Preparation of Substituted Methyl o-Nitrophenyl Sulfides

The nucleophilic substitution of substituted o-nitrochlorobenzenes with substituted methanethiolates, catalysed with triethylamine or pyridine, has been used to prepare a series of appropriately substituted methyl-o-nitrophenylsulfides. The prepared compounds were identified by their 1H- and 13C-NMR spectra. The base catalysed ring closure of methyl 2-(methoxycarbonylmethylsulfanyl)-3,5-dinitrobenzenecarboxylate only results in an attack of carbanion on the ester group, not on a nitro group as with the other compounds prepared. The cyclisation product is methyl 3-hydroxy-5,7-dinitro-benzo[b]thiophene-2-carboxylate (11).


Introduction
Wagner et al. [1] have described the base-catalysed reaction of substituted 2-nitrochlorobenzenes with esters of sulfanylethanoic acid giving the correspondingly substituted 2-alkoxycarbonylbenzo[d]thiazol-3-oxides. The expected intermediate of this reaction, i.e. a substituted alkyl (2nitrophenylsulfanyl)ethanoate, was isolated and identified in a single case -the case of ethyl (4-cyano-2,6-dinitrophenylsulfanyl)ethanoate [1]. It is interesting that 2,6-dinitrochlorobenzene reacts with methyl sulfanylethanoate in methanol (with triethylamine catalysis) to give 2-methoxycarbonyl-7nitrobenzo [ Janík [2] developed a method for preparation of substituted methyl (2,6-dinitrophenylsulfanyl)ethanoates and studied the kinetics of the cyclisation reactions of methyl (2,4,6trinitrophenylsulfanyl)ethanoate and methyl (4-methoxycarbonyl-2,6-dinitrophenylsulfanyl)ethanoate to the corresponding benzo[d]thiazol-3-oxides [2,3]. On the basis of the kinetic studies he suggested the mechanism of this cyclisation reaction. It proceeds as a multi-step reaction: the first step consists in base-catalysed splitting off of the proton from the methylene group of the substrate to give the corresponding carbanion, which in the second step attacks the nitrogen atom of nitro group of the same molecule. There then follows a splitting off of hydroxyl ion and final formation of the product (Scheme 2). The reaction rate is determined by the first two steps, which are specifically affected by oxygen and nitrogen bases (substituted phenoxides and tertiary amines) [2,3]. The kinetic measurements carried out so far have not allowed an unambiguous decision as to which of the first two reaction steps is rate-limiting.

Scheme 2
The aim of the present work was to synthesise a larger series of the "open" compounds of type I for subsequent more detailed kinetic studies.

Results and discussion
Compounds 1 -8 were prepared by nucleophilic substitution reaction of the chlorine substitutent (activated for S N Ar by the presence of several electron-withdrawing groups: NO 2 , COOCH 3 ) by substituted methanethiolate ions. The methanethiolate ions were generated from the corresponding substituted methanethiols by reaction with triethylamine or pyridine. It turned out that the whole amount of the base cannot be added at once because a high concentration of base causes an immediate cyclisation of the primary substituted alkyl 2-nitrophenyl sulfides to benzo[d]thiazol-3-oxides (Scheme 2), as described by Wagner et al. [1]. If the base concentration is kept at a low level throughout the reaction, then the alkyl 2-nitrophenyl sulfides can be obtained in relatively good yields. The above-described method fails in the case of compound 9. In the reaction of methyl sulfanylethanoate with methyl 2-chloro-3,5-dinitrobenzenecarboxylate in the presence of a base, the primary methyl 2-(methoxycarbonylmethylsulfanyl)-3,5-dinitrobenzenecarboxylate (9) produces a carbanion, which can attack either the nitro group (to give the substituted benzo[b]thiazol-3-oxide) or the ester group (to give methyl 3-hydroxy-5,7-dinitrobenzo[b]thiophene-2-carboxylate (11); Scheme 3). We have found that the attack on the ester group is so fast that the carbanion attacks this group exclusively. This rate of attack makes it impossible to prepare methyl 2-(methoxycarbonylmethylsulfanyl)-3,5-dinitrobenzenecarboxylate (9): even if the base was added very slowly, and we obtained mixtures of 9 and 11 with the latter substance predominating considerably. Therefore, compound 9 was prepared in a "roundabout" way: the nucleophilic substitution with anion of methyl sulfanylethanoate was carried out on the 2-chloro-3,5-dinitrobenzenecarboxylate anion that was formed by adding 1 equivalent of triethylamine to 2-chloro-3,5-dinitrobenzenecarboxylic acid. Of course, the nucleophilic substitution itself by action of sulfanylethanoate anion (formed by addition of the second equivalent of base) is somewhat slowed down by the presence of carboxylate group in the substrate, but it can be accomplished without subsequent cyclisation. The esterification of the carboxylic acid group in compound 5 was carried out only in the last step by reaction with diazomethane under mild conditions. Preparation of 2-(phenylmethylsulfanyl)-3,5-dinitrobenzenecarboxylic acid 6 and methyl 2-(phenylmethylsulfanyl)-3,5-dinitrobenzenecarboxylate 10 from 2-chloro-3,5-dinitrobenzoic acid are described in the literature [4] without any experimental specifications and physical constants.

