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.     

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).     

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 aminolysis of methyl 4-nitrophenyl,methyl 2,4-dinitrophenyl,and phenyl 2,4-dinitrophenyl carbonates

The reactions of methyl 4-nitrophenyl carbonate (MNPC) with a series of secondary alicyclic amines (SAA) and quinuclidines (QUIN), methyl 2,4-dinitrophenyl carbonate (MDNPC) with QUIN and 1-(2-hydroxyethyl)piperazinium ion (HPA), and phenyl 2,4-dinitrophenyl carbonate (PDNPC) with SAA are subjected to a kinetic investigation in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M. By following spectrophotometrically the nucleofuge release (330-400 nm) under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. Plots of k(obsd) vs [amine] at constant pH are linear, with the slope (k(N)) being pH independent. The Br?nsted-type plot (log k(N) vs amine pK(a)) for the reactions of SAA with MNPC is biphasic with slopes beta(1) = 0.3 (high pK(a) region) and beta(2) = 1.0 (low pK(a) region) and a curvature center at pK(a)(0) = 9.3. This plot is consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T(+/-)) and a change in the rate-determining step with SAA basicity. The Br?nsted plot for the quinuclidinolysis of MNPC is linear with slope beta(N) = 0.86, in line with a stepwise process where breakdown of T(+/-) to products is rate limiting. A previous work on the reactions of SAA with MDNPC was revised by including the reaction of HPA. The Br?nsted plots for the reactions of QUIN and SAA with MDNPC and SAA with PDNPC are linear with slopes beta = 0.51, 0.48, and 0.39, respectively, consistent with concerted mechanisms. Since quinuclidines are better leaving groups from T(+/-) than isobasic SAA, yielding a less stable T(+/-), it seems doubtful that the quinuclidinolysis of PDNPC is stepwise, as reported.     

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.     

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).     

Aminolyses of 4-nitrophenyl phenyl carbonate and thionocarbonate: effect of modification of electrophilic center from C=O to C=S on reactivity and mechanism

A kinetic study is reported for nucleophilic substitution reactions of 4-nitrophenyl phenyl carbonate (5) and 4-nitrophenyl phenyl thionocarbonate (6) with a series of primary amines. The thiono compound 6 is less reactive than its oxygen analogue 5 toward strongly basic amines but is more reactive toward weakly basic CF3CH2NH2. The Br?nsted-type plots obtained from the aminolyses of 5 and 6 are curved downwardly. The reactions are proposed to proceed through a stepwise mechanism with a change in the RDS on the basis of the curved Br?nsted-type plots. The microscopic rate constants (k(1) and k(2)/k(-1) ratio) associated with the current aminolyses are consistent with the proposed reaction mechanism. The replacement of the C=O bond in 5 by a polarizable C=S group results in a decrease in the k(1) value but an increase in the k(2)/k(-1) ratio. Besides, such a modification of the electrophilic center causes a decrease in pKa degrees , defined as the pK(a) at the curvature center of curved Br?nsted-type plots, but does not alter the reaction mechanism. The larger k(2)/k(-1) ratio for the reactions of 6 compared to those of 5 is proposed to be responsible for the decreased pK(a) degrees value.     

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.     

Kinetics and mechanism of the pyridinolysis of s-4-nitrophenyl 4-substituted thiobenzoates in aqueous ethanol

