A Kinetic Study on Nucleophilic Substitution Reactions of Phenyl Y-Substituted-Phenyl Carbonates with Z-Substituted-Phenoxides: Effect of Modification of Nonleaving Group from Benzoyl to Phenyloxycarbonyl on Reactivity and Reaction Mechanism

Abstract

Second-order rate constants for the reactions of phenyl Y-substituted-phenyl carbonates 5a-g with Z-substituted-phenoxides (<TEX>$k_{Z-PhO^-}$</TEX>) have been measured spectrophotometrically in 80 mol % <TEX>$H_2O$</TEX>/20 mol % DMSO at <TEX>$25.0{\pm}0.1^{\circ}C$</TEX>. 4-Nitrophenyl phenyl carbonate (5e) is up to 235 times more reactive than 4-nitrophenyl benzoate (4e). The Br<TEX>$\o$</TEX>nsted-type plot for the reactions of 5e with Z-substituted-phenoxides is linear with <TEX>${\beta}_{nuc}=0.54$</TEX>, which is typical for reactions reported previously to proceed through a concerted mechanism. Hammett plots correlated with <TEX>${\sigma}^o$</TEX> and <TEX>${\sigma}^-$</TEX> constants for the reactions of 5a-f with 4-chlorophenoxide exhibit highly scattered points. In contrast, the Yukawa-Tsuno plot results in an excellent linear correlation with <TEX>${\rho}_Y=1.51$</TEX> and r = 0.52, indicating that the leaving-group departure occurs at the rate-determining step (RDS). A stepwise mechanism, in which leaving-group departure occurs at RDS, has been excluded since the incoming 4-<TEX>$ClPhO^-$</TEX> is more basic and a poorer nucleofuge than the leaving Y-substituted-phenoxides. Thus, the reaction has been concluded to proceed through a concerted mechanism. Our study has shown that the modification of the nonleaving group from benzoyl to phenyloxycarbonyl causes a change in the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway) as well as an increase in the reactivity.