Kinetic Study on Nucleophilic Substitution Reactions of Aryl Diphenylphosphinates with Butane‐2,3‐dione Monoximate and Aryloxide Anions: Reaction Mechanism and Origin of the α‐Effect

Abstract

A kinetic study is reported for nucleophilic substitution reactions of X‐substituted‐phenyl diphenylphosphinates (3a–3f) with butane‐2,3‐dione monoximate (Ox−) and a series of Y‐substituted‐phenoxide (Y‐PhO−) ions in 50 mol % H2O/50 mol % DMSO at 25.0 ± 0.1°C. The reactions of 3a–3f with Ox− and 4‐chlorophenoxide (4‐ClPhO−) result in linear Brønsted‐type plots with βlg = −0.70 and −0.64, respectively, a typical βlg value for reactions reported previously to proceed through a concerted mechanism. The Brønsted‐type plots for the reactions of 4‐chloro‐2‐nitrophenyl diphenylphosphinate (3a), 4‐nitrophenyl diphenylphosphinate (3b), and 4‐acetylphenyl diphenylphosphinate (3d) with Y‐PhO− are also linear with βnuc = 0.15–0.35. The current reactions have been concluded to proceed through a concerted mechanism in which the bond formation is much less advanced than the bond rupture in the TS on the basis of the βlg and βnuc values. The α‐effect observed in this study is very small (i.e., the kOxprefix−/kp‐ClPhO− ratio = 16.4 – 43.5) and is independent of the leaving‐group basicity. It has been concluded that the α‐effect shown by Ox− in the current reactions is mainly due to desolvation of Ox− in the reaction medium (ground‐state contribution) rather than stabilization of the transition‐state (TS contribution) on the basis of the kinetic results.