Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. II. Metal ion catalysis in the reaction of p-nitrophenyl diphenylphosphinate with alkali metal phenoxides in ethanol

Abstract

The reactions of p-nitrophenyl diphenylphosphinate (1) with lithium, sodium, potassium, and benzyltrimethylammonium phenoxides (BTMAOPh) have been studied by spectrophotometric techniques in anhydrous ethanol at 25 °C. The reactivity (kobs) of the alkali metal phenoxides increases in the order BTMAOPh < KOPh < NaOPh < LiOPh. The rate of reaction of 1 with LiOPh is enhanced when lithium salts (LiSCN, LiNO3, LiClO4, LiOAc) are added to the reaction media. The addition of the alkali metal complexing agents dicyclohexyl-18-crown-6 ether or [2.2.2]cryptand for Na+, and [2.1.1]cryptand for Li+, to each of the alkali metal phenoxide reactions resulted in a decrease in rate, indicating catalysis by the alkali metal ions. The kinetic data are analyzed to obtain specific rate coefficients of reactions of phenoxide and ethoxide as the dissociated ions and as alkali metal – phenoxide ion pairs. Reactivities follow the order [Formula: see text]; [Formula: see text]; [Formula: see text]; [Formula: see text]. A mechanism is proposed in which the ion-paired metal phenoxide is more reactive towards the substrate than is the dissociated phenoxide. Analysis of the data in terms of initial state and transition state interactions with metal ions indicates that the increased reactivity of the ion-paired species results from greater stabilization of the negatively charged transition state relative to stabilization of the ion-paired nucleophile. Keywords: nucleophilic displacement at phosphorus by phenoxide, alkali-metal-ion catalysis.