Diazo compounds nitrile ylide generation

To use 1,3 dipolar cycloadditions in a retrosynthetic sense, it is necessary to know what 1,3 dipoles are available. The list on pages 319-320 is representative of die more common and useful examples, although many others have been reported. Azides, diazo compounds, and nitrones are normally isolable compounds which can be added to a solution of an olefin. Other 1,3 dipolar species such as nitrile oxides and azomethine ylides are not stable molecules they must be generated in the reaction mixture in the presence of the olefin. As might be expected, many different ways to generate 1,3 dipoles have been developed. 

Carbenes are readily obtained by the photodecomposition of diazo-compounds. The reactions of carbenes generated in this way in rigid matrices at low temperatures have been reviewed. Addition of singlet methylene to acetonitrile affords the nitrile ylide... 

Cycloadditions 1,3-dipoles readily react with the (6,6)-double bond of C o that acts as a highly reactive dipolarophile. Five-membered rings are generated on the fullerene s surface this way, and these can bear a multitude of functional groups. Suitable 1,3-dipoles include, for instance, nitrile oxide, azomethine ylides, alkylazides, diazo compounds or trimethylenemethane (Figure 2.60). 

Certain electron-deficient triazoles, such as l-sulfonyl-l,2,3-triazoles, could undergo ring-chain isomerism to generate diazo compounds. Based on this intrinsic ability of triazoles, Homeff et al. [44] developed Rh-catalyzed transannulation of l-sulfonyl-l,2,3-triazoles 66 with nitriles, which afforded imidazoles 67 in good to excellent yields (Scheme 5.46). In this process, l-sulfonyl-l,2,3-triazole serves as a precursor to diazoimine species A, which, in turn, could be converted to Rh-carbenoid B. Then a nucleophilic attack of nitrile at the Rh-carbenoid B leads to the ylide C, which undergoes cyclization and snbsequent metal loss to prodnce imidazole. 

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