Kinetics and mechanism of the aminolysis of 4-methylphenyl and 4-chlorophenyl 4-nitrophenyl carbonates in aqueous ethanol

Reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-chlorophenyl 4-nitrophenyl carbonate (ClPNPC) with a series of quinuclidines (QUIN) and the latter carbonate with a series of secondary alicyclic amines (SAA) are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically at 330 or 400 nm (4-nitrophenol or 4-nitrophenoxide anion appearance, respectively). Under excess amine, pseudo-first-order rate coefficients (k(obsd)) are found. For all these reactions, plots of k(obsd) vs free amine concentration at constant pH are linear, the slope (k(N)) being independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acids of the amines) for the reactions of the series of QUIN with MPNPC and ClPNPC are linear with slopes (beta(N)) 0.88 and 0.87, respectively, which are explained by a stepwise process where breakdown of a zwitterionic tetrahedral intermediate (T(+/-)) to products is rate limiting. The Br?nsted-type plot for the reactions of the series of SAA with ClPNPC is biphasic with slopes beta(1) = 0.2 (high pK(a) region) and beta(2) = 0.9 (low pK(a) region) and a curvature center at pK(a)(0) = 10.6. This plot is in accordance with a stepwise mechanism through T(+/-) and a change in the rate-determining step, from T(+/-) breakdown to T(+/-) formation as the basicity of the SAA increases. Two conclusions arise from these results: (i) QUIN are better leaving groups from T(+/-) than isobasic SAA, and (ii) the non-leaving group effect on k(N) for these reactions is small, since beta(nlg) ranges from -0.2 to - 0.3. From these values, it is deduced that ClPNPC is ca. 70% more reactive than MPNPC toward SAA and QUIN, when expulsion of the leaving group from T(+/-) is the rate determining step.     

Kinetic investigation of the reactions of S-4-nitrophenyl 4-substituted thiobenzoates with secondary alicyclic amines in aqueous ethanol

The reactions of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO(2): 1, 2, and 3, respectively) with a series of secondary alicyclic amines (SAA) were subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions were followed spectrophotometrically by monitoring the release of 4-nitrobenzenethiolate anion at 420-425 nm. Under excess amine, pseudo-first-order rate constants (k(obsd)) are obtained for all reactions. The plots of k(obsd) vs [SAA] at constant pH are linear with the slope (k(N)) independent of pH. The statistically corrected Br?nsted-type plots (log k(N)/q vs pK(a) + log p/q) for the reactions of 1 and 2 are nonlinear with slopes at high pK(a), beta(1) = 0.27 and 0.10, respectively, and slopes at low pK(a), beta(2) = 0.86 and 0.84, respectively. The Br?nsted curvature is centered at pK(a) (pK(a)(0)) 10.0 and 10.4, respectively. The reactions of SAA with 3 exhibit a linear Br?nsted-type plot of slope 0.81. These results are consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T(+/-)). For the reactions of 1 and 2, there is a change in rate-determining step with amine basicity, from T(+/-) breakdown to products at low pK(a), to T(+/-) formation at high pK(a). For the reactions of 3, breakdown to products of T(+/-) is rate limiting for all the SAA series (pK(a)(0) > 11). The increasing pK(a)(0) value as the substituent in the acyl group becomes more electron withdrawing is attributed to an increasing nucleofugality of SAA from T(+/-). The greater pK(a)(0) value for the reactions of SAA with 1, relative to that found in the pyridinolysis of 2,4-dinitrophenyl benzoate (pK(a)(0) = 9.5), is explained by the greater nucleofugality from T(+/-) of the former amines, compared to isobasic pyridines, and the greater leaving ability from T(+/-) of 2,4-dinitrophenoxide relative to 4-nitrobenzenethiolate.     

Kinetics and mechanism of the pyridinolysis of S-2,4-dinitrophenyl 4-substituted thiobenzoates

