Kinetics and mechanisms of the reactions of S-methyl chlorothioformate with pyridines and secondary alicyclic amines

The pyridinolysis of S-methyl chlorothioformate shows a biphasic Brønsted-type plot, in agreement with a stepwise mechanism and a change in the rate-limiting step, from formation of a zwitterionic tetrahedral intermediate (T^±) at high pK_a to its breakdown at low pK_a. The reaction of the same substrate with secondary alicyclic amines shows a linear Brønsted-type plot of slope 0.23, which is in accordance with a stepwise mechanism where formation of T^± is the rate-determining step for the whole pK_a range examined.     

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.     

Kinetics and mechanism of the reactions of S-2,4-dinitrophenyl 4-substituted thiobenzoates with secondary alicyclic amines

[reaction: see text] The title reactions, in 44 wt % ethanol-water at 25.0 degrees C, exhibit slightly curved Br?nsted-type plots (log kN versus pKa of amines) with slopes beta1 = 0.1-0.44 (at high pKa) and beta2 ca. 0.7 (at low pKa). The magnitude of some of these slopes, together with the fact that the curvature center (pKa(0) = 9.5-10.8) does not change with the electronic effects of the benzoyl substituent, suggests that these reactions are not stepwise, but concerted.     

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.     

Rationalization of the conflicting effects of hydrogen bond donor solvent on nucleophilic aromatic substitution reactions in non-polar aprotic solvent: reactions of phenyl 2,4,6-trinitrophenyl ether with primary and secondary amines in benzene-methanol mixtures

The kinetics of the reactions of phenyl 2,4,6-trinitrophenyl ether with piperidine and cyclohexylamine respectively were studied at different amine concentrations in benzene. The reaction of cyclohexylamine was not base-catalysed while that of piperidine was catalysed by one molecule of the nucleophilic amine. Addition of small amounts of hydrogen-bond donor solvent, methanol to the benzene medium of the reactions produced different effects—rate diminution followed by rate increase in one and continuous rate diminution in the other. These effects are compared with that of aniline (previously studied) in which a continuous rate increase was observed. The results are rationalized in terms of the effect of amine-solvent interaction on the nucleophilicity of the amines in addition to some other factors operating through cyclic transition states leading to products. It is evident from the rationalization that the idea of ‘dimer nucleophile’ in nucleophilic aromatic substitution reactions is erroneous.     

Acceleration of amine/epoxy reactions with N‐methyl secondary amines

Kinetic studies established that the monomethylation of a primary amine leads to significantly higher reaction rates with glycidyl ethers. The relative rates for approximately 25 amines were determined in an alcohol solvent under pseudo‐first‐order conditions (excess epoxy). The rates were referenced to aniline. For the aliphatic amines, reactivity consistently increased upon going from a primary amine to the corresponding N‐methyl secondary amine. This acceleration effect was not seen for aniline. The enhanced reactivity was also seen in curing systems, both with pure methylated amine curing agents and with complex mixtures obtained from the partial methylation of polyamines. Economically viable partially methylated amine curing agents were obtained by the reductive alkylation of commercial polyamines with formaldehyde and by the reaction of monomethylamine with 3‐(N‐methylamino)propionitrile in the presence of hydrogen and a hydrogenation catalyst. Although actual cure performance is based on a complex combination of several factors, the acceleration due to monomethylation could be a useful tool for enhancing amine/epoxy curing reactions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 921–930, 2000     

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 benzenethiolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates

The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Br?nsted-type plots for the nucleophilic rate constants are linear with slopes beta = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Br?nsted plot was found for the reactions of DNPC and DNPTC at pK(a) ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle.     

Catalytic effects of hydrogen-bond acceptor solvent on nucleophilic aromatic substitution reactions in non-polar aprotic solvent: reactions of phenyl 2,4,6-trinitrophenyl ether with amines in benzene-acetonitrile mixtures

The effect of addition of small amounts of hydrogen-bond acceptor solvent, acetonitrile, to the benzene medium of the reactions of phenyl 2,4,6-trinitrophenyl ether with aniline and cyclohexylamine, respectively have been investigated. The addition produced similar effects in the two reactions—continuous rate increase with increasing amounts of acetonitrile. The results are interpreted in terms of the effect of amine-solvent interaction on the nucleophilicity of the amines and are in accord with our expectations based on the effects observed for hydrogen-bond donor solvent, methanol on the same reactions. It is also established from the results that the role of hydrogen-bond acceptor co-solvent could be played by an added more basic non-nucleophilic amine.     

Kinetics and mechanisms of the reactions of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates with alicyclic amines

The reactions of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (1, 2, and 3, respectively) with a series of secondary alicyclic amines are studied kinetically in 44 wt % ethanol-water at 25.0 degrees C and an ionic strength of 0.2 M (KCl). Pseudo-first-order rate coefficients (k(obsd)) are obtained for all reactions (amine excess was used). The reactions of compound 1 with piperidine, piperazine, and 1-(2-hydroxyethyl)piperazine and of compounds 2 and 3 with these amines and morpholine exhibit linear k(obsd) versus amine concentration plots with slopes (k1) independent of pH. In contrast, the plots are nonlinear upward for the reactions of substrate 1 with morpholine, 1-formylpiperazine, and piperazinium ion and of substrates 2 and 3 with the two latter amines. For all these reactions, a reaction scheme is proposed with a zwitterionic tetrahedral intermediate (T+/-), which can be deprotonated by an amine to yield an anionic intermediate (T-). When the nonlinear plots are fit through an equation derived from the scheme, rate and equilibrium microcoefficients are obtained. The Br?nsted-type plots for k1 are linear with slopes of beta1 = 0.22, 0.20, and 0.24 for the aminolysis of 1, 2, and 3, respectively, indicating that the formation of T+/- (k1 step) is rate-determining. The k1 values for these reactions follow the sequence 3 > 2 > 1, which can be explained by the sequence of the electron-withdrawing effects from the substituents on the nonleaving group of the substrates.     