The pyridinolysis of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO2; 1, 2, and 3, respectively) is studied kinetically in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions are measured spectrophotometrically (420-425 nm) by following the appearance of 4-nitrobenzenethiolate anion. Pseudo-first-order rate coefficients (kobsd) are obtained throughout, under excess of amine over the substrate. Plots of kobsd vs [free amine] at constant pH are linear with the slope (kN) independent of pH. The Brnsted-type plot (log kN vs pKa0 of the conjugate acids of the pyridines) for the reactions of thiolbenzoate 1 is curved with a slope at high pKa, beta1 = 0.20, and slope at low pKa0, beta2 = 0.94. The pKa value for the center of the Brnsted curvature is pKa0 = 9.7. The pyridinolysis of thiolbenzoates 2 and 3 show linear Brnsted-type plots of slopes 0.94 and 1.0, respectively. These results and other evidence indicate that these reactions occur with the formation of a zwitterionic tetrahedral intermediate (T+/-). For the pyridinolysis of thiolbenzoate 1, breakdown of T+/- to products (k2 step) is rate-limiting for weakly basic pyridines and T+/- formation (k1 step) is rate-determining for very basic pyridines. The k2 step is rate-limiting for the reactions of thiolbenzoates 2 and 3. The smallest pKa0 value for the reaction of 1 is due to the weakest electron withdrawal of H (relative to Cl and NO2) in the acyl group, which results in the smallest k-1/k2 ratio. The pKa0 values for the title reactions are smaller than those for the reactions of secondary alicyclic amines with thiolbenzoates 1-3. This is attributed to a lower leaving ability from the T+/- of pyridines than isobasic alicyclic amines. The lower p value found for the pyridinolysis of 2,4-dinitrophenyl benzoate (pKa0 = 9.5), compared with that for the pyridinolysis of 1, is explained by the greater nucleofugality from T+/- of 2,4-dinitrophenoxide than 4-nitrobenzenethiolate, which renders the k-1/k2 ratio smaller for the reactions of the benzoate relative to thiolbenzoate 1. The title reactions are also compared with the aminolysis of similar thiolbenzoates in other solvents to assess the solvent effect.     

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.     

Effect of substituent on regioselectivity and reaction mechanism in aminolysis of 2,4-dinitrophenyl X-substituted benzenesulfonates

[reaction: see text] We report on a kinetic study for the nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzensulfonates (X = 4-MeO, 1a, and X = 4-NO(2), 1c) with a series of primary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 degrees C. The reactions proceed through S-O and C-O bond fission pathways competitively. The fraction of the S-O bond fission increases as the attaching amine becomes more basic and the substituent X changes from 4-MeO to 4-NO(2), indicating that the regioselectivity is governed by the electronic nature of the substituent X as well as the basicity of amines. The S-O bond fission has been suggested to proceed through an addition intermediate with a change in the rate-determining step (RDS) at pK(a) degrees = 8.9 +/- 0.1. The electronic nature of the substituent X influences k(N)(S-O) and k(1) values, but not the k(2)/k(-1) ratios and the pK(a) degrees value significantly. Stabilization of the ground state (GS) through resonance interaction between the electron-donating substituent and the electrophilic center has been suggested to be responsible for the decreased reactivity of 1a compared to 1c. The second-order rate constants for the C-O bond fission exhibit no correlation with the electronic nature of the substituent X. The distance effect and the nature of the reaction mechanism have been suggested to be responsible for the absence of the correlation.     

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 amine nature on reaction rate and mechanism in nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates with alicyclic secondary amines

Second-order rate constants have been measured for reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates with a series of alicyclic secondary amines. The reaction proceeds through S-O and C-O bond fission pathways competitively. The S-O bond fission occurs more dominantly as the amine basicity increases and the substituent X in the sulfonyl moiety becomes more strongly electron withdrawing, indicating that the regioselectivity is governed by the amine basicity as well as the electronic nature of the substituent X. The S-O bond fission proceeds through an addition intermediate with a change in the rate-determining step at pK(a) degrees = 9.1. The secondary amines are more reactive than primary amines of similar basicity for the S-O bond fission. The k(1) value has been determined to be larger for reactions with secondary amines than with primary amines of similar basicity, which fully accounts for their higher reactivity. The second-order rate constants for the S-O bond fission result in linear Yukawa-Tsuno plots while those for the C-O bond fission exhibit poor correlation with the electronic nature of the substituent X. The distance effect and the nature of reaction mechanism have been suggested to be responsible for the poor correlation for the C-O bond fission pathway.     

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 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.     

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.     