[reaction: see text] The reactions of S-2,4-dinitrophenyl 4-methyl (1), S-2,4-dinitrophenyl 4-H (2), S-2,4-dinitrophenyl 4-chloro (3), and S-2,4-dinitrophenyl 4-nitro (4) thiobenzoates with a structurally homogeneous series of pyridines are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions are studied spectrophotometrically (420 nm) by monitoring the appearance of 2,4-dinitrobenzenethiolate anion. Pseudo-first-order rate coefficients (k(obsd)) are obtained for all the reactions, employing excess of amine. The plots of k(obsd) vs [free pyridine] at constant pH are linear with the slopes (k(N)) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acid of the pyridines) are curved for all the reactions. The Br?nsted curves are in accordance with stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T(+/-)), and a change in the rate-limiting step. An equation based on this hypothesis accounts well for the experimental points. The Br?nsted lines were calculated with the following parameters: Reactions of thiolbenzoate 1: beta(1) 0.33 (slope at high pK(a)), beta(2) 0.95 (slope at low pK(a)), and pK(a)(0) = 8.5 (pK(a) at the curvature center); thiolbenzoate 2: beta(1) 0.30, beta(2) 0.88, and pK(a)(0) = 8.9; thiolbenzoate 3: beta(1) 0.33, beta(2) 0.89, and pK(a)(0) = 9.5; thiolbenzoate 4: beta(1) 0.21, beta(2) 0.97, and pK(a)(0) = 9.9. The increase of the pK(a)(0) value with the increase of the electron-withdrawing effect of the acyl substituent is explained by the argument that the rate of pyridine expulsion from T(+/-) (k(-)(1)) is favored over that of 2,4-dinitrobenzenethiolate leaving (k(2)), i.e., k(-)(1)/k(2) increases, as the acyl group becomes more electron withdrawing. The pK(a)(0) values for the title reactions are smaller than those for the reactions of the corresponding 4-nitrophenyl 4-substituted thiolbenzoates with the same pyridine series. This is explained by the larger k(2) value for 2,4-dinitrobenzenethiolate leaving from T(+/-) compared with 4-nitrobenzenethiolate, which results in lower k(-)(1)/k(2) ratios for the dinitro derivatives. The pK(a)(0) value obtained for the pyridinolysis of thiolbenzoate 2 (pK(a)(0) = 8.9) is smaller than that found for the same aminolysis of 2,4-dinitrophenyl benzoate (pK(a)(0) = 9.5). This is attributed to the greater nucleofugality from T(+/-) of 2,4-dinitrobenzenethiolate (pK(a) of conjugate acid 3.4) relative to 2,4-dinitrophenoxide (pK(a) of conjugate acid 4.1). The title reactions are also compared with the aminolysis of similar esters to assess the effect of the amine nature and leaving and acyl groups on the kinetics and mechanism.     

Concerted mechanisms of the reactions of 2,4,6-trinitrophenyl methyl carbonate and 2,4,6-trinitrophenyl acetate with secondary alicyclic amines

The reactions of secondary alicyclic amines with 2,4,6-trinitrophenyl methyl carbonate (TNPMC) and 2,4,6-trinitrophenyl acetate (TNPA) are subjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of the 2,4,6-trinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [amine] are linear, with the slope (kN) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acid of the amines) are linear, with slopes beta = 0.41 and beta = 0.36 for the reactions of TNPA and TNPMC, respectively. The predicted breaks of the Br?nsted plots for stepwise mechanisms are pK(a)0 = 6.8 and 7.3, respectively. The lack of Br?nsted breaks for these reactions and the values of the Br?nsted slopes are consistent with concerted mechanisms. By comparison of the reactions under investigation among them and with similar aminolysis and pyridinolysis, the following conclusions can be drawn: (i) Secondary alicyclic amines react with TNPA and TNPMC by concerted mechanisms. (ii) TNPA is more reactive toward these amines than TNPMC due to the greater electron release of MeO from the latter substrate. (iii) The change of 2,4-dinitrophenoxy to 2,4,6-trinitrophenoxy in the zwitterionic tetrahedral intermediate (T+/-) formed in the reactions of the title amines with 2,4-dinitrophenyl acetate greatly destabilizes T+/-. (iv) Secondary alicyclic amines destabilize T+/- relative to pyridines. (v) The intermediate T+/- formed in the reactions of the title amines with S-(2,4,6-trinitrophenyl) acetate is greatly destabilized by substitution of S-(2,4,6-trinitrophenyl) by O-(2,4,6-trinitrophenyl) as the leaving group.     

