The α-Effect in Nucleophilic Substitution Reactions of Y-Substituted-Phenyl Diphenylphosphinates with HOO-and OH-

Abstract

Second-order rate constants (<TEX>$k_{HOO^-}$</TEX>) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with <TEX>$HOO^-$</TEX> in <TEX>$H_2O$</TEX> have been measured spectrophotometrically. The <TEX>${\alpha}$</TEX>-nucleophile <TEX>$HOO^-$</TEX> is 10-70 times more reactive than the reference nucleophile <TEX>$OH^-$</TEX> although the former is ca. <TEX>$4pK_a$</TEX> units less basic than the latter, indicating the <TEX>${\alpha}$</TEX>-effect is operative. The Bronsted-type plot for the reactions of 4a-4i with <TEX>$HOO^-$</TEX> is linear with <TEX>${\beta}_{lg}=-0.51$</TEX>, a typical <TEX>${\beta}_{lg}$</TEX> value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with <TEX>${\rho}=1.40$</TEX> 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 <TEX>${\alpha}$</TEX>-effect (i.e., the <TEX>$k_{HOO^-}/k_{HO^-}$</TEX> ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the <TEX>${\alpha}$</TEX>-effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the <TEX>${\alpha}$</TEX>-effect.