Diazolium salts

Few examples on the reactivity of substituents attached to ring atoms are available. The desilylation of the 1,3,4-thia-diazolium salt 124 by CsF gave the unstable 1,3-dipole 125 which was trapped with N-substituted maleimides to afford exclusively the < . 

Enaminoesters and enaminoketones, for example (40), rearrange to imidazoles (41) or oxa-diazolium salts (42) (Scheme 14) <74T3859>. For R = H mixtures of imidazoles and pyrazoles, for example (43), have been obtained <78JA4208>. The formation of (43) is an example of a ring transformation, where the attacking nucleophile is the first atom of the side-chain in the 3-position of the oxadiazole (Scheme 14). 

A similar 1,3-dipolar cycloaddition occurs with diethyl azodicarboxylate, and the intermediate 1,3-cycloadduct was isolated. Amidines (101) and halogenoacetonitriles (XCHjCN) yield 4-amino-1,3-diazolium salts (102) directly. These salts (102) could not be transformed into the corresponding meso-ionic heterocycles (98) with base ... 

Malek, K., Skubel, M., Schroeder, G. et al.. Theoretical and experimental studies on selected 1,3-diazolium salts. Vibrational Spectr., 42, 317, 2006. 

An unusual reaction of a diazolium salt was reported during an attempt to form copper complexes of the NHC.100 The isolated products showed ring expansion of the carbene to form six-membered lactams. The authors verified the product by X-ray crystallography and independent synthesis. It is not clear where the extra carbon comes from. The authors consider a possible mechanism involving a carbene dimer formation, but point out that the carbenes are very hindered. Possible reaction with atmospheric carbon dioxide was not considered. Direct oxidation of the carbene to give a urea is also noted. 

Herrmann and Schiitz [43] constructed a carbene complex on a rigid purine framework (Scheme 2.156). The synthesis of the diazolium salt was carried out... 

A large number of differently bridged diazolium salts of type 24 (Figure 1.4) have been described. Compound 25, the precursor for an interesting bis-carbene containing carbene and alcoholato donor functions, was prepared by Arnold et al. by the reaction of two equivalents of an A-alkylimidazole with a functionalized epichlorhydrin (Figure 1.4). Dibenzimidazolium salts are also known. Trisimidazolium salt 26 was first described by Dias and Jin, " shortly thereafter followed by the description of trisimidazolium salts of type 27. Tripodal tris-imidazolium salts of types 26, 28 and 29 were prepared by Hu and Meyer. ... 

Diazolium and triazohum salts have been used in O- to C-acyl transfer reactions.101 The reaction works well (67-82% yield) in the presence of <1.5 mol% catalyst. The mechanism involves acyl transfer from the oxygen atom of an enol ether to the... 

The heating of 4-o-tolyl-l,3-dithia-2,5-diazolium hexafluoroarsenate 154 with a small excess of o-tolylcyanide 155 in liquid S02 leads to 1,2,4-dithiazolium hexafluoroarsenate 6 in low yield (Equation 24) <1996AXC2148>. The decomposition of the known pesticide, 3,5-dirncthyltctrahydro-2//-1,3,5-thiadiazine-2-thione 156 (DTTT), in water and in salt solutions was studied in detail. 4-Methyl-5-methylimino-l,2,4-dithiazolidine-3-thione 157 and 2,4-dimethyl-l,2,4-thiadiazolidine-3,5-dithione 158 were isolated by HPLC and identified among the noncyclic decomposition products (Equation 25) <1996MI503>. 

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