Pyridinium methylides

Pyridinium methylides react with alkynes such as DMAD, alkylpropiolates, cyano-acetylene and dicyanoacetylene to give indolizines (167) directly (Scheme 22). In most of the reactions with ethylenes, however, tetrahydroindolizines (168) and dihydroin-dolizines (169) and (170) are isolated as intermediates, which can be converted to indolizines in the presence of a dehydrogenating catalyst. 

The reaction of DMAD with the cyanocarbalkoxy pyridinium methylide 39 in various solvents has also been studied.64 In benzene the expected indolizine 40 was obtained. In dimethylformamide, the car-bethoxy methylide gave some elimination of CN instead of COOR. However, in acetonitrile no indolizine was formed at all. The 1 1 crystalline adducts that were isolated were formulated on the basis of chemical and spectroscopic evidence as the highly stabilized ylids (41), formed by an internal rearrangement (Scheme 8). 

Other heterocyclic ylids have been generated in the gas-phase by collisional reduction of stable ylid cation-radicals using synthetic strategies similar to that described for 35+, e.g., imidazol-2-ylidene [149], thiazol-2-ylidene [150], pyrazine ylids [151], pyrimidine ylids [152], and pyridinium methylids [153]. 

The [3 -I- 2] cycloadducts from pyridinium methylides and perfluoro-2-butyne as well as 3,3,3-trifluoropropyne in the presence of sodium hydride are spontaneously transformed into the trifluoromethyl-substituted indoli-zines [91 JFC(51)407] (Scheme 72). 

Pyridinium methylides. Trimethylsilylmethyl triflate converts pyridines into N-(trimethylsilylmethyl)pyridinium triflates in high yield. On treatment with cesium fluoride these products are converted into nonstabilizcd pyridinium methylides. which can be trapped by cycloaddition to dimethyl acetylenedicarboxylate to form indolizincs. 

A pyridinium methylide can undergo a novel reaction with an electron-deficient alkene to provide the next higher homolog in which the original double bond is saturated and a methylene group is added. This reaction fails with hindered alkenes, and apparently requires DME as solvent. A [3 + 2] cycloaddition may be the initial step. 

Banks and coworkers published a series of papers describing the reaction of pyridinium methylides with prefluoroalkenes, perfluoroazaolehns, and trifloroace-tonitrile to produce fluorinated heterocycles, exemplihed by the synthesis of indolizine 74 (Fig. 3.33). Modest yields isolated from complex mixtures were reported. 

More one example is connected with generation of unsubstituted ylides [35], As facile precursors for non stabilized pyridinium methylides N-(trimethylsilylmethyl) pyridinium triflates were synthesized. Cesium fluoride induced desilylation of the precursors liberated the nonstabilized pyridinium methylides which were trapped as the cycloadducts to dimethyl acetylenedicarboxylate. Trapping of 3-CF3-pyridinium ylide gave the mixture of isomers (50a 50b = 1 5) in 53 % overall yield. 

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