Azolium cations

In azolium cations, the electron-pull of the positively charged heteroatom is strong, and substituents attached a or y to positive poles in azolium rings show correspondingly enhanced reactivity. 

In accord with calculations performed by Cavell et al. [110], the oxidative addition of C2-X functionalized azolium cations (X = halogen) to metal centers proceeds faster and with a more favorable reaction enthalpy than the oxidative addition of the C2-H substimted imidazolium cations [118, 119]. The former reaction was applied successfully for the preparation of nickel and palladium complexes bearing a variety of different ylidene ligands [119]. 

Transition from a neutral azole to an azolium cation strongly increases the TT-deficiency. In azines, the HOMO is normally occupied by the nonbonded electron pair of a nitrogen atom. By contrast, in azoles the HOMO is of the rr-type. This is supported by the parallel behavior of calculated HOMO... 

R.C. Makino, Ibid. IV. CNDO/2 Calculations on Azolium Cations and on the Zwitterions Resulting from their Deportonation , BRL 1618 (1972) 7) R. Shaw, Structure-Property Cor-... 

The observed kinetic law, the type of rate profile (plot of log k vs. sulfuric acid concentration), the values of the Arrhenius parameters, the comparison of the observed reaction rates with the calculated encounter rates, and the agreement with the features of the nitration of quinoline127 are in favor of a reaction of nitronium ions with the azolium cations. Only at lower acidities (< 90% H2S04) can the reaction of the neutral azole molecules become important. 

As with pyridine, not only does the electronegative nitrogen atom withdraw electron density from the ring, but under the acidic conditions of many electrophilic reactions the azole nitrogen is protonated. The azolium cation is relatively inert to further attack by a positively charged electrophile. 

The structures of representative azolyl anions and azolium cations (one resonance form and one isomeric form only) and their names are shown in Figure 2. 

Different physical properties in both the ground and the excited states should provide deeper insight into the high dipolar nature of compounds of general type 1. When the acid-base equilibria of these heterocyclic betaines are discussed, two situations must be considered (i) there is resonance interaction between the pyridinium (azolium) cation and the azolate anion and (ii) the two moities are independent. 

Direct acylation of AT-protected imidazole involves formation of an AT-acyl azolium salt, which is then deprotonated at C-2 followed by rearrangement of the acylazolium ylide to the 2-acylated imidazole (for mechanism and a number of applications, see CHEC-II(1996)). An example is shown in Scheme 82 N-acylation of imidazole 350 followed by 2-deprotonation of the azolium cation, then migration of the acyl group to C-2 gives product 351 <2005JME2154>. 

Azolines 13C NMR, 5, 20 H NMR, 5, 17 NMR, 5, 14 Azolinethiones reactions, 5, 102-103 Azolinones acidity, 5, 72 13C NMR, 5, 16, 19 H NMR, 5, 14, 15 halogenation, 5, 58 hydrolysis, 5, 64 IR spectra, 5, 24, 27 reactions, 5, 81 with aldehydes, 5, 60 with ketones, 5, 60 Azolin-2-ones electrophilic attack, 5, 99 Azolium cations nucleophilic attack at carbon, 5, 61 reactivity, 5, 42 Azolium ions reactions with alkali, 5, 63 with amines, 5, 65 Azolium salts hydrogen exchange, 5, 70 nucleophilic attack at hydrogen, 5, 69-72 reactions... 

The generic structural formula for a l-alkyl-3-methylimid-azolium cation is... 

Lithium dicyano- 1,2,3-triazolate was reported as a useful electrolyte (38) [45]. 1-Butyl-3-methylimidazoliiun 3,5-dinitro-l,2,4-triazolate (39) (Fig. 2) (m.p. 32 °C, rmaterial Some novel ionic liquids made up of azolium cations and anions were also reported. These salts are 1-ethyl-3-methyUmidazohum 1,2,4-triazolate (40) (Tg - 76 °C, T 207 °C, 7] 60.2 cP at 25 "C) and tetrazolate (41) (Tg -89°C, t] 42.5 cP at 25 °C) (Fig. 2). Both 40 and 41 were prepared by the coupUng reactions of 1-ethyl-3-methylimidazolium hydroxide with triazole or tetrazole, respectively [50]. 

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