Aminolysis of 2,4-Dinitrophenyl and 3,4-Dinitrophenyl Diphenylphosphinothioates: Sterie Hindrance versus Nucleofugality in Nucleophilic Substitution Reactions

摘要

Aminolyses of esters have been reported to proceed through a concerted mechanism or through a stepwise pathway with one or two intermediates (i.e., a zwitterionic tetrahedral intermediate T~± and its deprotonated form T~-) depending on the nature of the electrophilic centers (e.g., C=O, C=S, P=O and P=S). Reactions of carboxylic esters (C=O) with amines have generally been reported to proceed through a stepwise mechanism. The rate-determining step (RDS) has been suggested to be dependent on the basicity of the incoming amine and the leaving group, i.e., it changes from breakdown of T~± to it formation as the incoming amine becomes more basic than the leaving group by 4 to 5 pK_a units on the basis of a curved Bronsted-type plot found for aminolysis of esters possessing a good leaving group such as 2,4- or 3,4-dinitrophenoxide. In contrast, aminolysis of thiono esters (C=S) has been shown to proceed through two intermediates (i.e., T~~± and T~-), while the corresponding reactions of phosphorus esters (P=O and P=S) proceed through a concerted mechanism.