Kinetics and mechanism of the aminolysis of O-aryl S-methyl thiocarbonates

[reaction: see text] The reactions of secondary alicyclic (SA) amines and quinuclidines (QUI) with 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) and those of SA amines with 2,3,4,5,6-pentafluorophenyl S-methyl thiocarbonate (3) are subjected to a kinetic study in aqueous solution, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). The reactions of thiocarbonates 1, 2, and 3 were followed spectrophotometrically at 400, 360, and 220 nm, respectively. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs amine concentration at constant pH are linear, with the slope (kN) independent of pH. The Br?nsted-type plots (log kN vs pKa of aminium ions) are linear for all the reactions, with slopes beta = 0.9 for those of 1 with SA amines and QUI, beta = 0.36 and 0.57 for the reactions of 2 with SA amines and QUI, respectively, and beta = 0.39 for the reactions of SA amines with 3. The magnitude of the slopes indicates that both aminolyses of 1 are governed by stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T+/-), where expulsion of the nucleofuge from T+/- is the rate-determining step. The values of the Br?nsted slopes found for the aminolyses of thiocarbonates 2 and 3 suggest that these reactions are concerted. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions arise: (i) Thiocarbonate 2 is more reactive than 1 toward the two amine series. (ii) The change of the nonleaving group from MeO in 4-nitrophenyl methyl carbonate to MeS in thiocarbonate 1 results in lower kN values. (iii) The greater reactivity of this carbonate than thiocarbonate 1 is attributed to steric hindrance of the MeS group, compared to MeO toward amine attack. (iv) The change of a pyridine to an isobasic SA amine or QUI destabilizes the T+/- intermediate formed in the aminolyses of 2. (v) The change of 4-nitrophenoxy to 2,3,4,5,6-pentafluorphenoxy or 2,4-dinitrophenoxy as the leaving group destabilizes the tetrahedral intermediate formed in the reactions with SA amines, changing the mechanism from a stepwise process to a concerted reaction.     

Kinetics and mechanism of the aminolysis of 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates in aqueous ethanol

The reactions of anilines with 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates (MPDNPC and ClPDNPC, respectively) and the latter substrate with secondary alicyclic (SA) amines are subjected to a kinetic study in 44 wt % ethanol-water solution, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of 2,4-dinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [amine] are linear and pH-independent, with slope k(N). The Br?nsted-type plots (log k(N) vs pK(a) of aminium ions) are linear, with slopes beta = 0.68 and 0.66 for the reactions of anilines with MPDNPC and ClPDNPC, respectively, and beta = 0.44 for the reactions of SA amines with ClPDNPC. The magnitude of the slope for the latter reaction indicates that its mechanism is concerted. The slope values for the reactions of anilines are in the borderline between stepwise and concerted mechanisms. The sensitivity of logk(N) to the basicity of the nonleaving group (beta(nlg)) is ca. -0.7 for the reactions of anilines, in agreement with that found for the SA reactions (beta(nlg) ca. -0.6). These results suggest that the reactions of anilines are concerted, although it is also possible that both mechanisms (stepwise and concerted) operate simultaneously. By comparison of the reactions under investigation between them and with similar aminolyses, the following conclusions can be drawn: (i) ClPDNPC is more reactive than MPDNPC toward the two amine series. (ii) The change of water to aqueous ethanol destabilizes a zwitterionic tetrahedral intermediate. (iii) The change of the nonleaving group from MeO to 4-methylphenoxy or 4-chlorophenoxy also destabilizes this intermediate.     

Concerted mechanisms of the reactions of methyl aryl carbonates with substituted phenoxide ions

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC, DNPC, and TNPC, respectively) with substituted phenoxide ions are subjected to a kinetic study in water at 25.0 degrees C, ionic strength 0.2 M (KCl). Production of the leaving groups (the nitro derivatives) is followed spectrophotometrically. Under excess of the phenoxide ions pseudo-first-order rate coefficients (k(obsd)) are found throughout. Plots of k(obsd) vs substituted phenoxide concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the phenols) are linear with slopes beta = 0.67, 0.48, and 0.52 for the phenolysis of NPC, DNPC, and TNPC, respectively. The magnitudes of these Br?nsted slopes are consistent with a concerted mechanism. In the particular case of the phenolysis of NPC the expected hypothetical curvature center of the Br?nsted plot for a stepwise mechanism should be pK(a)(0) = 7.1 (the pK(a) of 4-nitrophenol). This curvature does not appear within the pK(a) range of the substituted phenols studied (5.3--10.3), indicating that these reactions are concerted. The phenolysis of DNPC and TNPC should also be concerted in view of the even more unstable tetrahedral intermediates that would be formed if the reactions were stepwise. The reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate perhaps to the point of nonexistence. The k(N) values for the title reactions are larger than those for the concerted phenolysis of the corresponding ethyl S-aryl thiolcarbonates. The k(N) values found in the present reactions are subjected to a dual regression analysis as a function of the pK(a), of both the nucleophile and leaving group, the coefficients being beta(N) = 0.5 and beta(lg) = -0.3, respectively. These coefficients are consistent with a concerted mechanism.     

