Cucurbit[7]uril as a Supramolecular Catalyst in Base-Catalyzed Reactions. Experimental and Theoretical Studies on Carbonate and Thiocarbonate Hydrolysis Reactions.

Cucurbit[7]uril (CB7) catalyzes the hydrolysis reaction of bis(4-nitrophenyl)carbonate (1) but inhibits that of bis(4-nitrophenyl)thiocarbonate (2). Two relevant CB7 effects are proposed, a base-catalyst mediated by the CB7 portal and an inhibitory role attributed to the lower interaction of the thiocarbonyl group with the solvent in the host cavity, respectively.

Correction: Reaction mechanisms in ionic liquids: the kinetics and mechanism of the reaction of O,O-diethyl (2,4-dinitrophenyl) phosphate triester with secondary alicyclic amines.

Correction for 'Reaction mechanisms in ionic liquids: the kinetics and mechanism of the reaction of O,O-diethyl (2,4-dinitrophenyl) phosphate triester with secondary alicyclic amines' by Paulina Pavez et al., Org. Biomol. Chem., 2016, DOI: 10.1039/c5ob02128f.

Reaction mechanisms in ionic liquids: the kinetics and mechanism of the reaction of O,O-diethyl (2,4-dinitrophenyl) phosphate triester with secondary alicyclic amines.

The reactions of O,O-diethyl 2,4-dinitrophenyl phosphate triester (1) with secondary alicyclic (SA) amines in the ionic liquids [Bmim]BF4 and [Bmim]DCA were subjected to a kinetic study. Eyring plots were obtained for the title reactions in the above ionic liquids (ILs) and also in aqueous ethanol (44 wt% ethanol). Two different reaction pathways were observed in [Bmim]BF4: nucleophilic attack at the phosphoryl center, SN2(P), and at the C-1 aromatic carbon, SN(Ar), where the product distribution remained constant and independent of the amine nature. In contrast, in [Bmim]DCA only the SN2(P) pathway was found. From the kinetic analysis of the SN2(P) pathway in both ILs, curved upwards plots of kobsdvs. 1-formylpiperazine concentration were obtained. Based on the kinetic behavior, a change in the mechanism of the SN2(P) pathway is proposed for the aminolysis of 1, from a concerted process in aqueous ethanol to a stepwise mechanism, through a zwitterionic pentacoordinate intermediate, when [Bmim]BF4 and [Bmim]DCA are used as the solvents of the reaction.

Mechanisms of degradation of paraoxon in different ionic liquids.

Herein, the reactivity and selectivity of the reaction of O,O-diethyl 4-nitrophenyl phosphate triester (Paraxon, 1) with piperidine in ionic liquids (ILs), three conventional organic solvents (COS), and water is studied by (31)P NMR, UV-vis, and GC/MS. Three phosphorylated products are identified as follows: O,O-diethyl piperidinophosphate diester (2), O,O-diethyl phosphate (3), and O-ethyl 4-nitrophenyl phosphate diester (4). Compound 4 also reacts with piperidine to yield O-ethyl piperidinophosphate monoester (5). The results show that both the rate and products distribution of this reaction depend on peculiar features of ILs as reaction media and the polarity of COS.

Changes in Protonation Sites of 3-Styryl Derivatives of 7-(dialkylamino)-aza-coumarin Dyes Induced by Cucurbit[7]uril.

The incorporation of a guest, with different basic sites, into an organized system (host), such as macrocycles, could stabilize, detect, or promote the formation of a certain protomer. In this context, this work aimed to study the influence of cucurbit[7]uril (CB7) on dyes such as 7-(dimethylamino)-aza-coumarins, which have more than one basic site along their molecular structure. For this, three 3-styryl derivatives of 7-(dialkylamino)-aza-coumarin dyes (SAC1-3) were synthesized and characterized by NMR, ESI-HRMS and IR. The spectral behaviour of the SACs in the absence and presence of CB7 was studied. The results showed large shifts in the UV-vis spectrum in acid medium: a hypsochromic shift of ≈5400 cm-1 (SAC1-2) and ≈3500 cm-1 (SAC3) in the absence of CB7 and a bathochromic shift of ≈4500 cm-1 (SAC1-3) in the presence of CB7. The new absorptions at long and short wavelengths were assigned to the corresponding protomers by computational calculations at the density functional theory (DFT) level. Additionally, the binding mode was corroborated by molecular dynamics simulations. Findings revealed that in the presence of CB7 the heterocyclic nitrogen was preferably protonated instead of the dialkylamino group. Namely, CB7 induces a change in the protonation preference at the basic sites of the SACs, as consequence of the molecular recognition by the macrocycle.