Effect of amine nature on reaction mechanism: aminolyses of o-4-nitrophenyl thionobenzoate with primary and secondary amines

Pseudo-first-order rate constants (k(obs)) have been measured spectrophotometrically for reactions of O-4-nitrophenyl thionobenzoate (2) with a series of primary and acyclic secondary amines. The plots of k(obs) vs amine concentration are linear for the reaction of 2 with primary amines. The slope of the Br?nsted-type plot for the reaction of 2 with primary amines decreases from 0.77 to 0.17 as the amine basicity increases, indicating that the reaction proceeds through a zwitterionic addition intermediate in which the rate-determining step changes from the breakdown of the intermediate to the reaction products to the formation of the intermediate as the amine basicity increases. On the other hand, for reactions with all the acyclic secondary amines studied, the plot of k(obs) vs amine concentration exhibits an upward curvature, suggesting that the reaction proceeds through two intermediates, e.g., a zwitterionic addition intermediate and an anionic intermediate. The microscopic rate constants (k(1), k(-)(1), k(2), and k(3) where available) have been determined for the reactions of 2 with all the primary and secondary amines studied. The k(1) value is larger for the reaction with the primary amine than for the reaction with the isobasic acyclic secondary amines, while the k(-)(1) value is much larger for the latter reaction than for the former reaction. The k(3) value for the reaction with secondary amine is independent of the amine basicity. The small k(2)/k(-)(1) ratio is proposed to be responsible for the deprotonation process observed in aminolyses of carbonyl or thiocarbonyl derivatives.     

Aminolyses of aryl diphenylphosphinates and diphenylphosphinothioates: effect of modification of electrophilic center from P=O to P=S

A kinetic study is reported for aminolysis of aryl diphenylphosphinothioates (2a-i). The phosphinothioates 2a-i are less reactive than aryl diphenylphosphinates (1a-i), the oxygen analogues of 2a-i, regardless of the basicity of the leaving aryloxides or the attacking amines. The Yukawa-Tsuno plot for the reactions of 2b-i with piperidine exhibits good linearity with a small r value (r=0.28), indicating that the leaving group departs at the rate-determining step with a small degree of bond fission. Reactions of 2,4-dinitrophenyl diphenylphosphinothioate (2a) with alicyclic secondary amines result in a good linear Br?nsted-type plot with betanuc=0.52, implying that the reactions proceed through a concerted mechanism. The betanuc value determined for the reactions of 2a is slightly larger than that reported for the corresponding reactions of 2,4-dinitrophenyl diphenylphosphinate (1a, i.e., betanuc=0.38), suggesting that reactions of 2a proceed through a tighter transition state (TS) than that of 1a. The reaction of 2a with piperidine exhibits a ca. 0.4 kcal/mol more favorable enthalpy of activation (DeltaH) than that of 1a. On the contrary, the entropy of activation at 25.0 degrees C (TDeltaS) is ca. 1.5 kcal/mol more unfavorable for the reaction of 2a than for that of 1a. This result supports the proposal that the reaction of 2a proceeds through a tighter TS than that of 1a and explains why 2a-i are less reactive than 1a-i.     

Aminolysis of Y-substituted phenyl diphenylphosphinates and benzoates: effect of modification of electrophilic center from C=O to P=O

The effect of modification of the electrophilic center from C=O to P=O on reactivity and reaction mechanism has been investigated for aminolysis of Y-substituted phenyl diphenylphosphinates (1a-j) and benzoates (2a-i). The phosphinates 1a-j are less reactive than the benzoates 2a-i. The reactions of 2,4-dinitrophenyl diphenylphosphinate (1a) with alicyclic secondary amines resulted in a linear Br?nsted-type plot with a beta(nuc) value of 0.38, while the corresponding reactions of 2,4-dinitrophenyl benzoate (2a) yielded a curved Br?nsted-type plot. Similarly, a linear Br?nsted-type plot with a beta(lg) value of -0.66 was obtained for the reactions of 1a-j with piperidine, while the corresponding reactions of 2a-i gave a curved Br?nsted-type plot. The linear Br?nsted-type plots for the reactions of 1a-j have been taken as evidence for a concerted mechanism, while the curved Br?nsted-type plots for the reactions of 2a-i have been suggested to indicate a change in the rate-determining step of a stepwise mechanism. The Hammett plot for the reactions of 1b-j exhibited a poor correlation with sigma(-) constants (R(2) = 0.962) but slightly better correlation with sigma(o) (R(2) = 0.986). However, the Yukawa-Tsuno plot for the same reactions resulted in an excellent correlation (R(2) = 0.9993) with an r value of 0.30. The aminolysis of 1a-j has been suggested to proceed through a concerted mechanism with an early transition state on the basis of the small beta(nuc) and small r values.