Aminolyses of aryl diphenylphosphinates and diphenylphosphinothioates: Effect of modification of electrophilic center from P=O to P=S

Abstract

(Chemical Equation Presented) A kinetic study is reported for aminolysis of aryl diphenylphosphinothioates (2a-i). The phosphinothioates 2a-i are less reactive than aryl diphenylphosphinates (1a-i), the oxygen analogues of 2a-i, regardless of the basicity of the leaving aryloxides or the attacking amines. The Yukawa-Tsuno plot for the reactions of 2b-i with piperidine exhibits good linearity with a small r value (r = 0.28), indicating that the leaving group departs at the rate-determining step with a small degree of bond fission. Reactions of 2,4-dinitrophenyl diphenylphosphinothioate (2a) with alicyclic secondary amines result in a good linear Brønsted-type plot with β nuc = 0.52, implying that the reactions proceed through a concerted mechanism. The β nuc value determined for the reactions of 2a is slightly larger than that reported for the corresponding reactions of 2,4-dinitrophenyl diphenylphosphinate (1a, i.e., β nuc = 0.38), suggesting that reactions of 2a proceed through a tighter transition state (TS) than that of 1a. The reaction of 2a with piperidine exhibits a ca. 0.4 kcal/mol more favorable enthalpy of activation (ΔH ‡) than that of 1a. On the contrary, the entropy of activation at 25.0°C (TΔS ‡) is ca. 1.5 kcal/mol more unfavorable for the reaction of 2a than for that of 1a. This result supports the proposal that the reaction of 2a proceeds through a tighter TS than that of 1a and explains why 2a-i are less reactive than 1a-i. © 2007 American Chemical Society.