Supramolecular Control of Reactivity toward Hydrolysis of 7-Diethylaminocoumarin Schiff Bases by Cucurbit[7]uril Encapsulation.

A series of aromatic Schiff bases, featuring 7-diethylamino-coumarin and with five different substituents at an adjacent phenyl ring, were synthesized and characterized. With the aim of assessing the stability of these dyes in acidic medium, their hydrolysis reactions were kinetically studied in the absence and presence of the macrocycle cucurbit[7]uril (CB[7]). Our results are consistent with a model containing three different forms of substrates (un-, mono-, and diprotonated) and three parallel reaction pathways. The pK a values and the rate constants were estimated and discussed in terms of the presence of a hydroxyl group at the ortho position and electron-releasing groups on the phenyl ring of the dyes. The kinetic study in the presence of CB[7] led to two different behaviors. Promotion of the reaction by CB[7] was observed for the hydrolysis of the Schiff bases containing only one coordination site toward the macrocycle. Conversely, an inhibitor effect was observed for the hydrolysis of a Schiff base with two coordination sites toward CB[7]. The latter effect could be explained with a model as a function of a prototropic tautomeric equilibrium and the formation of a 2:1 host/guest complex, which prevents the attack of water. Therefore, the kinetic results demonstrated a supramolecular control of the macrocycle toward the reactivity and stability of 7-diethylaminocoumarin Schiff bases in acidic medium.

Concerted pyridinolysis of aryl 2,4,6-trinitrophenyl carbonates.

The Brønsted plots for the title reactions are linear with slopes of 0.53-0.56. The magnitude of the slopes and the fact that there are no breaks at the predicted pK(a) for stepwise mechanisms indicate that these reactions are concerted. This finding is in great contrast to the stepwise mechanisms found for the pyridinolysis of other carbonates. The concerted mechanism is attributed to the fact that the title carbonates possess two O-aryl groups, one of them being an exceptionally good nucleofuge.

Experimental and theoretical studies on the nucleofugality patterns in the aminolysis and phenolysis of S-aryl O-aryl thiocarbonates.

The reactions of S-phenyl, S-(4-chlorophenyl), and S-(2,3,4,5,6-pentafluorophenyl) 4-nitrophenyl thiocarbonates (9, 11, and 16, respectively) with a series of secondary alicyclic (SA) amines and those of S-(4-methylphenyl) 4-nitrophenyl thiocarbonate (8) and compounds 9 and 11 with a series of phenols 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. Under nucleophile excess, pseudo-first-order rate coefficients (k(obsd)) were found. For all these reactions, plots of k(obsd) vs. free amine or phenoxide anion concentration at constant pH are linear, the slope (k(N)) being independent of pH. The Brønsted-type plots (log k(N) vs. pK(a) of the conjugate acids of the nucleophiles) for the aminolysis of 9, 11, and 16 are linear with slopes beta = 0.85, 0.90, and 0.67, respectively. The two former slopes are consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate, which breaking to products is rate determining. The latter beta value is consistent with a concerted mechanism. The Brønsted-type plots for the phenolysis of thiocarbonates 8, 9, and 11 are linear with slopes beta = 0.62, 0.70, and 0.69, respectively. These beta values and the absence of curvature at pK(a) = 7.5 confirm a concerted mechanism. In all these reactions, except those of 16, the main nucleofuge is 4-nitrophenoxide, being the thio benzenethiolate the minor nucleofuge. For the reactions of thiocarbonate 16 the main nucleofuge is pentafluorobenzenethiolate whereas little 4-nitrophenoxide was found. The reactions of two SA amines with S-(3-chlorophenyl) 4-nitrophenyl thiocarbonate (10) were subjected to product analysis, showing 60% 4-nitrophenoxide and 40% 3-chlorobenzenethiolate. The study is completed with a theoretical analysis based on the group electrophilicity index, a reactivity descriptor that may be taken as a measure of the ability of a group or fragment to depart from a molecule with the bonding electron pair. The theoretical analysis is in accordance with the experimental results obtained and predicts relative nucleofugalities of O-aryl vs. S-aryl groups in a series of diaryl thiocarbonates not experimentally evaluated to date.

