Thiadiazoles decomposition

Then, as described in U.S. Patent 2,55416, the 2-acetylamido-5-mercapto-1,3,4-thiadiazole is converted to the sulfonyl chloride by passing chlorine gas into a cooled (5°-10°C) solution in 33% acetic acid (66 parts to 4 parts of mercapto compound) used as a reaction medium. Chlorine treatment is continued for two hours. The crude product can be dried and purified by recrystallization from ethylene chloride. The pure compound is a white crystalline solid, MP l94°C,with decomposition, when heated rapidly. The crude damp sulfonyl chloride is converted to the sulfonamide by addition to a large excess of liquid ammonia. The product is purified by recrystallization from water. The pure compound is a white, crystalline solid, MP 259°C, with decomposition. The yield of sulfonamide was 85% of theory based on mercapto compound. 

The thermal decomposition of some 3,5-disubstituted-l,2,4-thiadiazoles has been studied and some nonisothermal kinetic parameters have been reported <1986MI239>. Polarographic measurements of a series of methylated 5-amino-l,2,4-thiadiazoles show that thiadiazoles are not reducible in methanolic lithium chloride solution, while thiadiazolines are uniformily reduced at 0.5 = — 1.6 0.02 V. This technique has been used to assign structures to compounds which may exist theoretically as either thiadiazoles or thiadiazolines <1984CHEC(6)463>. The photoelectron spectrum for 1,2,4-thiadiazole has been published <1996CHEC-II(4)307>. 

Type G syntheses are typified by the 1,3-dipolar cycloaddition reactions of nitrile sulfides with nitriles. Nitrile sulfides are reactive 1,3-dipoles and they are prepared as intermediates by the thermolysis of 5-substituted-l,3,4-oxathiazol-2-ones 102. The use of nitriles as dipolarophiles has resulted in a general method for the synthesis of 3,5-disubstituted-l,2,4-thiadiazoles 103 (Scheme 11). The thermolysis is performed at 190°C with an excess of the nitrile. The yields are moderate, but are satisfactory when aromatic nitrile sulfides interact with electrophilic nitriles. A common side reaction results from the decomposition of the nitrile sulfide to give a nitrile and sulfur. This nitrile then reacts with the nitrile sulfide to yield symmetrical 1,2,4-thiadiazoles <2004HOU277>. Excellent yields have been obtained when tosyl cyanide has been used as the acceptor molecule <1993JHC357>. 

Alder cycloaddition with iV-phenylmaleimide to afford the colorless 1 1 adduct 110 (Equation 18) <1997T10169>. The adduct reverted to benzobis(thiadiazole) 109 and iV-phenylmaleimide at its decomposition point (>230 °C). 

JME538, 1997CH739>. The main thiadiazole product 185, however, suffered chlorination in the a-position. The isolation of 2-amino acrylonitrile 184 from the reaction mixture supported decomposition of the 2-oximino acetonitrile 183 furthermore, treatment of the pure acrylonitrile under typical reaction conditions gave exclusively ot-chloro-3-chloro-l,2,5-thiadiazole 185 (Scheme 27 Table 11). Mechanisms explaining the formation of both thiadiazoles were proposed <1998H(48)2111>. 

The thermal decomposition and cycloaddition reactions of 2,5-dihydro-1,3,4-thiadiazoles are reviewed in Sections... 

The reaction of tetrazole 132 with thiophosgene leads to [l,2,4]triazolo[3,4+][l,3,4]thiadiazoles 49. The reaction involves the in situ generation of aryldiazomethanes by decomposition of the tetrazole, followed by two cycloadditions (Equation 38) <1999CPA215>. 

No systematic study of 1,2,3-thiadiazoles by TLC or GC has appeared, but as long as the temperatures for GC analysis are below 200 °C (the approximate decomposition temperature for 1,2,3-thiadiazoles) this method should be satisfactory. Most simple thiadiazoles are isolated by distillation, and more complex structures are quite amenable to column chromatography <85JMC442, 88JHC1873>. 

