Ylides, nitrilium

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 first nitrile ylide stable enough to be isolated (i.e., 1) has been prepared by the carbene/nitrile method (1). For this dipole, the anionic component is stabilized by electron delocalization and the nitrilium component by the steric bulk of the adamantyl group to such an effect that it has a melting point of 230 °C. The X-ray structure showed that the nitrile ylide moiety is close to linear and much like the resonance structure shown below. 

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

Another example of a stable nitrile ylide (2 R,R = alkyl) was generated by the thermal decomposition of 7-azido-l,3-disubstituted lumazines in xylene (2). The product is formed via a complex multistep process. As in the case of 1, the stability is attributed to strong resonance stabilization of the anionic moiety but it is notable that this is the first isolable nitrile ylide that does not also rely on the presence of a bulky substituent at the nitrilium carbon. 

The proposed mechanism for formation of 151 is shown in (Scheme 27). Proton abstraction by the hydride base from the activated 2-position of the W-fluoropyridinum triflate yields a highly reactive carbene which undergoes attack by the acetonitrile solvent. The resulting nitrilium ylide eliminates fluoride and subsequently adds the isonitrile with cyclization. Finally, reduction by the hydride reagent and aromatization provide the imidazopyridine 151. The undesired amide 152 is a product of hydrolysis of the intermediate nitrilium compound. 

The preparation of 2,5-dihydrooxazoles from a-hydroxy ketones, aldehydes and gaseous ammonia is illustrated in equation (165). A derivative of 2,5-dihydrooxazole is obtained by the 1,3-dipolar cycloaddition of benzaldehyde to the nitrilium ylide (306), generated from a-chlorobenzylidene-p-nitrobenzylamine (equation 166). 

A T-WICOls complex of a 1-unsubstituted lf/-l,2-azaphosphole was assumed to be an intermediate in the [3-F2] cycloaddition of a nitrilium phosphane ylide P-W(CO)s complex, with dimethyl acetylenedicarboxylate. It reacts further with another equivalent of dimethyl acetylenedicarboxylate to give a 7-aza-l-phosphanorbornadiene as the final product <1999AGE215>. 

For several years, Streubel and coworkers have been exploring the chemistry of nitrilium phosphine ylides that contain low-valent organophosphorus fragments, such as complex (19) (Scheme 3). In their latest contributions to this area, they have investigated the reactions of complex (19) with phosphoranes and iminophosphoranes which have led to a number of new species containing ylidic moieties, such as the tricyclic complex (20), and 2H-l,2-azaphosphole complexes (21) and (22). Similarly, treatment of other low-valent organophosphorus complexes such as phosphinidine species (23) with stabilised phosphoranes produces new ylidic-aducts (24) and (25) (Scheme 4). ... 

Thermolysis of 2-diazo-4,5-dicyano-2//-imidazole 656 in benzonitrile gives nitrilium ylide 657, which reacts with a second benzonitrile molecule to afford [4+2]-cycloadduct 658 (79JOC1717). Intermediate formation of nitrile ylide 659 was also supported in a reaction of N-fluoropyridinium... 

Finally, we draw this section to a close by mentioning nitrilium phosphine ylide complex (164), which is perhaps more accurately described as a low-valent organophosphorus species (see Chapter 1) rather than a conventional ylide. Complex (164) has been identified as an intermediate in the thermally induced conversion of 7-phosphanorbornadiene complexes into 2H-l,2-azaphosphole complexes (Scheme 34). ... 

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>. 

If the reaction is done with acetonitrile as a solvent under reducing conditions, the carbene intermediate 78 eliminates fluoride and reacts with the nitrile generating nitrilium ylide 83 as illustrated in Scheme 23. This intermediate, in turn, undergoes addition of isonitrile followed by subsequent cyclization to the imidazopyridinium intermediate 84 that is then reduced with sodium triacetoxyborohydride to give imidazopyridine 85 <05TL4487>. 

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... 

Thermal rearrangement of 4-carbonyl substituted oxazoles via nitrilium ylide 2 (see 1st edition). 

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