Concerted Mechanisms of the Reactions of Phenyl and 4-Nitrophenyl Chlorothionoformates with Substituted Phenoxide Ions.

The title reactions are subjected to a kinetic study in 3% (v/v) dioxane in water, 25.0 degrees C, ionic strength 0.2 M (KCl). By following the reactions spectrophotometrically, pseudo-first-order rate coefficients (k(obsd)) are found under an 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 beta = 0.55 and 0.47 for the reactions of the phenyl and the 4-nitrophenyl derivatives, respectively. These Brönsted slopes are in agreement with the ones found in the concerted reactions of the same nucleophiles with reactive phenyl esters and acetic anhydride in water. In contrast to the concerted mechanism of the title reactions that of the same substrates with secondary alicyclic amines is stepwise, which means that substitution of an amino moiety in a tetrahedral intermediate with a phenoxy group by another phenoxy group destabilizes the intermediate to the point that it no longer exists.

Kinetics and Mechanism of the Reactions of Quinuclidines with Ethyl S-Aryl Thiolcarbonates.

The reactions of quinuclidines with ethyl S-(4-nitrophenyl) thiolcarbonate (NPTC), ethyl S-(2,4-dinitrophenyl) thiolcarbonate (DNPTC), and ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate (TNPTC) 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 (400 nm) the release of the corresponding substituted benzenethiolate anion. Under quinuclidine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [N] (N is the free substituted quinuclidine) are linear and pH independent, with slope k(N). The Brönsted-type plots (log k(N) vs pK(a) of quinuclidinium ions) are linear, with slope beta = 0.85 for NPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T(+/-)) is rate determining, and beta = 0.54 and 0.47 for DNPTC and TNPTC, respectively, consistent with a concerted mechanism. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) Substitution of the 4-nitrobenzenethio group in T(+/-) by 2,4-dinitrobenzenethio or 2,4,6-trinitrobenzenethio destabilizes the tetrahedral intermediate. (ii) Quinuclidines destabilize the tetrahedral intermediate relative to secondary alicyclic amines, anilines, and pyridines. The leaving abilities of isobasic amines from T(+/-) follow the sequence pyridines < anilines < secondary alicyclic amines < quinuclidines. (iii) Quinuclidines are more reactive toward the carbonyl group of phenyl 4-nitrophenyl carbonate than that of NPTC.

Kinetics and Mechanism of the Aminolysis of Phenyl and 4-Nitrophenyl Chlorothionoformates.

The reactions of a series of secondary alicyclic amines with the title substrates are subjected to a kinetic investigation in aqueous solution, 25 degrees C, ionic strength 0.2 M (maintained with KCl). Under amine excess pseudo-first-order rate coefficients (k(obsd)) are found. The plots of k(obsd) against concentration of free amine at constant pH are linear, with the slopes (k(N)) independent of pH. The Brönsted-type plots obtained (log k(N) vs amine pK(a)) for the aminolysis of both substrates are linear with the same slope, beta = 0.26. From this value, the kinetic law, and the analysis of products, it is deduced that these reactions proceed through a zwitterionic tetrahedral addition intermediate (T(+/-)) on the reaction path, and its formation is the rate-determining step. From a comparison of the present reactions with the concerted aminolysis of substituted phenyl chloroformates in acetonitrile it is inferred that the change of S(-) by O(-) in T(+/-) and that of water by acetonitrile as solvent destabilizes T(+/-) in such a way that the stepwise reaction becomes enforced concerted.

