Phenolysis of diaryl thiolcarbonates and thionocarbonates

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 °C, ionic strength 0.2 M. The Brønsted plots (log k N vs . p K a of phenols) for the reactions of 1 and 2 are linear and those of 3, 4 and 5 are curved. The Brønsted slopes for 1 and 2 are 0.64 and 0.60, respectively, which suggest a concerted mechanism for these reactions. The curved Brønsted plots for the reactions of 3 , 4 and 5 show slope values of 1.0, 1.1 and 1.1, respectively, at low p K a , and 0.39, 0.28 and 0.34, respectively, at high p K a . The shape and slope values of these Brønsted plots are in accordance with a stepwise mechanism. The reactivity towards phenoxides of the thiocarbonates increases with the increase in the Hammett σ 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. Copyright © 2008 John Wiley & Sons, Ltd.