Reaction Mechanism and Structure of Transition State Determined from Analysis of Bronsted βnucfor Aminolysis of 4-Nitrophenyl Diphenylphosphinate

Abstract

Useful information on reaction mechanism can be obtained from analysis of Bronsted-type plots. Curved Bronstedtype plots often found for reactions of esters possessing a good leaving group (e.g., 2,4-dinitrophenoxide) have been taken as evidence for a change in the rate-determining step (RDS) of a stepwise mechanism. In fact, a curved Bronsted-type plot has been reported for reactions of 2,4dinitrophenyl benzoate (1) with a series of alicyclic secondary amines, i.e., the βnuc value decreases from 0.74 to 0.34 as the amine basicity increases. In contrast, the corresponding reactions of 4-nitrophenyl benzoate (2) resulted in a linear Bronsted-type plot with βnuc = 0.88 in the pKa range 5.9511.02. Thus, it has been concluded that the aminolyses of 1 and 2 proceed through a stepwise mechanism and the RDS changes in the reactions of 1 but not in the reactions of 2 in that pKa range. 4a,b