Change in rate-determining step in an E1cB mechanism during aminolysis of sulfamate esters in acetonitrile

The kinetics of the reactions of the nitrogen-sulfur(VI) esters 4-nitrophenyl N-methylsulfamate (NPMS) with a series of pyridines and a series of alicyclic amines and of 4-nitrophenyl N-benzylsulfamate (NPBS) with pyridines, alicyclic amines, and a series of quinuclidines have been investigated in acetonitrile (ACN) in the presence of excess amine at various temperatures. Pseudo-first-order rate constants (k(obsd)) have been obtained by monitoring the release of 4-nitrophenol/4-nitrophenoxide. From the slope of a plot of k(obsd) vs [amine], second-order rate constants (k'(2)) have been obtained for the pyridinolysis of NPMS, and a Br?nsted plot of log k'(2) vs pK(a) of pyridine gave a straight line with beta = 0.45. However, aminolysis with alicyclic amines of NPMS gave a biphasic Br?nsted plot (beta(1) = 0.6, beta(2) approximately equal to 0). Pyridinolysis and aminolysis with alicyclic amines and quinuclidines of NPBS also gave similar biphasic Br?nsted plots. This biphasic behavior has been explained in terms of a mechanistic change within the E1cB mechanism from an (E1cB)(irrev) (less basic amines) to an (E1cB)(rev) (more basic amines), and the change occurs at approximately the pK(a)'s (in ACN) of NPMS (17.94) and NPBS (17.68). The straight line Br?nsted plot for NPMS with pyridines occurs because the later bases are not strong enough to substantially remove the substrate proton and initiate the mechanistic change observed in the reaction of NPMS with the strong alicyclic amines and quinuclidines. An entropy study supports the change from a bimolecular to a unimolecular mechanism. This is the first clear demonstration of this E1cB mechanistic changeover involving a nitrogen acid substrate.     

Kinetic study of the phenolysis of O-methyl and O-phenyl O-2,4-dinitrophenyl thiocarbonates and O-ethyl 2,4-dinitrophenyl dithiocarbonate

The reactions of a series of phenols with O-methyl O-2,4-dinitrophenyl thiocarbonate (MDNPTOC), O-phenyl O-2,4-dinitrophenyl thiocarbonate (PDNPTOC), and O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC) are studied kinetically in water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). All reactions show pseudo-first-order kinetics under an excess of phenol over the substrate, and are first order in phenoxide anion. The reactions of EDNPDTC show a linear Br?nsted-type plot of slope beta = 0.67, suggesting a concerted mechanism. On the other hand, the phenolyses of MDNPTOC and PDNPTOC exhibit linear Br?nsted-type plots of slopes beta = 0.27 and 0.28, respectively, consistent with stepwise mechanisms where the formation of an anionic tetrahedral intermediate (T(-)) is rate determining. By comparison of the kinetics and mechanisms of the reactions under investigation with similar reactions, the following conclusions arise: (i). Substitution of S(-) by O(-) in the intermediate T(-) destabilizes this species. (ii). The change of DNPO in T(-) to DNPS also destabilizes this intermediate. (iii). Substitution of MeO by PhO as the nonleaving group of the substrate does not affect the kinetics, probably by a compensation of electronic and steric effects. (iv). The change of an amino group in a tetrahedral intermediate to a phenoxy group destabilizes the intermediate.     

Kinetic study of the aminolysis and pyridinolysis of O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates. A remarkable leaving group effect

The reactions of a series of secondary alicyclic (SA) amines with O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates (1 and 2, respectively) and of a series of pyridines with the former substrate are subjected to a kinetic investigation in water, at 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, all the reactions obey pseudo-first-order kinetics and are first-order in amine. The Br?nsted-type plots are biphasic, with slopes (at high pK(a)) of beta(1) = 0.20 for the reactions of SA amines with 1 and 2 and beta(1) = 0.10 for the pyridinolysis of 1 and with slopes (at low pK(a)) of beta(2) = 0.80 for the reactions of SA amines with 1 and 2 and beta(2) = 1.0 for the pyridinolysis of 1. The pK(a) values at the curvature center (pK(a)(0)) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T++) and a change in the rate-determining step with the variation of amine basicity. The larger pK(a)(0) value for the pyridinolysis of 1 compared to that for 2 (pK(a)(0) = 6.8) and the larger pK(a)(0) value for the reactions of SA amines with 1 relative to 2 are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger pK(a)(0) values for the reactions of SA amines with 1 and 2, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller pK(a)(0) value for the reactions of SA amines with 2 than with O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (pK(a)(0) = 9.2) is explained by the greater nucleofugality from T(++) of 2,4-dinitrophenoxide (DNPO(-)) relative to the thio derivative. The stepwise reactions of SA amines with 1 and 2, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T(++) by the change of O(-) to S(-). The simple mechanism for the SA aminolysis of 2 (only one tetrahedral intermediate, T(++)) is in contrast to the more complex mechanism (two tetrahedral intermediates, T(++) and T(-), the latter formed by deprotonation of T(++) by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO(-)) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T by replacement of NPO(-) with DNPO(-) as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T(++) of DNPO(-) than NPO(-) and (ii) the similar rates of deprotonation of both T(++) (formed with DNPO and NPO).     