Scheme
Janík [2] prepared methyl (2,4,6-trinitrophenylsulfanyl)ethanoate by reaction of 2,4,6trinitrochlorobenzene with methyl sulfanylethanoate catalysed with triethylamine in heterogeneous phase (benzene). We have tested the possibility of preparing this substance in a homogeneous phase (methanol). It was found that in methanolic solution the substrate undergoes a side reaction with methoxide giving 2,4,6-trinitroanisole. The methoxide is formed at low concentration by solvolytic reaction of the solvent with triethylamine. In order to eliminate this reaction, we adopted 1,2dimethoxyethane as the solvent: it is sufficiently polar for the reactants to dissolve and does not contain acidic protons. The reactions in this solvent take place very cleanly.

Acknowledgements
The research was financially supported by the Grant Agency of the Czech Republic, Grant No. 203/01/0227.

General methods
The synthesised substances were identified by means of their 1 H-and 13 C-NMR spectra, elemental analyses and, if applicable, by comparison of their melting points with literature data. The 1 H-and 13 C-NMR spectra were measured at 25 °C with an AMX 360 Bruker spectrometer at the frequencies of 360.14 and 90.57 MHz, respectively. For the measurements the substances were dissolved in CDCl 3 or (CD 3 ) 2 SO (5% solutions). The δ 1 H chemical shifts are referenced to the signal of HMDSO in CDCl 3 solutions (δ 1 H: 0.05) and to the solvent signal in (CD 3 ) 2 SO solutions (δ 1 H: 2.55). The δ 13 C chemical shifts are referenced to the signals of the two solvents (δ 13 C: 77.0 and 39.6, respectively). The analysis of the proton spectra was carried out according to the rules for the first-order splitting with the help of integral intensities. The 13 C-NMR spectra were measured with full decoupling from the protons, and the signals were assigned with the help of SCS. The quaternary carbon atoms and CH groups were differentiated by means of the APT pulse sequence. The elemental analyses were carried out on an automatic analyser EA 1108 (Fisons). The yields, melting points and elemental analyses of the substances synthesised are presented in Table I. The 1 H-and 13 C-NMR spectra with assigned signals are given in Tables IIa and IIb. 2,4,6-Trinitrochlorbenzene was prepared by treatment of 2,4,6-trinitrophenol with POCl 3 in dry pyridine [5]. The product was recrystallized from methanol, yield 54 %, m.p. 82.5-83°C (lit. [5] m.p. 80-81°C).
2-Chloro-3,5-dinitrobenzenecarboxylic acid was prepared by dinitration and subsequent hydrolysis (one pot) of 2-chlorobenzonitrile [12] [15] by treatment of 4-nitrophenylchloromethane with thioethanoic S-acid and consequent hydrolysis of the S-acetyl derivative with diluted sulfuric acid. The structure of the crude product was verified by 1 H-NMR spectroscopy and it was pure enough for next synthesis.

2-(Methoxycarbonylmethylsulfanyl)-3,5-dinitrobenzenecarboxylic acid (5).
Methyl sulfanylethanoate (4.46 g, 0.042 mol) was added dropwise to a stirred solution of 2-chloro-3,5-dinitrobenzoic acid (9.86 g, 0.04 mol) in 1,2-dimethoxyethane (25 mL) in a 100 ml flask at room temperature under an inert atmosphere of Ar. Triethylamine (4.05 g, 0.04 mol) was added at once to neutralize the carboxy group. More triethylamine (4.05 g, 0.04 mol) was then added dropwise with stirring over a period of ca. 30 minutes. The mixture was stirred for an additional 10 minutes and then poured into dilute aqueous hydrochloric acid (1:1, 30 mL). The product was extracted with chloroform (3x50 mL), the organic phase was dried over Na 2 SO 4 and the solvent was removed under reduced pressure. The residue was recrystallized from chloroform yielding 7.8 g (62%) of the product, m.p. 109-111°C. (6) was prepared in similar fashion (two equivalents of triethylamine were added at once and the reaction time was lengthened to 4 h at room temperature). (7). Pyridine (0.79 g, 0.01 mol) was added in one portion at room temperature to a stirred solution of 2-chloro-3,5-dinitrobenzenecarboxylic acid (2.47 g, 0.01 mol) and 4-nitrophenylmethanethiol (1.7 g, 0.01 mol) in methanol (40 mL) in a 100 ml flask under an inert atmosphere of Ar. Additional pyridine (0.79 g, 0.01 mol) was then added dropwise with stirring. The mixture was stirred for 1 h at a temperature of 40-50°C and then poured into diluted aqueous hydrochloric acid (1:1, 60 mL). The organic phase was extracted with ether (3x50 mL), dried over Na 2 SO 4 and evaporated to dryness. The yield after recrystallization from chloroform was 2.3 g (60%); m.p. 179.5 -180°C.