Nitrilium betaines

Tnfluoromethyl-substUuted 1,3-dipoles of the propargyl-allenyl type and trifluoromethyl-substituted nitrilium betaines. Tnfluoromethyl- [164, 765] and bis(trifluoromethy])-substituted [166, 167] nitrile ylides have been generated by different methods and trapped with various dipolarophiles to yield [3+2] [768] and [3+1] cycloadducts [769], respectively... 

The early stages of the reaction of the quaternary salt can be regarded as proceeding in a manner exactly analogous to that by which the isoxazoles themselves are degraded, the j8-oxoketene imine structure (148) being one mesomeric form of a compound which could alternatively be formulated as a nitrilium betaine. However, by contrast with the products from the isoxazoles (i.e., enolates of /3-keto-nitriles), this is electrically neutral and susceptible to further nucleophilic attack. 

Independent work by Schmid93 and by Padwa94 on the photochemistry of 2H-azirines has shown that irradiation of such systems leads in the first instance to the formation of nitrile ylids (nitrilium betaines). Subsequent 1,3-addition to a variety of dipolarophiles affords five-membered heterocycles. These additions take place in a stereospecific and regioselective manner thus, irradiation of the diphenyl-2f/-azirine 117 in the presence of dimethyl maleate leads to the formation of the two isomeric 1-pyrrolines... 

Probably, the diversity of nitrile oxide chemistry is not conducive to writing reviews related to all aspects of their reactivity. Therefore, only several references can be mentioned, which are connected with several topics in this section. Among these are the reviews devoted to the photochemistry of N-oxides (including nitrile oxides) (100) and reactions of nitrilium betaines with heteroaromatic compounds (101). Other references on reviews will be given in corresponding subsections or paragraphs. 

Padwa and Carlsen32 photolyzed 3-allyl-3-methyl-2-phenylazirine (4) in the presence of excess DMAD and obtained the pyrrole 7 in high yield in the absence of trapping agent, compound 6 was obtained. The interesting intermediate nitrilium betaine 5 was trapped by the... 

There is a delicate balance between the relative energies of the planar and non-planar species as the component atoms of, or substituents on, various nitrilium betaines are varied. Table 2 summarizes the calculations for substituted derivatives of the parent nitrile ylide32 ... 

The exocyclic C=S bond of 5-thioxo-2-thiazolines (203) can act as a dipolarophile towards several compounds. For instance, cycloaddition reactions have been reported with nitrilium betaines (204) to give heterocyclic spiro compounds <84HCA534>, with thiocarbonyl ylides (205) to give spirocyclic 1,3-dithiolanes <9IHCA1386>, and with 2-diazopropane (206) to give thiadiazoles <92HCA1825>. On the other hand, the reaction of (203) with diazomethane leads to a mixture of five products (Scheme 49) <93hcai715>. 

The iminocarbenes 33 (Scheme 7) may also be regarded as nitrile ylides (35). Ab initio calculations45 indicate that nitrilium betaines are best described as hybrids of the bent dipolar and carbenic structures (e.g., 33 and 35). Consequently, the generation of the nitrile ylide 35 in the gas phase should give the same product as obtained from the azirine 31. This was confirmed by flash vacuum pyrolysis of the appropriate oxazol-5(2//)-one (36), which gave a quantitative yield of the azadiene 34.46... 

The cycloaddition behavior of phosphaalkynes toward 1,3-dipoles is particularly pronounced. Thus, reactions with diazo compounds give rise to the 37/-l,2,4-diazaphospholes 12 or their l//-isomers, respectively [16,18]. Azides [18a, 19], nitrilium betaines [18a, 20] mesoionic species [21], and sextet dipoles such as selenoxocarbenes [22] react analogously to form heteroatom-substituted phospholes. 

Scheme 1. Nitrilium betaines A, their cyclic isomers B and complexes thereof C and D (R=ubiquitous organic substituents [M] = metal complex fragment)...

This alternate possibility was discounted by Huisgen, however, who showed that three-membered rings are not primary products in the 1,3-dipolar additions leading to five-membered heterocycles with nitrilium betaines. ... 

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