Triazolium salts, ring opening

Disubstituted triazolium salts are prepared from alkyl or aryl hydrazines via an oxadiazolium salt 28 (Scheme 16). Addition of a chiral amine on this salt resulted in a ring opening - ring closure reaction affording the triazolium salts 29. 

Both the inner salts 6 of anhydro 177-[l,2,4]triazolo[4,3-6][l,2,4]triazolium hydroxides and their methylation products 7, when heated above the melting points, suffer loss of sulfur or of methyl iodide and sulfur, respectively, and concomitant ring opening into 1-methyl-3-methylthio-5 (arylcyanamino)[l,2,4]triazoles 8 takes place (Equation 1) <1984TL5427,1986T2121>. 

The ring transformation of the fused triazolium salt 133 to the pyrazolopyridine derivative 134 proceeds via ring opening according to a valence-bond isomerization <1999JOC5499> and was already discussed in Section 11.16.5.1. 

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. 

Certain AMmines formed by deprotonation of N-aminoazolium salts also failed to undergo cycloaddition to give azapentalenes. TV-Amino-thiazolium mesylate on treatment with potassium carbonate and DMAD added 2 moles of dipolarophile to give the fused diazepine 219 2°5 4-Amino-l-methyl-s-triazolium iodide and DMAD in the presence of base gave the pyrazole 221, probably by ring opening of the first-formed adduct 220. Dehydrogenation to 222 did not occur.297... 

The reaction has to be carried out in the absence of oxygen and water, otherwise the intermediately formed nucleophilic carbene is oxidized to the tria-zolinone or suffers hydrolysis with subsequent aminal-type ring opening. Attempts to apply catalyst 7 to the synthesis of aliphatic acyloins gave very low yields and low enantioselectivities. Optimization of the catalyst structure with regard to activity and enantioselectivity yielded triazolium salt 8 as the best-suited system for the condensation of aliphatic aldehydes (Scheme 5) [37]. However, the enantiomeric excesses obtained, only up to 26%, and the rather low total turnover numbers, ranging from 4 to 8, are modest and a search for new, more active catalyst systems is highly desirable. 

In the actual implementation of this concept, Bode and coworkers used 3-hydroxy-2-pyranone derivatives (kojic acids) as the enol components, and the lactone product was submitted in situ to ring-opening with methanol to afford the more stable methyl esters. Both the yields and the enantioselectivities were excellent, using the chiral triazolium salt 26 as the preotalyst (Scheme 40.40). 

---