Kinetics and Mechanism of the Reactions of Anilines with Ethyl S-Aryl Thiocarbonates.

The reactions of anilines with ethyl S-(2,4-dinitrophenyl) thiocarbonate (DNPTC) and ethyl S-(2,4,6-trinitrophenyl) thiocarbonate (TNPTC) are subjected to a kinetic study in aqueous solution at 25.0 degrees C and ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under aniline excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [N] (N is the free substituted aniline) are linear and pH independent, with slope k(N). The Brönsted-type plots (log k(N) vs pK(a) of anilinium ions) are linear, with slopes beta = 0.9 for DNPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T(+/-)) is rate determining, and beta = 0.54 for TNPTC, consistent with a concerted mechanism. Consideration of the results for aminolysis from the present work and those from previous studies leads to the following conclusions. (i) The tetrahedral intermediate possessing a 2,4-dinitrobenzenethio group is more stable than that including the 2,4,6-trinitrobenzenethio group. (ii) The tetrahedral intermediate possessing the 2,4,6-trinitrobenzenethio group has no existence beyond the limit of a vibration period (concerted mechanism). (iii) Tetrahedral intermediates possessing anilino groups are less stable than those derived from pyridines but are more stable than the tetrahedral intermediates derived from secondary alicyclic amines. (iv) Anilines are more reactive toward the carbonyl group of methyl 2,4-dinitrophenyl carbonate than toward the carbonyl of DNPTC.

Kinetics and Mechanism of the Aminolysis of Phenyl and 4-Nitrophenyl Chloroformates in Aqueous Solution.

The reactions of secondary alicyclic amines with phenyl and 4-nitrophenyl chloroformates (PClF and NPClF, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl). The reactions are followed spectrophotometrically at 210-270 nm (PClF) and at 310-400 nm (NPClF). Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. From linear plots of k(obsd) vs free amine concentration, the second-order rate coefficients (k(N)) for aminolysis are obtained. For the aminolysis of both substrates, linear Brönsted-type plots (log k(N) vs amine pK(a)) of slopes 0.23 (PClF) and 0.26 (NPClF) are found. The values of the slopes are consistent with stepwise mechanisms where the formation of a zwitterionic tetrahedral intermediate (T(+/-)) is the rate-determining step (k(1) step). In contrast, the aminolysis (anilines) of the same substrates in acetonitrile are concerted, which is attributed to destabilization of T(+/-) in the latter solvent due to a faster expulsion of the amine from T(+/-) in acetonitrile compared to water. The values of k(1) are larger for the title reactions compared to the same aminolysis of the corresponding thionochloroformates, and this is attributed to the relatively hard character of these amines which prefer to bind to the harder carbonyl group (relative to thiocarbonyl). There is no change in mechanism by the change of S(-) by O(-) in T(+/-), which should destabilize this intermediate. By comparison with the stepwise pyridinolysis of methyl chloroformate, it is concluded that the changes of methoxy by phenoxy (or 4-nitrophenoxy) and a pyridine by an alicyclic amine in T(+/-) do not greatly affect the stability of these intermediates.

Kinetics and Mechanism of the Aminolysis of Phenyl and Methyl 4-Nitrophenyl Thionocarbonates.

