Carbenes heteroaromatic compound reactions

Heteroaromatic compounds, reactions with carbenes 82KGS723. 

Few examples of the preparation of six-membered heteroaromatic compounds using Fischer-type carbene complexes have been reported [224,251,381]. One intriguing pyridine synthesis, reported by de Meijere, is sketched in Figure 2.35. In this sequence a (2-aminovinyl)carbene complex first rearranges to yield a complexed 1 -azadiene, which undergoes intermolecular Diels-Alder reaction with phenylacetylene. Elimination of ethanol from the initially formed adduct leads to the final pyridine. 

Carboxamidation of heteroaromatic compounds, promoted by fluorine, provides a convenient alternative to the well-known Chichibabin amination reaction (Fig. 86) [228]. The mechanism of this reaction was suggested to proceed via a carbene intermediate (Fig 87). 

Electron-rich aromatic and heteroaromatic compounds add to 1,6-enynes in the presence of Au(I) catalysts (equations 62 and 63). This reaction leads stereospecifically to adducts related to the products in the aUcoxycychzation by nucleophihc addition on cyclopropyl gold carbene intermediates. Interestingly, in a few cases, addition of the nucleophile takes place at the carbene carbon to give adducts of the type in equation (63). ... 

The unsaturated substituent in the carbene complex 1 often is aromatic or heteroaromatic, but can also be olefinic. The reaction conditions of the Dotz procedure are mild various functional groups are tolerated. Yields are often high. The use of chromium hexacarbonyl is disadvantageous, since this compound is considered to be carcinogenic however to date it cannot be replaced by a less toxic compound. Of particular interest is the benzo-anellation procedure for the synthesis of anthra-cyclinones, which are potentially cytostatic agents. ... 

Among the known classes of heteroaromatic carbenes, the benzimidazole derivatives have received the least attention thus far. They [89] present new results in terms of the synthesis and properties of stable heteroaromatic monocarbenes and biscarbenes of the benzimidazole series. One of the major aims in this area was the attachment of sterically bulky groups to the benzimidazole nucleus. The introduction of the 1-adamantyl substituent into the benzimidazole system was achieved by direct adamantylation of benzimidazole using 1-bromoadamantane in the presence of sodium acetate in acetic acid. However, in this case the reaction was incomplete and yields of only 33% of pure salt 134 were realized. In o-dichlorobenzene in the presence of potassium carbonate, a 54% yield of l-(l-adamantyl)benzimidazole (86) can be achieved and this compound can be further quatemized by treatment with 1 -bromoadamantane in o-dichlorobenzene to afford a high yield of salt 134 (90%).As is well known, in the... 

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