1.2.3- triazolium tetrafluoroborate

Analogously to carboxylic esters, thionocarboxylic esters can be synthesized by the azolide method. An example is the synthesis of thionobenzoic acid methyl ester via a 1 -thioacyl-4-alkyl-1,2,4-triazolium tetrafluoroborate and methanol.[175]... 

Li et al. used a similar rearrangement to synthesize 1,4-oxazepine 252, but instead of antimony pentachloride, tetrafluoroboric acid was employed as the acid. In the solution of bicyclic geminal arylazo isocyanate 250 in diethyl ether, a solution of tetrafluoroboric acid was added to form triazolium tetrafluoroborate salt 251. The rearrangement of 251 occurred quickly under mild conditions with ring expansion to yield the tricyclic 252 in less than 5 min. The isomeric form 253 is not observed (Scheme 45) <2004SC1691>. 

The 3-substituted 1,2,3-triazole 1-oxides 448 were alkylated at the oxygen by trimethyloxonium tetrafluoroborate using liquid sulfur dioxide as the solvent affording hygroscopic 3-substituted l-methoxy-l,2,3-triazolium tetrafluoroborate 507 in high yield (1987ACSA(B)724).The reactivity of these salts has not been reported (Scheme 147). 

Mesoionic l,2,3,4-oxatriazole-5-thiones (86) and thiatriazol-5-ones (88) are alkylated with triethyloxonium tetrafluoroborate to give 3,5-disubstituted oxa- and thia-triazolium tetrafluoroborates (80 equation 51), while oxatriazolethiones (86) were found to be inert towards methyl iodide (76CC306,79JCS(Pl)732>. 

Hydroxytriazole coupled with diazotized aromatic amines in the 4-position to give phenylazotriazoles, which could be reduced to primary amines (58ACS1236). 3-Methoxy-2-phenyl-l,2,3-triazolium tetrafluoroborate (80) (made from 2-phenyltriazole 3-oxide and trimethyloxonium tetrafluoroborate) was converted by ammonia in acetonitrile to 4-amino-2-phenyltriazole (20°C, 3 days, 79%) [81JCS(P1)503]. 

Triazolium tetrafluoroborates, obtained from the cycloaddition reac-... 

The nitration of the 2,7-diamino-77/-[l,2,4]triazolo[3,4-c][l,2,4]triazolium ion 44 with nitronium tetrafluoroborate affords after workup with base the corresponding potassium bis(iV-nitro imides) 45 <1991IZV1239>. 

V-(4-triazolyl)-2,4,6-trimethylpyridinium tetrafluoroborate decomposes at high temperature to give products consistent with the formation of the 4-triazolium cation <91JCS(P1)1761,94TL232l>. 

The meso-ionic l,2,4-triazol-3-ones (200) are stable to acid, but alkaline hydrolysis gives 1,4-disubstituted semicarbazides. They do not normally participate in 1,3-dipolar cycloadditions, but the meso-ionic 1,4-diphenyl-l,2,4-triazol-3-one (200, R = R = Ph, R = H) and benzyne yielded 2-phenylindazole. 1,2,4-Triazolium salts (211) are formed with triethyloxonium tetrafluoroborate. Reduction of the meso-ionic compound 200, R = Me, R = R = Ph, with lithium aluminum hydride gives the triazolidinone 212. ... 

Triazoles with alkyl, aryl or acyl substituents on N-1 or N-4 can be quaternized. Because of the mesomeric distribution of the positive charge on triazolium compounds, representations such as (63) are convenient but the equivalent formula (64) may be used to denote the site at which quaternization has taken place (the mesomeric nature of (64) must be kept in mind of course). Trialkyloxonium tetrafluoroborates are powerful quaternizing reagents (70JOC2246). 

Surprisingly, isolable azomethine ylide 151 reacts with aryldiazonium tetrafluoroborates, at — 30°C in acetonitrile, to produce fused triazolium salts 231 after the elimination of the formyl moiety from the initial cycloadducts (84TL65). 

Similarly, the halide triazolium salts are solids at room temperature, but metathesis to salts, such as bis(trifluoromethanesulphonyl)amide, triflate or tetrafluoroborate, lowers the melting point to near or below room temperature. 

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