Effects of amine nature and nonleaving group substituents on rate and mechanism in aminolyses of 2,4-dinitrophenyl X-substituted benzoates

Second-order rate constants have been measured for the reactions of 2,4-dinitrophenyl X-substituted benzoates (1a-f) with a series of primary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Br?nsted-type plot for the reactions of 1d with primary amines is biphasic with slopes beta(1) = 0.36 at the high pK(a) region and beta(2) = 0.78 at the low pK(a) region and the curvature center at pK(a) degrees = 9.2, indicating that the reaction proceeds through an addition intermediate with a change in the rate-determining step as the basicity of amines increases. The corresponding Br?nsted-type plot for the reactions with secondary amines is also biphasic with beta(1) = 0.34, beta(2) = 0.74, and pK(a) degrees = 9.1, indicating that the effect of amine nature on the reaction mechanism and pK(a) degrees is insignificant. However, primary amines have been found to be less reactive than isobasic secondary amines. The microscopic rate constants associated with the aminolysis have revealed that the smaller k(1) for the reactions with primary amines is fully responsible for their lower reactivity. The electron-donating substituent in the nonleaving group exhibits a negative deviation from the Hammett plots for the reactions of 1a-f with primary and secondary amines, while the corresponding Yukawa-Tsuno plots are linear. The negative deviation has been ascribed to stabilization of the ground state of the substrate through resonance interaction between the electron-donating substituent and the carbonyl functionality.     

Kinetics and mechanisms of the pyridinolysis of phenyl and 4-nitrophenyl chlorothionoformates. Formation and hydrolysis of 1-(aryloxythiocarbonyl)pyridinium cations

The title reactions are subjected to a kinetic study in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). By following the reactions spectrophotometrically two consecutive reactions are observed: the first is formation of the corresponding thionocarbamates (1-(aryloxythiocarbonyl)pyridinium cations) and the second is their decomposition to the corresponding phenol and pyridine, and COS. Pseudo-first-order rate coefficients (k(obsd1) and k(obsd2), respectively) are found under excess amine. Plots of k(obsd1) vs free pyridine concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acids of the pyridines) are linear with slopes beta = 0.07 and 0.11 for the reactions of phenyl and 4-nitrophenyl chlorothionoformates, respectively. These Br?nsted slopes are in agreement with those found in other stepwise reactions of the same pyridines in water, where the formation of a tetrahedral intermediate is the rate-determining step. In contrast to the stepwise mechanism of the title reactions that for the reactions of the same substrates with phenols is concerted, which means that substitution of a pyridino moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate. The second reaction corresponds to the pyridine-catalyzed hydrolysis of the corresponding 1-(aryloxythiocarbonyl)pyridinium cation. Plots of k(obsd2) vs free pyridine concentration at constant pH are linear, with the slope (k(H)) independent of pH. The Br?nsted plots for k(H) are linear with slopes beta = 0.19 and 0.26 for the reactions of the phenyl and 4-nitrophenyl derivatives, respectively. These low values are explained by the fact that as pK(a) increases the effect of a better pyridine catalyst is compensated by a worse leaving pyridine from the corresponding thionocarbamate     

Kinetics and mechanisms of the reactions of 4-nitro- and 3-nitrophenyl 4-methylphenyl thionocarbonates with alicyclic amines and pyridines

The reactions of the title thionocarbonates (1 and 2, respectively) with a series of secondary alicyclic amines and pyridines are subjected to a kinetic investigation in 44 wt % ethanol-water, 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess over the substrates pseudo-first-order rate coefficients (k(obsd)) are obtained for all the reactions. Those of the alicyclic amines with the two substrates show nonlinear upward plots of k(obsd) vs [amine], except the reactions of piperidine, which exhibit linear plots. For these reactions a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic (T(+/-)) and the other anionic (T(-)), with a kinetically significant proton transfer from T(+/-) to an amine to give T(-). From an equation derived from the scheme the rate microcoefficients are obtained through fitting. The rate coefficient for formation of T(+/-) (k(1)) is larger for 1 compared to 2, which can be explained by a stronger electron-withdrawal of 4-nitro in 1 than 3-nitro in 2, which leaves the thiocarbonyl carbon of 1 more positive and, therefore, more susceptible to nucleophilic attack. For the pyridinolyses of both thionocarbonates the plots of k(obsd) vs [amine] are linear, with the slope (k(N)) independent of pH. The Bronsted plots (log k(N) vs pyridine pK(a)) for these reactions are linear with slopes beta = 0.9 and 1.2 for the pyridinolysis of 1 and 2, respectively. These slopes are consistent with a mechanism through a T(+/-) intermediate on the reaction path, whereby decomposition of T(+/-) to products is the rate-determining step. The k(N) values are larger for the reactions of 1 than those of 2. This is attributed to a larger equilibrium formation of T(+/-) and a larger expulsion rate of the nucleofuge from T(+/-) in the reactions of 1 compared to those of 2.     