Thiadiazoles are generally quite stable to heat due to the aromatic nature of the ring. Their thermal stability is influenced by the nature of the 3- and 5-substituents <65AHC(5)l 19>. Mass spectral decomposition patterns of substituted 1,2,4-thiadiazoles are discussed in Section 4.08.3.5. Photochemical behavior of 1,2,4-thiadiazoles has not been studied to date. 

In general, amino-1,2,4-thiadiazoles are resistant to the action of acids under mild conditions. Prolonged treatment with hot mineral acids, however, can result in decomposition with liberation of sulfur <65AHC(5)119>. 5-Amino-1,2,4-thiadiazoles are decomposed by hot alkalis, with evolution of ammonia, the rate of degradation depending on the nature of the substituent in position 3. The corresponding 3-amino isomers are much more stable to alkali <84CHEC-I(6)463>. 

The halogens in 1,2,5-thiadiazoles are reactive but 3-chloro-l,2,5-thiadiazole usually does not produce high yields in displacement reactions, probably because of ring decomposition via proton abstractions <68AHC(9)107>. 

The nonclassical benzobis(thiadiazole) 54 undergoes Diels-Alder reaction when heated with ACphenylmaleimide in refluxing xylene to produce the 1 1 cycloadduct 55 in 89% yield (Equation 5) <1997T10169>. The cycloadduct reverts to starting materials by a retro-Diels-Alder reaction at its decomposition point (>230 °C). 

Amidoximes (46) were first used as a source of 1,2,4-thiadiazoles in 1889 their condensation with carbon disulfide or with an excess of aryl isothiocyanate yields 3-aryl-5-mercapto- (47)6 -73 or 3-aryl-5-aryl-amino- 1,2,4-thiadiazoles (50),71,74,75 respectively. The latter reaction has been reexamined and discussed by Gheorgiu and Barbos76 who suggest that an initial addition of two moles of phenyl isothiocyanate to one of benzamidoxime is followed by cyclization of the intermediate (49), with elimination of phenylthiocarbamic acid (51). Decomposition of the latter gives rise to the by-products observed (cf. following scheme). 

D. Amino-1,2,4-thiadiazoles 1. 3- and 5-Amino-1,2,4-thiadiazoles a. Physical Properties. 3-Substituted 5-amino-1,2,4-thiadiazoles are generally stable, colorless, odorless compounds.6,88,86,128 Their thermal stability (as reflected by their melting points), solubility, and resistance to acids are influenced by the nature of the 3-substituent. 5-Alkoxy-3-amino-l,2,4-thiadiazoleB sublime without decomposition.83... 

The 3-amino isomers are more resistant towards reduction 3-amino-5-phenyl-l,2,4-thiadiazole is not affected by hydrogen sulfide or acidified potassium iodide, but does undergo decomposition under the influence of stannous chloride in acids or sodium in alcohol.126 Zinc in hydrochloric acid cleaves the heterocyclic nucleus completely, benzoic acid and hydrogen sulfide being the main products,126 while, under the same conditions, the 3-toluene-p-sulfonyl derivative yields benzal-dehyde and toluene-p-sulfonylguanidine.125... 

Interaction with diethyl sulfide occurs with simultaneous elimination of nitrogen and formation of a sulfonium salt of type 306 its hydrolytic decomposition yields 5-hydroxy- (307) and 5-ethylthio-3-phenyl-l,2,4-thiadiazole (308), in proportions dependent upon the hydrogen ion concentration of the medium.160... 

In connection with a study of the electron impact-induced fragmentations of 1,2,3-thiadiazoles, the mass spectrum of 5-phenylthiatriazole has been scrutinized.13 Jensen et al.1 have undertaken a detailed investigation including 5-aryl-, 5-amino-, and 5-alkylthiothiatriazoles. The electron impact-induced decompositions resemble the pyrolytic loss of N2S (Section III, A). In all cases the M—N2S ion together with its fragmentation is responsible for the major part of the total ion current. A detailed discussion of the spectra is outside the scope of this review. 

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