Pyridinium ylides, addition

Another example with porphyrinic dipolar species uses pyridinium salt derivatives as precursors of porphyrinic pyridinium ylides (Scheme 18) <05TL5487>. The procedure involves the reaction of porphyrin 58 with methyl bromoacetate, in refluxing chloroform, to give pyridinium salt 59. The latter, in the presence of K2CO3, reacts with 1,4-benzoquinone to yield only the mono-addition compound 60. Notably, when the reaction was performed in the presence of DBU, bis-addition occurred and the porphyrinic dimer 61 was the only isolated addition product. 

Yields from 1,3-dipolar addition reactions with pyridinium ylides are enhanced by the addition of phase-transfer catalysts [60]. The ylides are produced in situ under basic two-phase conditions and react at room temperature with acetylenic dipolarophiles. 

In a one-pot reaction (Scheme 126) the iminophosphorane of 6-aminoura-cil (346) (92AHC129) is transformed with isocyanate in the presence of pyridine into a nonisolable carbodiimide. Spontaneous addition of pyridine follows to give a 1,6-dipolar pyridinium ylide (347), which cyclizes to the... 

CHMe, cyclopropylidene, and CMe2 to activated double bonds.1075 Similar reactions have been performed with phosphorus ylides, 076 with pyridinium ylides,1077 and with the compounds (PhS)3CLi and Me3Si(PhS)2CLi.1078 The reactions with ylides are of course nucleophilic addition. 

The reactions of (aryhnethylene)isoxazol-5-ones (97) with pyridinium ylides (98) leads to the diastereoselective formation of olates (99) as a result of preferential Michael addition rather than 1,3-cycloaddition (Scheme 22). The stereoselectivity of the reaction has been rationalized by the involvement of the preferential transition state [(100) over (101)].86... 

First a Michael addition of the pyridinium ylide to cyclopropenone occurs. This is followed by a reorganization of the intermediate 33 with pyridine elimination giving the keto ketene 34, which by ring closure yields the 2//-pyran-2-one system. Sulfonium and phosphonium ylides can be used instead of pyridinium enol betaines. 

Indolizines have been made via carbene addition to 2-vinylpyridine or dipolar addition of pyridinium ylides. 3-substituted indolizines have been obtained in a one step procedure from reaction of chlorocarbenes with 2-vinylpyridine (94CC509). Microwave irradiation has been used in 1,3-dipolar cycloaddition of pyridinium dicyanomethylide with alkynes <94H(38)785>. 

This reaction was first reported by Krohnke et al. in 1961. It is the synthesis of 2,4,6-trisubstituted pyridine derivatives involving the formation of pyridinium ylide from pyridine and a-bromoketone, which undergoes the 1,4-Michael addition to an a, -unsaturaled compound to form 1,5-dicarbonyl compounds and cyclizes with ammonium acetate. Therefore, it is generally known as the Krohnke pyridine synthesis or Krohnke reaction. In this reaction, the intermediate 1,5-dicarbonyl compounds do not need to be isolated from reaction mixture. Because three different substituents can be introduced into pyridine ring, this reaction becomes the ideal model for combinatorial synthesis, and a library pool containing pyridine from 9 to over 200 has been generated by this reaction. 

Pyrone formation is likely to occur via primary Michael addition of the pyridinium ylide to the cyclopropenone, followed by reorganization of the intermediate 33 to the ketocarbene 34 with pyridine elimination and concluded by electrocyclization of 34 to... 

An interesting intermediate 30 was proposed to result from the sequential addition of pyridine to tetrachlorocyclopropene (31). Compound 30 represents an alkyl nitrogen ylide with two 1-chloroalkyl pyridinium moieties in the same molecule. Pyridines with electron-withdrawing groups and heterocycles with an electron-deficient nitrogen, for example, pyridine-3-carbaldehyde or quinoline, react with 31 to yield the corresponding mono-substituted products 32a and 32b (83JOC2629) (Scheme 8). 

Thermolysis of 1-imidoyliminopyridinium N-ylides 109 gave the pyrido-triazines 110 in addition to 111 and 112 (76CL413 77JOC443). Compounds 109 were obtained by the reaction of pyridinium salts 107 with 108. 

Ylide can be viewed as a special carbanion in which the negative charge on carbon is stabilized by an adjacent positively charged heteroatom. The most common ylides are phospho-nium ylides, sulfur ylides (sulfonium and sulfoxonium ylides) and certain nitrogen-based ylides (ammonium, azomethine, pyridinium and nitrile ylides). In addition to synthetically important phosphorus, sulfur and nitrogen, ylides of tin (Sn) and iodine (I) have been developed in recent years. 

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