Aminolysis of X-Substituted Phenyl Diphenylphosphinates: Effect of Amine Nature on Reactivity and Transition-State Structure

Abstract

A kinetic study is reported for aminolysis of X-substituted phenyl diphenylphosphinates (1a-i) in 80 mol % H(2)O/20 mol % dimethyl sulfoxide at 25.0 +/- 0.1 degrees C. The Brønsted-type plot for the reactions of 2,4-dinitrophenyl diphenylphosphinate (1a) with primary amines is linear with beta(nuc) = 0.53. The reactions of 1a-i with ethylamine also result in a linear Brønsted-type plot with beta(lg) = -0.81. These beta(nuc) and beta(lg) values are slightly larger than those reported previously for the reactions of 1a with secondary amines (beta(nuc) = 0.38) and for those of 1a-i with piperidine (beta(lg) = -0.66) but typical for reactions that proceed through a concerted mechanism. It has been concluded that aminolysis of 1a-i proceed through a concerted mechanism and the nature of amines does not affect the reaction mechanism. However, the reactions with primary amines have been suggested to proceed through a later transition state (i.e., more bond formation and bond rupture in the transition state) on the basis of the larger beta(nuc) and beta(lg) values. The concerted mechanism has been further supported from the fact that the Yukawa-Tsuno plot for the reactions of 1a-i with ethylamine exhibits an excellent linear correlation with rho = 2.24 and r = 0.22. Weakly basic primary amines are less reactive than secondary amines of similar basicity. However, strongly basic ethylamine is ca. 2-fold more reactive than piperidine toward 1a, although the former is 0.35 pK(a) units less basic than the latter.