V. Štěrba

Influence of substitution on kinetics and mechanism of ring transformation of substituted S-[1-phenylpyrrolidin-2-on-3-yl]isothiuronium salts.

Twelve new substituted S-(1-phenylpyrrolidin-2-on-3-yl)isothiuronium bromides and twelve corresponding 2-imino-5-(2-phenylaminoethyl)thiazolidin-4-ones have been prepared and characterised. Kinetics and mechanism of transformation reaction of S-[1-(4-methoxyphenyl)pyrrolidin-2-on-3-yl]isothiuronium bromide and its N,N-dimethyl derivative 5a into corresponding substituted thiazolidin-4-ones 2a and 6a have been studied in aqueous solutions of amine buffers (pH 8.1-11.5) and sodium hydroxide solutions (0.005-0.5 mol l(-1)) at 25 degrees C and at I= 1 mol l(-1) under pseudo-first-order reaction conditions. The kinetics observed show that the transformation reaction is subject to general acid-base, and hydroxide ion catalyses. Acid catalysis does not operate in the transformation of 1a; the rate-limiting step of the base-catalysed transformation is the decomposition of bicyclic tetrahedral intermediate In(+/-) and the Brønsted dependence is non-linear (pK(a) approximately 9.8). In the case of derivative 5a both base and acid catalyses make themselves felt. In the base catalysis, the rate-limiting step consists of the decomposition of bicyclic intermediate In, and the Brønsted dependence is linear (beta = 0.9; pK(a) > 11.5). The acid-catalysed transformation of 5a also proceeds via the intermediate In, and the reaction is controlled by diffusion (alpha approximately equal to 0). With compound 5a in triethylamine and butylamine buffers, the general base catalysis changes into specific base catalysis. The effect of substitution in aromatic moiety of compounds 1a-h and 3a-h on the course of the transformation reaction has been studied in solutions of sodium hydroxide (0.005-0.5 mol l(-1)) at 25 degrees C by the stopped-flow method. The electron-acceptor substituents 4-NO(2) and 4-CN do not obey the Hammett correlation, which is due to a suppression of cross-conjugation in the ring-closure step of the transformation reaction.

13 C and 15N nuclear magnetic resonance spectra of Meisenheimer complexes of 1,3,5-trinitrobenzene

13 C and 15N n.m.r. spectra (at the natural abundance level of 15N) have been measured and interpreted for the Meisenheimer complexes (I)–(XII) of 1,3,5-trinitrobenzene (TNB) with anions of cyclopentanone, cyclohexanone, cycloheptanone, acetone, dimethyl malonate, methyl cyanoacetate, methyl acetoacetate, pentane-2,4-dione, 3-methylpentane-2,4-dione, phenol, and methanol. 13C N.m.r. spectra of the corresponding carbon-acids have been measured in [2H6]DMSO. The complexes derived from ketones and esters are present in the keto-form, that of (IX) exists in the enol form. Nitrogen atoms of nitro-groups at the 2- and 6-positions of complexes derived from carbon-acids of the type CH2XY are anisochronous as also are C(2) and C(6), C(3) and C(5), and H(3) and H(5).

Kinetics and mechanism of base-catalysed degradations of substituted aryl-N-hydroxycarbamates, their N-methyl and N-phenyl analogues.

The kinetics and mechanism of the degradation reactions of substituted phenyl N-hydroxycarbamates and their N-methyl and N-phenyl analogues have been studied at pseudo-first-order reaction conditions in aqueous buffers and sodium hydroxide solutions at 20 [degree]C and 60 [degree]C and at I= 1 mol[middle dot]l(-1). The dependence of log k(obs) on pH for phenyl N-hydroxycarbamates at pH < 9 and pH > 13 is linear with the unit slope; at pH 10-12 log k(obs) is pH independent. The Bronsted coefficient [small beta](lg) is about -1 (pH 7-13) and -1.53 (pH > 13) indicating that the degradation reaction of phenyl N-hydroxycarbamates follows an E1cB mechanism giving the corresponding phenol/phenolate and HO-N[double bond, length as m-dash]C[double bond, length as m-dash]O. The latter species undergoes further decomposition to give carbonate, nitrogen and ammonia as final products. In contrast to the phenyl N-hydroxycarbamates the N-methyl derivatives at pH 7-9 undergo degradation to the corresponding phenol/phenolate, carbonate and methylamine via a concerted mechanism ([small beta](lg) is about -0.75). The only exception is 4-nitrophenyl N-hydroxy-N-methylcarbamate in which the predominant break down pathway proceeds via the Smiles rearrangement to give sodium N-methyl-(4-nitrophenoxy)carbamate. At pH > 9 the reaction of N-hydroxy-N-methylcarbamates is kinetically complex: the dependence of absorbance on time is not exponential and it proceeds as a consecutive two-step reaction. N-Hydroxy-N-phenylcarbamate under the same conditions undergoes degradation to phenol, carbonate, aniline and azoxybenzene.

