Triazolium cations

Further examples of the ease of exchange of substituents on carbon by nucleophiles under mild conditions have been reported. The reactivity of the ring towards nucleophiles is increased in triazolium cations and mesoionic derivatives. 

The importance of fluconazole as an antifungal agent has led to a renewed interest in triazole derivatives and a further exploration of methods of alkylation. Reaction of 4-amino triazole (27) with chloroacetophenones (28) gave the corresponding triazolium cations (29) which could be deaminated to (30) with nitrous acid (Scheme 4) <93JHC1405>. 

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>. 

Our contribution to the reactivity of aromatic heterocycles involves only two papers [136], In the first one, we carried out IRC calculations (HF/6-31G ) to confirm the connecting structures of each transition state for the 1,2,3-triazole/iminodia-zomethane isomerizations. For the triazolate anion, both tautomers of 1,2,3-triazole and several 1,2,3-triazolium cations were studied. The below structure reports the case of the neutral molecules 30a and 30b. 

Substituents on C are exchanged in nucleophilic reactions under mild conditions and in the absence of activating groups. Reactivity of the ring towards nucleophiles is enhanced in triazolium cations and mesoionic triazole derivatives. 

C-Lithiations can be easily effected on A -1-protected 1,2,4-triazoles, the resulting 5-lithio derivatives being much more stable than C-hthiated 1,2,3-triazoles 5-Silylation can even be achieved using triethyl-amine with trimethylsilyl bromide - deprotonation of an iV -trimethylsilyl triazolium cation is presumably involved Exactly comparable silylations can be achieved with 2-aryl-l,3,4-oxa- and -thia-diazoles application of this regime to 1-phenyltetrazole produced phenyl trimethylsilyl carbodiimide in 90% yield ... 

Scheme 15. Structure N,N-dimethyl-l,3-diazolium and N,N-dimethyl-1,3,4-triazolium cations used as dienes for the Diels Alder reactions.

A = cyclopentadiene B = furan C = 1,1-dimethyl-1,3-diazolium cation D = 1,1-dimethyl-l,3,4-triazolium cation BOx-Y = bond order between atoms X and Y in the heterocyclic ring SBO = sum of ring bond orders ABO = average bond order BOD = sum of bond order deviation from the average ring bond order. 

Certainly a better insight into the reactivity of these two quaternized heterocycles can be obtained through evaluation of the activation barriers for cycloaddition (Table 51). It must be mentioned that transition state structures with these two quaternized heterocycles in reaction with acetylene, ethylene, and cyclopentene are very similar to the transition state structure with unquatemized heterocycles. As predicted on the basis of the bond order distribution in the heterocyclic ring, these two heterocycles were very reactive dienes for the Diels-Alder reactions. The computed energies were in full agreement with the bond order uniformity presented in Table 50. Both heterocycles had low 7t-density on the ring, but the A //-dimethyl-l,3,4-triazolium cation was more electron deficient (SBO = 6.530, Table 50), with a higher 7t-orbital localization (BOD = 2.330, Table 50). Because the reaction is LUMO diene controlled, the activation barriers with the A(,A(-dimethyl-l,3,4-triazolium cation must be lower in comparison to activation... 

All the qualitative evaluations of the Diels-Alder reactions with these two dienes are perfectly incorporated in the evaluated activation barriers. Even dienophiles with a low reactivity such as acetylene should add easily to both NJ -dimethyl-l,2-diazolium and Ar,A -dimethyl-l,3,4-triazolium cations. Later it becomes much more reactive. The exo cycloaddition had an activation barrier that was 9-10 kcal/mol higher, indicating that the endo cycloadduct should be the sole product of the reaction. In this way, we have demonstrated that cycloaddition reactions with quaternized heterocycles should be a very powerful method for utilizing heterocycles in organic syntheses. 

I = TS with ATA -dimethyl-l,3-diazolium cation II = TS with A, Af-dimethyl-1,3,4-triazolium cation a = for addition of acetylene b = for addition of ethylene c = for exo addition of cyclopropene d = for endo addition of cyclopropene HOF = heat of formation computed by AMI E = total energy (a.u.) computed with B3LYP/6-31G(d)/AMl AEi = activation barrier (kcal/mol) computed by AMI AEn = activation barrier (kcal/mol) computed with B3LYP/6-31G(d)/AMl. 

The synthesis and characterization of a series of nitrate salts, lOa-d, based on l-alkyl-4-amino-l,2,4-triazolium cations (alkyl = methyldecyl, isopropyl. 

A high-yield, efficient procedure to synthesize functionahzed alkyl/fluoro-alkyl-containing triazolium quaternary salts and triazolium ionic Uquids led to triazolium cations that contained covalently bound anionic sites, such as, fluorocarboxy, sulfonate, fluorohomoallyhc, and fluoroalkanol groups [43, 44]. These were converted by metathetical reactions with fluorine-containing anions to low-melting salts (26a,b, 27,28a,b, 29c,d, 30) (Scheme 8) [43]. These functionalized triazolium salts exhibit many characteristics associated with excellent ionic liquids with respect to stability in air and water, and when heated. All are liquids at 25 °C. 

To conclude this section just a quick word about triazolium cation derivatives of 1,2,3-triazoles [87]. They have also been investigated and show application for functional ionic liquids and precursors of mesoionic carbenes for catalytic purposes, or as units of supramolecular assemblies and molecular machines. 

The ionic liquids formed by the 1,2,4-triazolium cation and dinitramide anion as well as charge-diffuse tetrazolium cation with a variety of substituents coupled with various (usually oxygen containing) anions have been studied by ab initio quantum chemistry calculations [21, 180]. 

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