Phenolysis of diaryl thiolcarbonates and thionocarbonates

Abstract

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 8C, ionic strength 0.2 M. The Brønsted 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 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 pK a , and 0.39, 0.28 and 0.34, respectively, at high pK 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 s 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 À . 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 À 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. RESULTS AND DISCUSSIONFor all reactions, pseudo-first-order rate coefficients (k obs ) were obtained (under total phenol excess). The experimental (www.interscience.wiley.com)

The nucleofuge in the pyridinolysis of O‐(4‐nitrophenyl) S‐aryl thio and dithiocarbonates

“…From the results obtained in the reaction of compound 2 , it can be concluded that 4‐nitrophenoxide is a better nucleofuge than 4‐nitrobenzenethiolate from the same tetrahedral intermediate (I), although the former is three p K a units more basic than the latter (the p K a values of 4‐nitrophenol and 4‐nitrobenzenethiolate are 7.5 and 4.5, respectively) …”

“…The p K a of 4‐nitrophenol and 4‐nitrobenzenethiol were determined previously under the same conditions. These values are 7.5 and 4.5, respectively …”

The reactions of O‐(4‐nitrophenyl) S‐aryl dithiocarbonates with anilines: Effects on the relative nucleofugality

“…This result is in accordance with the fact that, at the isobasic point, RO − is a better nucleofuge than RS − . [8d, 13] The pK a of 4-nitrophenol is 7.5, [14] that of benzenethiol is 7.2, [8d] and that of 4chlorobenzenethiol is 7.0. [8d] To propose a mechanism for the anilinolysis of 5, it is not possible to use the same argument above-mentioned for 6, considering the stabilization of the tetrahedral intermediate by change of the amine nature.…”

“…[9] The presence of the 2 nucleofuges in the anilinolysis reaction of 5 shows that the nucleofugacity of 4-nitrophenolate ion is similar to that of 4-nitrophenylbenzenethiolate from the tetrahedral intermediate I (Scheme 4), despite the differences in their basicities (pK a values of 4-nitrophenol and 4nitrobenzenethiol are 7.5 and 4.5, respectively). [14]…”

Reaction Mechanism in Ionic Liquids: Kinetics and Mechanism of the Aminolysis of 4-Nitrophenyl Acetate

“…Particularly in the case of nucleophilic reactions of aryl esters, these can react by two possible mechanisms: (i) a concerted pathway, where the nucleophilic attack at the electrophilic carbonyl carbon occurs simultaneously with the leaving group departure within a single step; and (ii) a stepwise mechanism, where the interaction of the nucleophile with the electrophilic carbon leads to the formation of a tetrahedral intermediate, from which the leaving group detaches. In nucleophilic substitutions reactions, the Brønsted equation (log k vs. p K a of amine) and particularly the charge developed on the nucleophilic atom from reactants to the transition state (the slope of the Brønsted‐type plot, β) has been a very good tool to determine reaction mechanisms in water and in organic solvents . There is a great deal of information in the literature about β values for these reactions in conventional solvents; nevertheless, the information is scarce for the same reactions in ILs .…”