Study of formation of the spiro-Meisenheimer adduct of NN′-dimethyl-N-(2,4,6-trinitrophenyl)glycinamide and its rearrangement to 2-methylamino-N-methyl-N-(2,4,6-trinitrophenyl)acetamide

NN′-Dimethyl-N-(2,4,6-trinitrophenyl)glycinamide (5) is cyclized in methanol under specific base catalysis and gives the spiro-adduct (6). The spiro-adduct is opened by the action of methanolic hydrogen chloride to give the Z- and E-isomers of 2-methylamino-N-methyl-N-(2,4,6-trinitrophenyl)-acetamide hydrochloride (7). In aniline-anilinium chloride buffers the E-isomer of (7) is cyclized to the spiro-adduct (6) with a half-life <1 s, the rate-limiting step of the cyclization of Z-(7) isomer being its isomerization to E-(7). In methanolic acetate buffers the rate-limiting step is gradually shifted to the isomerization of the neutral (Z)-2-methylamino-N-methyl-N-(2,4,6-trinitrophenyl)acetamide Z-(8) to the E-(8) isomer.

The base-catalysed cyclisation of phenyl N-(2-hydroxybenzyl)-N-methylcarbamates is concerted

The kinetics of the cyclisation in aqueous solution of phenyl-(2-hydroxybenzyl)-N-methylcarbamates to 3-methyl- 3,4-dihydrobenzo[e][1,3]oxazin-2-ones and phenolate ions fit the rate law: kobs = kc/(1 + [H3O+]/Ka) The values of kc and pKa fit Brønsted equations against the pKas of the corresponding free phenols but the system does not conform to the reactivity-selectivity hypothesis. The values of the Brønsted parameters beta Y and beta X vary as a function of Y and X according to the equations: beta X = -0.179pKaHY + 0.87 beta Y = -0.179pKaHX + 2.30 The magnitude and sign of the Cordes-Thornton cross-interaction coefficient pXY (-0.179) rule out a stepwise mechanism involving a tetrahedral intermediate and is consistent with a concerted displacement mechanism. A similar concerted mechanism is proposed for the base-catalysed cyclisation of phenyl-N-(2-hydroxyphenyl)-N-methylcarbamate esters to benzoxazol-2-ones.

Kinetics and mechanism of the reaction of substituted O‐benzoylbenzamidoximes with sodium methoxide in methanol

The kinetics of reaction of substituted O-benzoylbenzamidoximes with sodium methoxide in methanol were studied at 25 °C. The only reaction products are substituted benzamidoximes and methyl benzoates. The slope of the dependence of rate constant on sodium methoxide concentration gradually increases, but in the presence of C18 crown ether the dependence becomes linear and the rate constant is lower than in the absence of the crown ether, which means that the reaction is catalysed by sodium cation. The rate constants of reactions with the ion pair and with methoxide ion were determined with the presumption that the rate-limited step of the catalysed reaction is the reaction of substituted O-benzoylamidoximes with the ion pair of sodium methoxide. The rate constants of the reaction with the ion pair are about 20 times higher than those of the non-catalysed process. The slopes of the dependence of log k of the non-catalysed and catalysed reactions on the pKa of substituted benzamidoximes are 1.05 and 0.94, respectively. These high values indicate the rate-limiting step involving the splitting off of substituted benzamidoxime from the tetrahedral intermediate. On the basis of the relatively high ρ constant of methanolysis at the benzoyl group substituted derivatives (2.17 and 1.98 for the non-catalysed and catalysed reactions, respectively), it can be presumed that the transition state structure will be close to the tetrahedral intermediate. Copyright

Formation of 4,6-dinitro-2-nitrosoaniline by intramolecular redox reaction of esters and amides of 2-(2,4,6-trinitroaniline) carboxylic acids

The reaction of methyl and ethyl esters of N-(2,4,6-trinitrophenyl) glycine and N-(2,4,6-Trinitrophenyl)α-alanine (3a–d) with methoxide ion in methanol produces 4,6-dinitro-2-nitrosoaniline (4). N-(2,4,6-Trinitrophenyl) glycine methylamide (1d) gives, under the same conditions, the nitroso compound (4) in addition to 12% of the spiro adduct (2d). A reaction mechanism is suggested, and the influence of the side chain on the reaction kinetics has been studied.

Coupling Kinetics of Benzenediazonium Ions with 2,6-Dioxo-3-( p -substituted phenylhydrazono)-1,2,3,6-tetrahydropyridine-4-carboxylic Acid

The kinetics have been measured of the reactions of 4-nitro-, 4-chloro-, and 4-methoxybenzenediazonium ions with substituted phenylazo derivatives of citrazinic acid in buffer solutions, and the p K a values of the corresponding monoazo and bisazo compounds have been estimated. The reactions of 4-nitrobenzenediazonium ion with 4-chloro- and 4-methoxyphenylazo derivatives and of 4-chlorobenzenediazonium ion with 4-methoxyphenylazo derivative were accompanied by a partial replacement of the substituted phenylazo group by the 4-nitro- and 4-chlorophenylazo groups, respectively. The reactions of 4-chloro- and 4-methoxybenzenediazonium ions are subject to general base catalysis, the rate-limiting step consisting in the splitting off of the proton from the tetrahedral intermediate; with 4-nitrobenzenediazonium ion the reaction rate is limited by the formation of the tetrahedral intermediate.