Analysis of linear free-energy relationships combined with activation parameters assigns a concerted mechanism to alkaline hydrolysis of X-substituted phenyl diphenylphosphinates

摘要

A kinetic study is reported for alkaline hydrolysis of X-substituted phenyl diphenylphosphinates (1a-i). The Bronsted-type plot for the reactions of 1a-i is linear over 4.5 pK(a) units with beta(1g) = -0.49, a typical beta(1g) value for reactions which proceed through a concerted mechanism. the Hammett plots correlated with sigma degrees and sigma(-) constants are linear but exhibit many scattered points. while the corre-sponding Yukawa-Tsuno plot results in excellent linear correlation with rho = 1.42 and r = 0.25. The r value of 0.35 implies that leaving-group departure is partially advanced at the rate-determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO- ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (D-N + A(S)) mechanism is also ruled out on the basis of the small beta(1g) value. As the substituent X in the leaving group changes from H to 4-NO2 and 3,4-(NO2)2, triangle H-not equal decreases from 11.3 kcal mol(-1) to 9.7 and 8.7 kcal mol(-1,) respectively, while triangle S* varies from -22.6 cal mol(-1) K-1 to -21.4 and -20.2 cal mol(-1) K--1,K- respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hhydrolysis of 1a-i.