Aminolysis of Y-substituted phenyl diphenylphosphinates and benzoates : Effect of modification of electrophilic center from C=O to P=O

The effect of modification of the electrophilic center from C=O to P=O on reactivity and reaction mechanism has been investigated for aminolysis of Y-substituted phenyl diphenylphosphinates (la-j) and benzoates (2a-i). The phosphinates la-j are less reactive than the benzoates 2a-i. The reactions of 2,4-dinitrophenyl diphenylphosphinate (la) with alicyclic secondary amines resulted in a linear Bronsted-type plot with a β nuc value of 0.38, while the corresponding reactions of 2,4-dinitrophenyl benzoate (2a) yielded a curved Bronsted-type plot. Similarly, a linear Bronsted-type plot with a β lg value of -0.66 was obtained for the reactions of la-j with piperidine, while the corresponding reactions of 2a-i gave a curved Bronsted-type plot. The linear Bronsted-type plots for the reactions of 1a-j have been taken as evidence for a concerted mechanism, while the curved Bronsted-type plots for the reactions of 2a-i have been suggested to indicate a change in the rate-determining step of a stepwise mechanism. The Hammett plot for the reactions of 1b-j exhibited a poor correlation with σ - constants (R 2 = 0.962) but slightly better correlation with o° (R 2 = 0.986). However, the Yukawa-Tsuno plot for the same reactions resulted in an excellent correlation (R 2 = 0.9993) with an r value of 0.30. The aminolysis of la-j has been suggested to proceed through a concerted mechanism with an early transition state on the basis of the small β nuc and small r values.