Kinetic Study on Nucleophilic Substitution Reactions of Aryl Diphenylphosphinates with Butane-2,3-dione Monoximate and Aryloxide Anions: Reaction Mechanism and Origin of the -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 degrees C. The reactions of 3a-3f with Ox(-) and 4-chlorophenoxide (4-ClPhO-) result in linear BrOnsted-type plots with (lg) = -0.70 and -0.64, respectively, a typical (lg) value for reactions reported previously to proceed through a concerted mechanism. The BrOnsted-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 (nuc) = 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 (lg) and (nuc) values. The -effect observed in this study is very small (i.e., the kOx-/kp-ClPhO- ratio = 16.4 - 43.5) and is independent of the leaving-group basicity. It has been concluded that the -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.