Azolium ylides, reactions with

Phenyl-1,2,4-triazolium dicyanomethylide (399) undergoes with DMAD 1,3-dipolar cycloaddition (83JCS(P2)1317). The primary product of the reaction (400) can be isolated under carefully controlled conditions. On gentle heating it is transformed into a l,2,4-triazolo[3,4-a]pyridine derivative (401). Many other azolium ylides undergo similar reactions. 

Reaction of Azolium Ylides with Dialkyl Acylphosphonate New Synthesis of Heterocyclic Compounds A. Takamizawa et al., Heterocycles, 1974, 2, 521-554. 

Acyl azoliums generated from enals have been converted to cyclopropyl carboxylic esters with ee < 99% by reaction with sulfur ylides. Some FLPs have been found to i react by conjugate P/B addition to unsaturated ketones and esters, whereas 1,2-addition to corresponding aldehydes is usual. ... 

The C-2-exchange of azolium salts via an ylide mechanism was discussed in Section 24.1.2.1. Thiamin pyrophosphate acts as a coenzyme in several biochemical processes and in these, its mode of action depends on the intermediacy of a 2-deprotonated species (32.2.4). In the laboratory, thiazolium salts (3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride is commercially available) will act as catalysts for the benzoin condensation, and in contrast to cyanide, the classical catalyst, allow such reactions to proceed with alkanals, as opposed to araldehydes the key steps in thiazolium ion catalysis for the synthesis of 2-hydroxy-ketones are shown below and depend on the formation and nucleophilic reactivity of the C-2-ylide. Such catalysis provides acyl-anion equivalents. 

The kinetics of reactions of substituted ethyl arylacetates with quinone methides and structurally related diethyl benzylidenemalonates and of reactions of pyridinium, iso-quinolinium, and quinolinium ylides with diarylcarbenium ions, quinone methides, and arylidene malonates have been studied using solvent DMSO. Pyridinium substitution is found to have a similar effect on the reactivity of carbanionic reaction centres as alkoxycarbonyl substitution. The structures and ambident reactivities of azolium eno-lates have been investigated. ... 

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