Phenolysis of diaryl thiolcarbonates and thionocarbonates

摘要

The phenolysis of phenyl S-4-nitrophenyl thiolcarbonate (1), 4-chlorophenyl S-4-nitrophenyl thiolcarbonate (2), phenyl 4-nitrophenyl thionocarbonate (3), 3-chlorophenyl 4-nitrophenyl thionocarbonate (4) and 3-methoxyphenyl 4-nitrophenyl thionocarbonate (5) are studied kinetically in 44 wt% ethanol-water, at 25.0 degrees C, ionic strength 0.2 M. The Bronsted plots (log k(N) vs. pK(a) of phenols) for the reactions of 1 and 2 are linear and those of 3, 4 and 5 are curved. The Bronsted slopes for 1 and 2 are 0.64 and 0.60, respectively, which suggest a concerted mechanism for these reactions. The curved Bronsted plots for the reactions of 3, 4 and 5 show slope values of 1.0, 1.1 and 1.1, respectively, at low pKa, and 0.39, 0.28 and 0.34, respectively, at high pKa. The shape and slope values of these Bronsted plots are in accordance with a stepwise mechanism. The reactivity towards phenoxides of the thiocarbonates increases with the increase in the Hammett sigma value for the substituents in the non-leaving group. The fact that the phenolysis of 3 is stepwise, whereas that of phenyl 4-nitrophenyl carbonate is concerted can be attributed to the destabilization of the intermediate T-. A similar argument explains the stepwise phenolysis of 3, in contrast to the concerted phenolysis of 1, where there is an additional destabilization of T- caused by the change of the leaving group, from 4-nitrophenoxide in 3 to 4-nitrobenzenethiolate in 1. Thiolcarbonates 1 and 2 in 44 wt% ethanol-water are less reactive towards phenoxide anions (ca. 100-fold) than their corresponding oxycarbonates in water.