Triazoles thermal rearrangements

L abbe has studied the rearrangement reactions of 1,2,3-thiadiazoles to differently substituted 1,2,3-thiadiazoles <1983CC588>. He also studied many 5-azido-l,2,3-thiadiazoles 33 that rearranged to 1,2,3,4-thiatriazoles 34 (Equation 5) <1988BSB163>. He even found that l,2,3-thiadiazole-4-carboxaldehydes 35 upon treatment with amines underwent thermal rearrangement to 1,2,3-triazoles 36 (Equation 6) <1993J(P1)1719>. 

C-Nitration of 1,2,3-triazole and 1,2,4-triazole rings can be achieved with either mixed acid or solutions of nitric acid in acetic anhydride. V-Nitration is usually achieved with nitric acid in acetic anhydride at ambient to subambient temperatures. Thermal rearrangement of the N-nitro product to the more stable C-nitro product often occurs at higher nitration temperature. 

The method has also been applied to the synthesis of alkyl-substituted quinoxalin-2-amines. Thermal rearrangement of l-(2-aminophenyl)-4,5-dihydro-5-morpholino-l,2,3-triazoles 15 affords unstable 2-alkyl-l,2,3,4-tetrahydroquinoxalin-3-amines, which undergo deamination and consequent oxidation to afford 2-alkylquinoxalines 16. ... 

Bromination occurs readily in alkaline solution giving 3,5-dibromo-l,2,4-triazole the 3-monochloro derivative can be obtained by thermal rearrangement of the A-chloro isomer an analogous N—>C 1,5-sigmatropic shift followed by tautomerisation converts the 1- into the 3-nitro compound. ... 

Reports describing 1,2,4,5-tetrazine ring contractions are scarce. However, photochemically induced ring contraction of 3,6-diphenyl-l,2-dihydro-l,2,4,5-tetrazine 363 into triazole 364 has been reported in the presence of hydrochloric acid <2000JPR281>. Similarly, thermal rearrangement of 3,6-bis(trifluoromethyl)-l,2-dihydro-l,2,4,5-tetrazine 365 in acetic acid afforded 1,2,4-triazoles 366 and 367 (Scheme 89) <1995JFC(72)95>. 

Maquestiau and co-workers investigated thermal rearrangement of iV(l)-acyl-1,2,4-triazoles to oxazoles using flash vacuum pyrolysis (FVP). In this case, FVP of an A (l)-acyl-1,2,4-triazole 83 leads to a 5-aryloxazole 86. It was assumed that a [1,5] sigmatropic shift of the acyl group in 83 produced 84, followed by loss of nitrogen to yield the diradical 85, which cycUzed to 86 (Scheme 1.25). A limited number of examples were investigated, and the results are shown in Table 1.4. The yields of 86 are excellent for iV(l)-aroyl-l,2,4-triazoles but modest in the case of A(l)-acetyl-1,2,4-triazole. Nonetheless, the author noted that this represented a one-step synthesis of a 5-susbstituted oxazole in contrast to the conventional four-step syntheses usually employed. 

Gas-phase pyrolysis of 4-amino-3-allylthio-l. 2,4-tna oles provided a new route to [13]thiazolo[3,2-f)][l,2,4]triazoles <01JCS(P1)424>. The kinetics of the thermal rearrangement of 4-ethyl-3,5-diphenyl-4//-l,2,4-triazolcs have been investigated <01JHC955>. The counteranion-dependent symmetry of Cu(lI)-4-amino-l,2,4-triazole polymeric chains was investigated <01CC1254>. 

The Dimroth rearrangement was originally reported by Otto Dimroth in 1902 with the second report occurring seven years later in 1909. The original reaction was discovered when 1,2,3-triazole derivatives rearranged under thermal conditions in the presence of base. 

One of the first thermal rearrangements of 1,2,3-triazoles was reported in 1902 by Dimroth. The original reaction involved heating 1,2,3-triazoles such as 5-amino-4-ethoxycarbonyl-l-phenyl-1,2,3-triazole 181 in absolute ethanol and benzene at 150 "C for 3 h, with the reaction favoring formation of the corresponding isomer 182. This work was later expanded by Lieber... 

Nitropyrazoles rearrange to 4-nitropyrazoles in H2SO4 and to 3-nitropyrazoles thermally. Similar rearrangements are known for 7V-nitro-l,2,4-triazoles. 

The 1-azirines obtained from the vapor phase pyrolysis of 4,5-disubstituted 1-phthalimido-1,2,3-triazoles (157) have been found to undergo further thermal reactions (71CC1S18). Those azirines which contain a methyl group in the 2-position of the ring are cleaved to nitriles and phthalimidocarbenes, whereas those azirines which possess a phenyl substituent in the 2-position rearrange to indoles. 

Another example of this rearrangement has been used to prepare 1,2,3-triazole 146 from furazanic phenylhydrazone 147 (Scheme 84) [93JCS(P1)2491]. Interestingly, furoxanic Z-phenylhydrazones 150 underwent thermal recyclization to 1,2,3-triazole A-oxides 152, evidently through intermediate 151. Treatment of the hydrazone 150 with rerr-BuOK leads to the nitromethyl derivative 149 [OOOMIl] (Scheme 84). Lead tetraacetate oxidation of 147 with subsequent Lewis acid treatment of the initially formed intermediate afforded indazole 148 (Scheme 84) (85JHC29). 

The kinetics of the thermally induced rearrangement of 4-ethyl-3,5-diphenyl-4//-l,2,4-triazole 28 to the corresponding 1-ethyl-substituted compound 29 in the presence of 15-crown-5 in octadecane at 330°C has been studied (Equation 5). A mechanism for the rearrangement was proposed that involved an intermediate triazolium triazolate species <2001JHC955>. 

In the presence of various metal ions, 2-(fluoroenone)benzothiazoline has been found to rearrange to A-2-mercaptophenylenimine, while a free radical mechanism involving the homolysis of C-S and C-N bonds has been invoked to explain the formation of 3-phenyl-1,2,4-triazole derivatives from the thermal fragmentation and rearrangement of 2-(arylidenehydrazino)-4-(5//)-thiazolone derivatives. The cycloadducts (36) formed from the reaction of 3-diethylamino-4-(4-methoxyphenyl)-5-vinyl-isothiazole 1,1-dioxide (34) with nitric oxides or miinchnones (35) have been found to undergo pyrolytic transformation into a, jS-unsaturated nitriles (38) by way of pyrrole-isothiazoline 1,1-dioxide intermediates (37). 

The oxatriazine (178) rearranges thermally or under acidic conditions to give triazole (179) (Equation (67)) <93H(36)455>. 

Simple triazoles are thermally stable to ca. 300°C. However, triazole carboxamides, when heated to 150°C in sulfolane, rearrange with the elimination of nitrogen to give 2-substituted oxazoles. The reaction is general and it is useful for the synthesis of oxazoles with diverse 2-substitutents in excellent yields even for bulky substituents (Scheme 1). This reaction does not occur photochemically... 

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