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.     

Reactions of 1-nitrocyclohexene with alicyclic amines

The interaction of 1-nitrocyclohexene with alicyclic amines can proceed via nucleophilic addition at the carbon-carbon double bond (morpholine, piperazine) to form aza-Michael products or as deprotonation (piperidine, azepane) to give ammonium salts, 2-cyclohexene-1-nitronates. The preferred reaction pathway is determined by the amine basicity.     

Competitive reactions of coordinated carbon monoxide and methyl isocyanide with amines

The relative reactivities of CO and CNR ligands with CH₃NH₂ were investigated in complexes which contained both ligands. Like (C₅H₅Fe(CO)₃⁺; the (C₅H₅)Fe(CO)₂(CNCH₃)⁺ complex reacts with CH₃NH₂ to give the carbamoyl complex (C₅)Fe(CO)(CNCH₃)(CONHCH₃); this is a readily reversible reaction. In contrast, (C₅H₅)Fe(CO)(CNCH₃)₂⁺ reacts with CH₃NH₂ to give the amidinium or carbene complex, (C₅H₅)Fe(CO)(CNCH₃)[C(NHCH₃)₂]⁺]. In a slow reaction, (C₅H₅)Fe(PPh₃)(CO)(CNCH₃)⁺ forms the amidinium complex, (C₅H₅)Fe(PPh₃)(CO)[C(NHCH₃)₂]⁺. Factors that affect the site of CH₃NH₂ reaction are discussed. The complexes have been characterized by IR and NMR spectroscopy; a variable temperature NMR study of (C₅H₅)Fe(CO)(CNCH₃)[C(NHCH₃)₂]⁺ indicates restricted rotation around the CN bonds of the amidinium ligand.     

Nonlinear structure–reactivity correlations. The hydrolysis of 2,4,6-trinitrophenyl acetate catalyzed by general bases

The hydrolysis of 2,4,6-trinitrophenyl acetate was studied in the presence of several carboxylic bases and tertiary amines. Carboxylic bases pyridine and imidazole react as nucleophilic catalysts, while 2,4-dimethyl and 2,4,6-trimethyl pyridine react as general-base catalysts. The nonlinear structure–reactivity correlations (log k_cat versus log k_OH and log k_cat versus pK of the leaving group) for the series of aryl acetates are discussed, and it is suggested that there is a change in transition-state structure along the series.     

Reaction of 2,4,6-trimethyl- and 2,4,6-triphenylpyrylium perchlorates with heterocyclic amines

2,4,6-Triphenylpyrylium perchlorate reacts with most heterocyclic amines in dimethylformamide to give quaternary pyridinium salts. 2,4,6-Trimethylpyrylium perchlorate forms similar products only up to a certain limit of the basicity of the amine, below which a proton is transferred from the-methyl group of the pyrylium salt to the pyridine nitrogen atom of the heteroring to give the perchlorate of the starting heterocycle. The residual 2,6-dimethyl--methylenepyran is polymerized to a hexamer.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1461–1467, November, 1974.     

Uranyl complexes with methyl derivatives of alicyclic dioximes

A reaction of uranyl dioxalate complexes with methyl derivatives of alicyclic ??-dioximes, 3-methyl-1,2-cyclohexanedione dioxime and 3-methyl-1,2-cyclopentanedione dioxime, was studied. The structure of (CN₃H₆)₄[(UO₂)₂(C₆H₈N₂O₂)(CO₃)(C₂O₄)₂] · (C₆H₁₀N₂O₂) · 2H₂O and NH₄(CN₃H₆)₃[(UO₂)₂(C₇H₁₀N₂O₂)(CO₃)(C₂O₄)₂] · 2H₂O based on binuclear complex anions with carbonate-dioximate fragment was studied by X-ray diffraction.     

Unimolecular reactions of ionized methyl acetate and its hydrogen-rearranged isomers

The proposed formation of [CH₃C(OH)OCH₂]⁺˙ (b) as the intermediate in the isomerization [CH₂?C(OH)OCH₃]⁺˙ (c)?b?[CH₃COOCH₃]⁺˙ (c has been confirmed by preparation of b from CH₃COOCH₂OCH₃. For the three isomers a–c the dominant metastable ion (MI) dissociation, CH₃O˙ loss, involves identical kinetic energy release values. The kinetic barriers for a?b and b?c must be nearly as high as that for CH₃O˙ loss from c, as shown by the insensitivity of the mass spectra from collisionally activated dissociation (CAD) of a–c to ionizing electron energy. The H/D scrambling of metastable [CH₂?C(OD)OCH₃]⁺˙ and c–D₃ ions confirm this, indicating that the barrier for a?b is slightly below that for b?c. Minor low-energy dissociations include losses of CH₄ and CH₃OH from a and losses of ˙CHO and CH₂O from b. Comparison of MI and CAD spectra of a–c with those from [CH₃(OH)CH₂O]⁺˙ (d) and [CH₃COCH₂OH]⁺˙ (e) give no evidence for skeletal rearrangement of a–c to d or e.