Concerted Mechanisms of the Reactions of Ethyl S-Aryl Thiocarbonates with Substituted Phenoxide Ions

Abstract

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl O-ethyl thiolcarbonates with substituted phenoxide ions are subjected to a kinetic study in water, 25.0 °C, ionic strength 0.2 M (KCl). By following the reactions spectrophotometrically, pseudo-first-order rate coefficients (k obsd) are found under excess of the nucleophile. Plots of k obsd vs phenoxide anion concentration at constant pH are linear, with the slope (k N) independent of pH. The Brönsted-type plots (log k N vs pK a of the phenols) are linear with slopes β = 0.92, 0.77, and 0.61 for the reactions of the 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl derivatives, respectively. For these reactions, a concerted mechanism is proposed since the slope values are similar to those found in the concerted phenolysis of aryl acetates; the slope magnitudes are not consistent with a stepwise mechanism where the formation of a tetrahedral intermediate is rate limiting. Our results are in line with the finding that the reactions of the 2,4-dinitrophenyl and 2,4,6-trinitrophenyl derivatives with secondary alicyclic amines in water are concerted. In contrast, the reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate with a benzenethio group by a phenoxy group destabilizes the intermediate.

Phenolysis of diaryl thiolcarbonates and thionocarbonates

“…A comparison of the nucleophilic rate constants (k N ) for the phenolysis of thiolcarbonates 1 and 2 in 44 wt% ethanol-water (this work) with those of the same reactions of phenyl and 4-chlorophenyl 4-nitrophenyl carbonates in water [10,12] shows that the carbonates are more reactive (about 80-100-fold) than the corresponding thiolcarbonates towards a given phenoxide. This is in accordance with the larger k N values found in the following reactions when a benzenethiolate leaving group is substituted by its phenoxide analogue: (i) the phenolysis of alkyl aryl carbonates [9] compared with that of alkyl aryl thiolcarbonates [13] (10-40-fold); (ii) the phenolysis of methyl 2,4-dinitrophenyl thionocarbonate, [14] relative to that of O-ethyl 2,4-dinitrophenyl dithiocarbonate; [14] (iii) the benzenethiolysis of methyl aryl carbonates [19] compared with that of ethyl S-aryl thiolcarbonates [19] (2-5-fold). Also, the reactions of secondary alicyclic amines with methyl 2,4,6-trinitrophenyl carbonate [21] show greater k N values than those with ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate.…”

“…This result is in accordance with the greater electron-withdrawing ability of 4-chlorophenoxy compared to phenoxy, [20] which leaves the carbonyl carbon atom of the former substrate more positive and, in consequence, more prone to nucleophilic attack. On the other hand, the change of the ethyl group in ethyl S-(4-nitrophenyl) thiolcarbonate [13] for an aryl group (phenyl and 4-chlorophenyl in 1 and 2, respectively, this study) does not change the mechanism of the phenolysis (all concerted), although increases the nucleophilic rate constant by more than 10-fold. Since the former reaction was measured in water (which should increase the k N value compared with aqueous ethanol) and the two latter in 44% aqueous ethanol, the increase of k N can be attributed to the greater electron withdrawal of the aryl groups in 1 and 2 relative to the ethyl group in the former thiolcarbonate.…”

“…[17,18] For the reactions of 1 and 2, where 4-nitrobenzenethiolate anion is the leaving group, it is difficult to predict the position of the centre of the curvature if the mechanism were stepwise, due to the fact that nucleophile (phenoxide anions) and nucleofuge (4-nitrobenzenethiolate) are of different nature. Nonetheless, we are inclined towards a concerted mechanism based on the following grounds: (i) The phenolysis of ethyl S-aryl thiolcarbonates [13] and the benzenethiolysis of methyl aryl carbonates [19] were found to be concerted, showing that the presence of one alkyl (methyl or ethyl), one phenoxy and one benzenethio groups in an anionic tetrahedral intermediate destabilizes the putative tetrahedral intermediate. (ii) The phenolysis reactions of a series of diaryl carbonates are concerted, showing that the presence of three phenoxy groups destabilizes the putative tetrahedral intermediate.…”

“…There have been many reports on the kinetics and mechanisms of the phenolysis of esters [1][2][3][4][5][6] and thioesters, [7,8] but less regarding the phenolysis of methyl aryl carbonates, [9] diaryl carbonates, [10][11][12] O-alkyl S-aryl thiolcarbonates [8,13] alkyl aryl thionocarbonates [8,11,14] and diaryl thionocarbonates. [8,11,14] The kinetic studies on the phenolysis of methyl aryl carbonates [9] and O-ethyl S-aryl thiolcarbonates [13] show concerted mechanisms, in contrast to those of methyl aryl thionocarbonates, which show stepwise mechanisms. [11,14] To our knowledge, there are only two works in the literature on the mechanism of the phenolysis of diaryl thionocarbonates (that of bis(4-nitrophenyl) thionocarbonate [11] and phenyl 2,4-dinitrophenyl thionocarbonate [14] ) and there are no reports on the phenolysis of O-aryl S-aryl thiolcarbonates.…”

Effect of substitution of oxygen by sulfur in the nonleaving group of a carbonate: kinetics of the phenolysis and benzenethiolysis of S‐methyl aryl thiocarbonates

“…This is also true when the S and O atoms are in the leaving group: the nucleophilic rate constants (k N ) for the phenolysis of methyl aryl carbonates are larger than those of the corresponding S-aryl thiocarbonates. 3,5 Comparison of the nucleophilic rate constants for the benzenethiolysis of 2 (this study) with those for the corresponding carbonate (4) 4 shows that the reactivities of these compounds toward benzenethiolates are similar.…”