1.2.3.4- Thiatriazoles decomposition

The 1,2,3,4-thiatriazoles are unstable. They decompose on heating— in some cases even at room temperature—and in many cases they melt with detonation. Accordingly the Ng-group has not been stabilized much by ring closure. The compounds behave in this respect similarly to azides and this fact doubtlessly delayed the recognition of their true nature. On heating with a solvent the thermal decomposition of 5-aryl-1,2,3,4-thiatriazoles proceeds according to Eq. (4). By the photochemical decomposition small amounts of the isothiocyanate, RNCS, are formed in addition to the nitrile. ... 

An analogous decomposition of 5-amino-l,2,3,4-thiatriazole or 5-alkylamino-1,2,3,4-thiatriazoles gives cyanamide or an alkyl-cyanamide. ... 

Similar decomposition of alkylthio- and alkoxy-thiatriazoles should yield thiocyanates and cyanates, but they may rearrange to the isocompounds and polymerize (see Sections IV, D and V). 

Sodium azide does not react with carbonyl sulfide to form 5-hydroxy-1,2,3,4-thiatriazole, nor with carboxymethyl xanthates, RO-CS SCH2COOH, to form 5-alkoxy-l,2,3,4-thiatriazoles. The latter, however, could be prepared from xanthogenhydrazides (RO-CS NHNH2) and nitrous acid. They are very unstable and may decompose explosively at room temperature only the ethoxy compound (6) has been examined in detail. This is a solid which decomposes rapidly at room temperature and even at 0°C is transformed after some months into a mixture of sulfur and triethyl isocyanurate. In ethereal solution at 20° C the decomposition takes place according to Eq. (16)... 

Isothiocyanates (27) are observed as by-products when the decomposition of 5-alkyl- (23 X = alkyl) and 5-aryl-thiatriazoles (23 X = aryl) is carried out at elevated (> 100°C) temperature, while nitriles (26) only are formed at RT (Scheme 2) <67RTC670. 78JOC4816). 

Decomposition of propargyloxy-l,2,3,4-thiatriazole (154) resulted in almost quantitative formation of the allenyl isocyanate (156). If the same reaction was carried out in the presence of hydrogen sulfide compound (157) was obtained, clearly demonstrating propargyl cyanate (155) as... 

Phenyl isothiocyanate (benzonitrile sulfide) is formed photolytically from 5-phenyl-l,2,3,4-thiatriazole (50) and related heterocycles (51) when extrusion of a small inorganic fragment such as N2, CO, C02, COS, or CS2 is possible [78JCS(P1)1445]. Rapid decomposition to benzonitrile takes place at room temperature, but the sulfide may be trapped as a cycloaddition product by carrying out the photolysis in neat dimethyl acetylenedi-carboxylate. Among the compounds that have been studied are 5-phenyl-substituted l,3,4-oxathiazol-2-one (51a), l,3,4-dithiazol-2-thione (51b), l,3,4-oxathiazol-2-thione (51c), 3,l,4-oxathiazol-2-one (51d), and meso-ionic 3,l,2-oxathiazol-2-one (51e) (Scheme 6). 

In addition, there are [5 -> 3 + 2] fragmentations leading to acyclic products that result from the original fragments by rearrangement. Examples are 5-phenyl-l,2,3,4-thiatriazole (50) and related compounds with three ring heteroatoms and an exocyclic double bond (51),THF (52), y-butyrolactone (53), y-thiobutyrolactone (54), pyrroles (8,61), pyrrolidine (55), furans (56, 58), pyrazole (62), and isoxazoles (11, 63,64). Most of these molecules are thermally rather stable and their decomposition requires drastic conditions. 

This survey of the chemistry of the heteroaromatic thiatriazoles is a continuation of the 1964 review by K. A. Jensen and C. Pedersen.1 In the meantime more work on this type of compound has appeared, and some interesting developments have taken place that justify a presentation of the results from the past decade. In particular the facile decomposition of thiatriazoles and thiatriazolines has led to several types of hitherto unknown or little known compounds. In this review, references preceded by an asterisk are taken from the article by Jensen and Pedersen. Since only... 

No conclusions with regard to the thermal stability of a true thiatriazole 5-oxide can be drawn with safety from the above results. They may indicate only that decomposition to nitrogen, sulfur monoxide, and benzonitrile is much faster than ring closure. 

In Section III, A the catalytic action of A1C13 and BBr3 on the thermal decomposition of thiatriazoles was mentioned. This effect is evidently connected with complex formation between a thiatriazole and a Lewis acid since the catalytic activity is lost on addition of compounds that complex more effectively with the Lewis acid.19 It is remarkable that titanium tetrachloride, in contrast to this, does not catalyze decomposition, but instead forms a thermally stable, orange 1 1 complex with 5-phenylthiatriazole.19 The complex is sensitive to atmospheric moisture and is hydrolyzed in high yield to the starting thiatriazole on addition of water. 

