Nitrile ylides generation mechanisms

The generation of electrophilic carbene complexes in the presence of nitriles or other cyano-group-containing compounds can lead to the formation of nitrile ylides. With acylcarbene complexes the final products are often 1,3-oxazoles [1194], presumably formed by the mechanism sketched in Figure 4.10. 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

The mechanism of the thermal conversion of 4-cyclopropenyl-4-substituted-5(4f/)-oxazolones to pyridines has been studied. A stepwise cycloaddition of the initially generated nitrile ylide has been proposed to account for the observed products. 

In 1988, Vedejs and Fields prepared 3-thiazolines from 5-methoxy-2-methyl-oxazole 249 and a series of thioaldehydes, generated in situ from their cyclopenta-diene adducts. Thus heating 249 and thioaldehyde 250 in a sealed tube at 140°C for 48 h afforded a 95% yield of 251 as a 1 1 mixture of diastereomers (Fig. 3.74). The unactivated oxazole, 5-methyl-2-phenyloxazole was unreactive under the same conditions. A Diels-Alder mechanism was proposed for this reaction but no intermediate Diels-Alder adduct was observed. A nitrile ylide pathway was discounted, since these reactions proceed at room temperature if the thioaldehyde is generated photolytically and oxazoles do not form nitrile ylides under those conditions. Thioformaldehyde, thioacetaldehyde, thiobenzaldehyde, and thioace-tone all successfully underwent similar reactions to provide thiazohnes. 

Thermolysis of triazole 520 in acetonitrile affords imidazole 521 in 75% yield. Its formation was rationalized in terms of a 1,3-dipolar cycloaddition of the intermediate a-cyaniminocarbene 522 to the C=N bond of the solvent [81AG(E)113], although the mechanism of ylide formation with concomitant cyclization must not be ruled out. The similar nitrile with carbene 523, generated from diazo tetrazole 524, leads to imidazotetrazolic systems 525 (R = Me, Ph) in 42 and 51% yield, respectively (85T4621). 

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