Kinetics and mechanism of the benzenethiolysis of O-ethylS-(2,4-dinitrophenyl) and O-ethyl S-(2,4,6-trinitrophenyl) dithiocarbonates and O-methyl O-(2,4-dinitrophenyl) thiocarbonate

Reactions of O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC), O-ethyl 2,4,6-trinitrophenyl dithiocarbonate (ETNPDTC), and O-methyl O-(2,4-dinitrophenyl) thiocarbonate (MDNPTOC) with a series of benzenethiolate anions in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M (KCl), are subjected to a kinetic investigation. Under excess benzenethiolate, these reactions obey pseudo-first-order kinetics and are first order in benzenethiolate. Nonetheless, similar reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with the ethyl trinitrophenyl ester (ETNPDTC), which showed overall second-order kinetics. The nucleophilic rate constants (k(N)) are pH independent, except those for the reactions of ETNPDTC with the X-benzenethiolates with X = H, 4-Cl, and 3-Cl, which increase as pH decreases. The Br?nsted-type plots (log k(N) vs pK(a) of benzenethiols) are linear with slopes beta = 0.66 for the reactions of both ethyl dinitrophenyl ester (EDNPDTC) and ethyl trinitrophenyl ester (ETNPDTC) and beta = 0.58 for those of the thiocarbonate ester (MDNPTOC). For the benzenethiolysis of MDNPTOC and EDNPDTC, no breaks were found in the Br?nsted-type plots at pK(a) 4.1 and 3.4, respectively, consistent with concerted mechanisms. Benzenethiolysis of the ethyl trinitrophenyl ester (ETNPDTC) should also be concerted in view of the even more unstable tetrahedral "intermediate" that would have been formed had this reaction been stepwise. ETNPDTC is more reactive toward benzenethiolate anions than EDNPDTC due to the better leaving group involved in the former substrate. The k(N) values found for the reactions of EDNPDTC with benzenethiolates are larger than those obtained for the concerted reactions of the same substrate with isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle. The concerted mechanism for the benzenethiolysis of MDNPTOC, in contrast to the stepwise mechanism found for the phenolysis of this substrate, is attributed to the greater kinetic instability of the hypothetical tetrahedral "intermediate" formed in the former reaction, due to the greater nucleofugality of ArS(-) compared with an isobasic ArO(-). Benzenethiolates are more reactive toward MDNPTOC and EDNPDTC than the corresponding carbonate and thiolcarbonate, respectively. This is also in accordance with the HSAB principle, since benzenthiolates are relatively soft bases that prefer to bind to a relatively soft thiocarbonyl center rather than a relatively hard carbonyl center.     

Kinetics and mechanism of the phenolysis of asymmetric diaryl carbonates

The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in aqueous solution (25.0 degrees C, ionic strength 0.2 M (KCI)). Under an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order kinetics and are first order in phenoxide. The Br?nsted-type plots for the nucleophilic rate constants (k(N)) are linear, with slopes beta = 0.48 (MPNPC), 0.67 (ClPNPC), 0.41 (MPDNPC), and 0.32 (ClPDNPC). The magnitude of these slopes and the absence of a curvature in the Br?nsted plot at pK(a) = 7.1 for the CIPNPC reactions are consistent with concerted mechanisms (one step). The carbonates MPDNPC and ClPDNPC are more reactive than MPNPC and CIPNPC, respectively, toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding 4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than 4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is deduced for log k(N) in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving group. This equation shows that for these reactions, the sensitivity of log k(N) to the basicity of the nonleaving moiety (beta(nlg) = -0.27) is very similar to that of the nucleofuge (beta(lg) = -0.25).     

