Triazolium salts rearrangement

In the 1,2,4-triazole series, base-induced degenerate rearrangements have been reported with the aroylarylazo-l,2,4-triazolium salts 185 (87MI3) (Scheme IV.72). The reaction product is 3-aroyl-l-aryl-1,2,4-triazole 187, and its formation can be described as involving an ANRORC mechanism, initiated by an addition of the nucleophile at C-5, ring opening into 186, and subsequent heterocyclization with the former CNN side chain of the rearranging 1,2,4-triazole. 

Arylhydrazones of A -acylbenzimidazoles (441) react with perchloric acid to rearrange into 1,2,4-triazolium salts 443, which can be isolated when R = Ar = Ph (Scheme 70). The protonated cycloadduct 442 represents a key intermediate. A reverse process was also pointed out neutralization of the triazolium salt 443 (R = Ar =Ph) with aqueous sodium carbonate gives back the corresponding 441, likely via a preliminary heter-ocyclization into a neutral cycloadduct. When R is a COMe or COOEt the unisolated triazolium salts 443 transform into final products by a condensation between the amino group and COMe or COOEt (89H339). 

Af, A -Disubstituted hydrazones rearrange smoothly to furnish 1,2,3-triazolium salts, hydrazides give triazolones (35), and amidrazones (36) afford (37) (Scheme 12) <91KGS822,92KGS969). 

Scheme 61, yielded thiazole 200 as the major product, along with minor amounts of carbinol 201 [152]. On the other hand, treatment of the imine formed from 199 and p-methoxyphenylamine with catalytic tetrabutylammonium cyanide, produced suc-cinimide derivative 202. In both cases, the process is initiated by nucleophilic attack to the carbaldehyde C=0 (or azomethine s C=N) group, which is followed up by an anionic rearrangement. A variation of the above process using as catalysts /V-heterocyclic carbenes (NHC) derived from base treatment of azolium, imidazo-lium, or triazolium salts, has also been developed to access gem-disubstituted succinimides [153, 154]. Unfortunately, an attempt of kinetic resolution of racemic 4-formyl (3-lactams by using chiral NHC resulted in moderate selectivities only [154]. 

Alkyl- and aryl-triazolium compounds are formed mostly by quaternization of preformed triazoles, more rarely by rearrangement of oxadiazoles proceeding through acylamidrazone intermediates. An unusual method (Scheme 136) is the addition of the alkoxydiazenium fluoroborate (312) to the Schiff base (313) to form the triazolium salt (314) through a mechanism that may involve a triazolidine intermediate (315) (69CB3176). 

Sodium hydride in DMF at room temperature induces rearrangement of some l,4-dialkyl-4 T-l,2,4-triazolium salts to 5-aminoimidazoles the reaction can be generalized to permit synthesis of 4-acyl-5-aminoimidazoles from readily accessible starting materials, but yields are only moderate (21-60%) [22]. 

Heterocumulenes, such as isocyanates and carbodiimides also react with l-aza-2-azoniaallenes across the C=N bond to give [3+2] cycloadducts. For example, aliphatic and aromatic isocyanates add at -20 °C to l-aza-2-azoniaallene salts 50 to give 4,5-dihydro-5-oxo-3/f-l,2,4-triazolium salts 51 or the rearranged triazolium salts 52. ... 

Reaction of l-aza-2-azoniaallene salts 425 with phenyl isocyanate occurs via a [3-1-2] cycloaddition reaction with subsequent rearrangement to give triazolium salts 426" . 

Intramolecular addition processes involving latent carbenes have been reported twice. On one hand, the thermolysis of 2,2-dialkoxy-5,5-dimethyl-A -l,3,4-oxadiazolines (38) as latent carbenes formed bicyclic compounds (39) via an efficient and highly stereoselective formal 4- -1-cycloaddition process, carbene (40) being postulated as the key intermediate. On the other hand, the deprotonated carbenic forms of alkyne-tethered imidazolium (41) and 1,2,4-triazolium salts (42) have been found to undergo 6-exo-dig intramolecular addition, leading to zwitterionic intermediates (43) and (44) that rearrange to give 2-substituted imidazole (45) and bicyclic 1,2,4-triazole (46), respectively. ... 

Smith and coworkers have investigated the ability of chiral N-heterocychc carbenes (NHCs) to promote the asymmetric Steglich rearrangement [28]. After an extensive study, it was found that the carbene derived from the oxazoline-triazolium salt 15 was the best catalyst (Scheme 40.21). 

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>. 

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