The reactions of secondary alicyclic amines with the title substrates are subjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl), by following spectrophotometrically the release of 4-nitrophenoxide ion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. For the reactions of phenyl 4-nitrophenyl thionocarbonate (1), linear plots of k(obsd) vs [NH] (NH is the free amine) are obtained, except for the reaction with piperazinium ion, which shows nonlinear upward plots. The aminolysis of methyl 4-nitrophenyl thionocarbonate (2) exhibits nonlinear plots of k(obsd) vs [NH], except that with piperidine, which is linear. The Brönsted-type plot for 1 is linear with slope beta = 0.25, indicating that the formation (k(1) step) of a tetrahedral addition intermediate (T(+/-)) is rate determining. For the aminolysis of 2 (except piperidine), k(-)(1) approximately k(3)[NH] > k(2), where k(-)(1), k(3), and k(2) are the rate coefficients for amine expulsion, amine deprotonation, and leaving group expulsion from T(+/-), respectively. For the reaction of 2 with piperidine, k(-)(1) < k(3)[NH]; therefore, the k(1) step is rate limiting. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) The change of MeO by EtO in 2 does not affect the k(1), k(-)(1), or k(2) values. (ii) Substitution of MeO by PhO in 2 results in lower k(1) values due to steric hindrance. (iii) The change of 4-nitrophenoxy (NPO) by PhO in 2 lowers the k(1) values and enlarges those of k(-)(1). (iv) Secondary alicyclic amines are less reactive toward 2 than isobasic pyridines when the breakdown of T(+/-) is rate determining; this is mainly due to larger k(-)(1) values for the former amines. (v) The change of PhO by NPO in 1 changes the mechanism from stepwise to concerted. (vi) Substitution of NPO by PhO in 1 does not alter the k(1) values significantly. (vii) The change of NPO by Cl in 1 increases the k(1) values. (viii) Substitution of C=S by C=O in 1 shifts the rate-limiting step from k(1) to k(2) due to a larger k(-)(1)/k(2) ratio by this change.

Kinetics and Mechanism of the Pyridinolysis of 2,4-Dinitrophenyl and 2,4,6-Trinitrophenyl O-Ethyl Dithiocarbonates.

The title reactions are subjected to a kinetic study in water, 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order rate coefficients are found, which are linearly related to the free amine concentration. No dependence of the slopes of these plots (k(N)) on the pH values was observed. The Brönsted-type plots (log k(N) vs pK(a) of the pyridines) are biphasic with slopes beta(1) = 0.2 (high pK(a)) for both reaction series and beta(2) = 1.0 and 0.9 (low pK(a)) for the dinitro and the trinitro derivatives, respectively, and with the curvature center at pK(a) = pK(a)(0) = 6.9 and 5.6 for the dinitro and the trinitro compounds, respectively. These results can be explained by the formation of a zwitterionic tetrahedral intermediate (T(+/-)) in a stepwise reaction. Comparison of these Brönsted-type plots with those in the reactions of the same substrates with secondary alicyclic amines shows that the latter amines are better nucleofuges from T(+/-) than isobasic pyridines. Comparison of the Brönsted-type plots for the dinitro and trinitro derivatives obtained in this work with those for the pyridinolysis of S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) O-ethyl thiocarbonates indicates that substitution of S(-) by O(-) in T(+/-) increases the amine/ArS(-) nucleofugality ratio from T(+/-).

Kinetics and Mechanism of the Pyridinolysis of Alkyl Aryl Thionocarbonates.

