Kinetics and mechanism of the pyridinolysis of N‐aryl‐P,P‐diphenyl phosphinic amides in dimethyl sulfoxide

Abstract

Kinetic studies for the reactions of Z‐N‐aryl‐P,P‐diphenyl phosphinic amides with X‐pyridines have been carried out in dimethyl sulfoxide at 85.0 °C. The two strong π‐acceptor substituents, X = 4‐Ac and 4‐CN in the X‐pyridine, exhibit positive deviations from both the Hammett and Brönsted plots. The Hammett plots for substituent Z variations are anomalously biphasic concave upwards with a break point at Z = H, and the sign of ρZ changes from negative for electron‐donating to positive for electron‐withdrawing substituents Z in the leaving group. The negative sign of the cross‐interaction constants (ρXZ) for both electron‐donating (large magnitude of ρXZ = −1.54) and ‐withdrawing (ρXZ = −0.27) substituents Z indicates that the reaction proceeds through a concerted mechanism. These results are indicative of frontside and backside nucleophilic attacks at the P reaction center for electron‐donating and ‐withdrawing substituents Z, respectively. The poor leaving group mobility of NHPhZ yields the change of the nucleophilic attacking direction, resulting in biphasic concave upwards Hammett plots for substituent Z variations. Copyright © 2010 John Wiley & Sons, Ltd.

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