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

作者：Min Se Won; Kim Min Young; Um Ik Hwan^*

来源：Bulletin of the Korean Chemical Society, 2012, 33(10): 3253-3257.

DOI：10.5012/bkcs.2012.33.10.3253

• Nonleaving group
• Phenolysis
• Bronsted-type plot
• Yukawa-Tsuno plot
• Concerted mechanism

Second-order rate constants for the reactions of phenyl Y-substituted-phenyl carbonates 5a-g with Z-substituted-phenoxides (k(Z-PHO-)) have been measured spectrophotometrically in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. 4-Nitrophenyl phenyl carbonate (5e) is up to 235 times more reactive than 4-nitrophenyl benzoate (4e). The Bronsted-type plot for the reactions of 5e with Z-substituted-phenoxides is linear with beta(nuc) = 0.54, which is typical for reactions reported previously to proceed through a concerted mechanism. Hammett plots correlated with sigma degrees and sigma(-) 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 rho(Y) = 1.51 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-ClPhO- 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.

• 出版日期2012-10-20