion–ethoxide pairs, k_MOEt. Derived δG_ip, δG_ts and ΔG_cat values quantify ground state and transition state stabilization by the metal ions to give δG_ts > δG_ip for Li⁺ and δG_ts ∼ δG_ip for K⁺. These results indicate moderate catalysis by Li⁺, with 4a manifesting lesser susceptibility to catalysis than other substrates previously studied. Second-order rate constants for the reaction of the aryl dimethylphosphinates 4a–f with free EtO⁻ were obtained from plots of log k_obsvs. [KOEt], measured in the presence of excess 18-C-6. Hammett plots with σ and σ° substituent constants give significantly better correlation of rates than σ⁻ and yield a moderately large ρ(ρ°) value; this is interpreted in terms of a stepwise mechanism involving rate-limiting formation of a pentacoordinate intermediate. Comparison of the present results with those of Williams on the aqueous alkaline hydrolysis of Me₂P(O)–OPhX and Ph₂P(O)–OPhX esters, establishes the rationale for a change in mechanism in the more basic EtO⁻/EtOH nucleophile/solvent system by a stepwise mechanism instead of a concerted one in aqueous base. Structure–reactivity correlations following Jencks show that the change in mechanism is accounted for by cross interactions between the nucleophile and the leaving group in the transition state. The observed duality of mechanism is rationalized on the basis of the More O'Ferrall–Jencks diagram, as a spectrum of transition states covering a wide range of nucleophile and leaving group basicities.