Effect of changing electrophilic center from C=O to C=S on rates and mechanism: pyridinolyses of O-2,4-dinitrophenyl thionobenzoate and its oxygen analogue

Second-order rate constants have been measured spectrophotometrically for the reactions of O-2,4-dinitrophenyl thionobenzoate (1) and 2,4-dinitrophenyl benzoate (2) with a series of substituted pyridines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Br?nsted-type plots obtained are nonlinear with beta(1) = 0.26, beta(2) = 1.07, and pK(a) degrees = 7.5 for the reactions of 1 and beta(1) = 0.40, beta(2) = 0.90, and pK(a) degrees = 9.5 for the reactions of 2, suggesting that the pyridinolyses of 1 and 2 proceed through a zwiterionic tetrahedral intermediate T(+/-) with a change in the rate-determining step at pK(a) degrees = 7.5 and 9.5, respectively. The thiono ester 1 is more reactive than its oxygen analogue 2 except for the reaction with the strongest basic pyridine studied (pK(a) = 11.30). The k(1) value is larger for the reactions of 1 than for those of 2 in the low pK(a) region, but the difference in the k(1) value becomes negligible with increasing the basicity of pyridines. On the other hand, 1 exhibits slightly larger k(2)/k(-1) ratio than 2 in the low pK(a) region but the difference in the k(2)/k(-1) ratio becomes more significant with increasing the basicity of pyridines. Pyridines are more reactive than alicyclic secondary amines of similar basicity toward 2 in the pK(a) above ca. 7.2 but less reactive in the pK(a) below ca. 7.2. The k(1) value is slightly larger, but the k(2)/k(-1) ratio is much smaller for the reactions of 2 with pyridines than with isobasic secondary amines in the low pK(a) region, which is responsible for the fact that the weakly basic pyridines are less reactive than isobasic secondary amines.     

Kinetic study of the phenolysis of bis(4-nitrophenyl) carbonate, bis(4-nitrophenyl) thionocarbonate, and methyl 4-nitrophenyl thionocarbonate

The reactions of a homogeneous series of phenols with bis(4-nitrophenyl) carbonate (BNPC), bis(4-nitrophenyl) thionocarbonate (BNPTOC), and methyl 4-nitrophenyl thionocarbonate (MNPTOC) are subjected to a kinetic investigation in water, at 25.0 degrees C and ionic strength of 0.2 M (KCl). Under excess of phenol over the substrate, all the reactions obey pseudo-first-order kinetics and are first order in phenoxide anion. The reactions of BNPC show a linear Br?nsted-type plot with slope beta = 0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOC and MNPTOC show biphasic Br?nsted-type plots with slopes beta = 0.30 and 0.44, respectively, at high pK(a), and beta = 1.25 and 1.60, respectively, at low pK(a), consistent with stepwise mechanisms. For the reactions of both thionocarbonates, the pK(a) value at the center of the Br?nsted plot (pK(a)(0)) is 7.1, which corresponds to the pK(a) of 4-nitrophenol. This confirms that the phenolyses of the thionocarbonates are stepwise processes, with the formation of an anionic tetrahedral intermediate. By the comparison of the kinetics and mechanisms of the title reactions with similar reactions, the following conclusions can be drawn: (i) Substitution of S(-) by O(-) in an anionic tetrahedral intermediate (T(-)) destabilizes it. (ii) The change of MeO by 4-nitrophenoxy in T(-) results in an increase of both the rate constant and equilibrium constant, for the formation of T(-), and also in an enlargement of the rate coefficient for the expulsion of 4-nitrophenoxide from T(-). (iii) Substitution of an amino group in a tetrahedral intermediate by ArO destabilizes it. (iv) Secondary alicyclic amines and other amines show greater reactivity toward MNPTOC than isobasic phenoxide anions.     

Kinetics and mechanism of the pyridinolysis of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates

The reactions of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) with a series of 3- and/or 4-substituted pyridines in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M (KCl), are subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the release of 4-nitrophenoxide (400 nm) or 2,4-dinitrophenoxide (360 nm) anions. Under amine excess, pseudo-first-order rate coefficients (kobsd) are found. Plots of kobsd vs [pyridine] are linear and pH-independent, with slope kN. The Br?nsted-type plot (log kN vs pKa of pyridinium ions) for the reactions of 1 is linear, with slope beta = 1.1, in contrast to the plot for the reactions of 2, which is biphasic, with slopes beta1 = 0.25 (high pKa) and beta2 = 0.90 (low pKa) and the curvature center at pKa = p = 7.3. The latter Br?nsted plot is consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T+/-) on the reaction path, and a change of the rate-determining step, from breakdown to formation of T+/-, as pyridine basicity increases. For the reactions of 1 the beta value indicates that the mechanism is also stepwise with expulsion of the nucleofuge from T+/- as the rate-determining step. By comparison of the reactions under investigation among each other and with similar aminolyses, the following conclusions can be drawn. (i) Thiocarbonate 2 is more reactive than 1 toward pyridines. (ii) The pka0 value for the pyridinolysis of 2,4-dinitrophenyl methyl carbonate (4) is larger than that for thiocarbonate 2. (iii) The k1 values (pyridine attack to form T+/-) are smaller for thiocarbonates 1 and 2 than the corresponding oxy carbonates 3 and 4, respectively. This is not in accordance with the electronic effects of MeS and MeO and could be attributed to steric hindrance of the MeS group toward pyridine attack. (iv) The kN values for the pyridinolysis of carbonates 3 and 4 are larger than those for thiocarbonates 1 and 2, respectively, when the k2 step is rate-limiting.     