Benzonitrile sulfide has previously been trapped by the same solvent as a product of the thermal decomposition of 5-phenyl-1,3,4-oxathiazol-2-one.28 Spectral studies of this elusive substance can be made when photolyzing either thiatriazole or the other precursors mentioned in an EPA glass at 85°K.20... 

The 5-(2-alkenyloxy)thiatriazoles on thermal degradation gave alkenyl isocyanates by a presumed allylic rearrangement of the expected 2-alkenyl cyanates.54 Although the data do not exclude decomposition via intramolecular cyclic attack at the 4-position in the ring followed by very rapid decomposition of the 4-substituted thiatriazolines, these... 

The thermal degradation of isotopically labeled 5-isobutoxy-thiatriazole15 and the kinetics of the decomposition of 5-alkoxy thiatriazoles have been studied16 (Section III, A). 

Textile clothing static charges, 394 Theory without detailed thought, 394 Thermal explosions, 394 Thermal stability of reaction mixtures and systems, 394 Thermite reactions, 395 Thermochemistry and exothermic decomposition, 396 Thiatriazoles, 400 Thionoesters, 401 Thiophenoxides, 401 Thorium furnace residues, 401 Tollens reagent, 401 Toxic hazards, 402 Trialkylaluminiums, 402 Trialkylantimony halides, 403 Trialkylbismuths, 403... 

The mechanism of the decomposition reaction of 5-methoxy- 1,2,3,4-thiatriazole to dinitrogen sulfide and methoxy-nitrile was studied by the DFT method at the CCSD(T)//MP2/6-31+G level of theory <2003JOC6049>. The calculations indicated that this is a concerted retro-[2+3]-dipolar cycloaddition process with an activation energy of 28.9 kcal mol 1 and a reaction energy of 1.9 kcal mol. This unimolecular decomposition is favored due to the entropy gain (25.8 eu) involved in the overall reaction (Scheme 1 and Table 2). 

Table 2 Calculated data for the activation and reaction energy in kcal mol 1 for the thermal decomposition of 5-methoxy-1,2,3,4-thiatriazole to dinitrogen sulfide and methoxycarbonitrile...

The most common thermal reaction of 1,2,3,4-thiatriazoles 9 is decomposition forming molecular nitrogen, sulfur, and an organic fragment <1984CHEC(6)579, 1996CHEC-II(4)691>. This decomposition may follow different pathways depending on factors such as substituents, solvent, and temperature. 

In some cases at higher decomposition temperatures (above 100 °C), isothiocyanates 11 may be formed after concerted Curtius-type decomposition/rearrangement of a (Z)-thioacyl azide 10-Z, while the corresponding ( )-isomer would give the nitrile 12 in a Grob-like fragmentation. The nitrenes are formed after nitrogen extrusion from the thioacyl azides, which are the open-chain isomers of the 1,2,3,4-thiatriazoles (Scheme 3). 

Reaction kinetics for the interaction of 5-alkyliminothiatriazoles 52 or 58 with heterocumulenes, nitriles, ketones, imines, or other dipolarophiles a=b show that the decomposition of the thiatriazole is bimolecular, and new heterocyclic five-membered rings 71 are formed (Scheme 15). The term masked 1,3-dipolar cycloaddition was used by L abbe and co-workers for this type of reaction <1978JOC4951>, the thioimidate function being the masked 1,3-dipole. The reaction is thought to involve a thiapentalenic intermediate 70 with hypervalent sulfur. The product 71 is itself a masked dipole and often further reactions take place. 

The 5-allyloxy-l,2,3,4-thiatriazoles and some analogues can decompose with the formation of intermediate cyanates that undergo [3,3]-sigmatropic rearrangement to isocyanates as described in Section 6.09.5.1. Banert reported that the [3,3]-sigmatropic rearrangement of the dimethylallyloxy derivative 120 could occur before the decomposition,... 

The thermal decomposition of 5-morpholino-l,2,3,4-thiatriazole leads to the extrusion of an active form of sulfur, that is intercepted by thioketones, leading to 1,2,4-trithiolanes in a regioselective manner <2007HCA594>. 

In comparison with the isomeric 1,2,3,4-thiatriazole, much less work has been done on the thermal decomposition reaction of the 1,2,3,5-isomers and its mechanism. In contrast to the aromatic 1,2,3,5-thiatriazole 12 that decomposed to benzonitrile, sulfur, and nitrogen during an attempted preparation, aromatic aminides 9, 1,2,3,5-thiatriazolium salts 10, and compounds of type 11 are stable and were isolated and identified by spectroscopic methods. 

The procedure described is a modification of that first presented by Freund1,2 involving the diazotization of thio-semicarbazide with emphasis on the removal of product so as to prevent its subsequent decomposition by the nitrous acid. The 1,2,3,4-thiatriazole ring is number 51 in the ring... 

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