The reactions of methyl 4-nitrophenyl, ethyl 4-nitrophenyl, and ethyl 2,4-dinitrophenyl thionocarbonates (MNPTOC, ENPTOC, and EDNPTOC, respectively) with a series of 3- and 4-substituted pyridines are subjected to a kinetic investigation in water, 25.0 degrees C, ionic strength 0.2 M (maintained with KCl). Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained, which are linearly proportional to the free-pyridine concentration. The second-order rate coefficients (k(N)) are obtained as the slopes of these plots. The Brönsted-type plots found for the two mononitro derivatives coincide in one straight line (same slope and intercept) of slope beta = 1.0. The EDNPTOC pyridinolysis shows a curved Brönsted-type plot with slopes beta(1) = 0.1 (high pK(a)), beta(2) = 1.0 (low pK(a)), and pK(a)(o) = 6.8 (pK(a) value at the center of curvature). These plots are consistent with the existence of a zwitterionic tetrahedral intermediate (T(+/-)) on the reaction pathway whereby expulsion of aryloxide anion from T(+/-) is rate determining (k(2) step) at low pK(a) for EDNPTOC (and in the whole pK(a) range for MNPTOC and ENPTOC), and there is a change to rate-limiting formation of T(+/-) (k(1) step) at high pK(a) for EDNPTOC. Comparison of these Brönsted plots among them and with similar ones permits the following conclusions: (i) There is no variation of k(N) by substitution of methoxy by ethoxy as the nonleaving group of the substrate. (ii) The pK(a)(o) value is smaller for the less basic aryloxide nucleofuge due to a larger k(2) value. (iii) The change of C=S by C=O as the electrophilic center of the substrate results in larger values for both k(-)(1) (amine expulsion rate) and k(2), and also a larger k(-)(1)/k(2) ratio for the carbonyl derivative. There is also an increase of k(1) by the same change. The K(1)k(2) (= k(1)k(2)/k(-)(1)) values are larger for the pyridinolysis of methyl 2,4-dinitrophenyl and methyl 4-nitrophenyl carbonates compared to the corresponding thionocarbonates (EDNPTOC and MNPTOC, respectively). (iv) Pyridines are more reactive than isobasic secondary alicyclic amines toward ENPTOC when either the k(1) step or the k(2) step is rate limiting. This is explained by the softer nature of pyridines than alicyclic amines (k(1) step) and the greater nucleofugality (k(-)(1)) of the latter amines than isobasic pyridines, leading to a larger k(2)/k(-)(1) ratio for pyridines (k(2) is little affected by the amine nature), and therefore a larger K(1)k(2) value when the k(2) step is rate determining.

Toward a pK(a) scale of N-base amines in ionic liquids.

An electrochemical technique was used to investigate pKa values of some substituted secondary alicyclic (SA) amines, pyridines (py), anilines (AN), and triethylamine (Et3N) in different ionic liquids. The method involves cyclic voltammetry at a platinized Pt electrode. The experimental data were correlated with pKa values reported previously in aqueous solution, and Hammett parameters were correlated with pKa values in ionic liquids to determine ρ values in these media.

Hydrogen bond contribution to preferential solvation in S(N)Ar reactions.

Preferential solvation in aromatic nucleophilic substitution reactions is discussed using a kinetic study complemented with quantum chemical calculations. The model system is the reaction of a series of secondary alicyclic amines toward phenyl 2,4,6-trinitrophenyl ether in aqueous ethanol mixtures of different compositions. From solvent effect studies, it is found that only piperidine is sensitive to solvation effects, a result that may be traced to the polarity of the solvent composition in the ethanol/water mixture, which points to a specific electrophilic solvation in the aqueous phase.

Kinetics and mechanism of the anilinolysis of aryl 4-nitrophenyl carbonates in aqueous ethanol.

[Chemical reaction: See text] The reactions of anilines with 4-nitrophenyl, 4-methylphenyl, and 4-chlorophenyl 4-nitrophenyl carbonates (BNPC, MPNPC and ClPNPC, respectively) are studied kinetically in 44 wt % ethanol-water, at 25.0 degrees C, with an ionic strength of 0.2 M (KCl). Plots of k(obsd) vs [amine] are linear, with the slopes (kN) independent of pH. The Brønsted-type plots (log k(N) vs pKa of conjugate acids of anilines) are linear, with slopes beta = 0.65, 0.85, and 0.78 for the reactions of anilines with BNPC, MPNPC, and ClPNPC, respectively. The values of the slopes for the two latter reactions are in accordance with those obtained in stepwise mechanism where breakdown to product of a zwitterionic tetrahedral intermediate is the rate-determining step. On the other hand, the beta value for the reactions of BNPC is at the upper limit of those found for concerted mechanisms. The kinetic results for the reactions of anilines with BNPC correlates well with those for the concerted reactions of the same amines with 4-methylphenyl and 4-chlorophenyl 2,4-dinitrophenyl carbonates: A plot of the calculated log k(N) values (through a multiple parametric equation) vs the experimental log k(N) values is linear with unity slope and zero intercept, which confirms the concerted mechanism for the latter three reactions.

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 Brønsted-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 Brønsted 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.