The α-effect in nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates with HOO- and OH -

Abstract

Second-order rate constants (kHOO-) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with HOO- in H2O have been measured spectrophotometrically. The a-nucleophile HOO- is 10-70 times more reactive than the reference nucleophile OH- although the former is ca. 4 pKa units less basic than the latter, indicating the a-effect is operative. The Brønsted-type plot for the reactions of 4a-4i with HOO- is linear with βlg = -0.51, a typical βlg value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with ρ = 1.40 and r = 0.47, indicating that a negative charge develops partially on the O atom of the leaving group, which can be delocalized to the substituent Y through resonance interactions. Thus, the reactions have been proposed to proceed through a concerted mechanism. The magnitude of the α-effect (i.e., the kHOO-/kHO- ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the α-effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the α-effect.