Kinetic and mechanistic investigation of the aminolysis of 3-methoxyphenyl 3-nitrophenyl thionocarbonate, 3-chlorophenyl 3-nitrophenyl thionocarbonate,and bis(3-nitrophenyl) thionocarbonate

The reactions of the title thionocarbonates (6, 7, and 8, respectively) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt % ethanol-water, 25.0 degrees C, ionic strength 0.2 M (KCl). Under excess amine, pseudo-first-order rate coefficients (k(obsd)) are obtained for all reactions. Reactions of substrates 6 and 7 with piperidine and of thionocarbonate 8 with the same amine and piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine show linear k(obsd) vs [amine] plots, with slopes (k(1)) independent of pH. On the other hand, these plots are nonlinear upward for the reactions of substrates 6 and 7 with all the amines, except piperidine, and also for the reactions of compound 8 with 1-formylpiperazine and piperazinium ion. For all these reactions a mechanistic scheme is proposed with the formation of a zwitterionic tetrahedral intermediate (T(+/-)), which can transfer a proton to an amine to give an anionic intermediate (T(-)). Rate and equilibrium microcoefficients of this scheme, k(1), k(-)(1), K(1) (= k(1)/k(-)(1)), and k(2), are obtained by fitting the nonlinear plots through an equation derived from the scheme. The Br?nsted-type plots for k(1) are linear with slopes beta(1) = 0.19, 0.21, and 0.26 for the aminolysis of 6, 7, and 8, respectively. This is consistent with the hypothesis that the formation of T(+/-) (k(1) step) is the rate-determining step. The k(1) values for these reactions follow the sequence 8 > 7 > 6, consistent with the sequence of the electron-withdrawing effects from the substituents on the "nonleaving" group of the substrates. The k(1) values for the aminolysis of 6, 7, and 8 are smaller than those for the same aminolysis of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (2, 3, and 4, respectively). The k(2) values (expulsion of the nucleofuge from T(+/-)) increase as the electron withdrawal from the nonleaving group increases. These values are smaller for the aminolysis of 6, 7, and 8 compared to those for the same aminolysis of 2, 3, and 4, respectively.     

Aminolysis of 4-nitrophenyl phenyl carbonate and thionocarbonate: effects of amine nature and modification of electrophilic center from C[double bond]O to C[double bond]S on reactivity and mechanism

A kinetic study is reported for the reactions of 4-nitrophenyl phenyl carbonate (5) and thionocarbonate (6) with a series of alicyclic secondary amines in 80 mol% H(2)O-20 mol% DMSO at 25.0 +/- 0.1 degrees C. The plots of k(obsd) vs. amine concentration are linear for the reactions of 5. On the contrary, the plots for the corresponding reactions of 6 curve upward as a function of increasing amine concentration, indicating that the reactions proceed through two intermediates (i.e., a zwitterionic tetrahedral intermediate T(+/-) and its deprotonated form T(-)). The Br?nsted-type plot for 5 the reactions of with secondary amines exhibits a downward curvature, i.e., the slope decreases from 0.98 to 0.26 as the pK(a) of the conjugate acid of amines increases, implying that the reactions proceed through T(+/-) with a change in the rate-determining step (RDS). The k(N) values are larger for the reactions of with secondary amines than for those with primary amines of similar basicity. Dissection of k(N) values for the reactions of 5 into the microscopic rate constants (i.e., k(1) and k(2)/k(-1) ratio) has revealed that k(1) is larger for the reactions with secondary amines than for those with isobasic primary amines, while the k(2)/k(-1) ratio is nearly identical. On the other hand, for reactions of 6, secondary amines exhibit larger k(1) values but smaller k(2)/k(-1) ratios than primary amines. The current study has shown that the reactivity and reaction mechanism are strongly influenced by the nature of amines (primary vs. secondary amines) and electrophilic centers (C[double bond]O vs. C[double bond]S).