Diazoalkanes, cycloaddition with

If the C=N function is attached to an electron-withdrawing group, 1,3-dipolar cycloaddition with diazoalkanes occurs leading to 1,2,3-triazoles (5, 276). When diazomethane is used, the initially formed NH-triazole is not isolated due to a rapid subsequent NH deprotonation followed by N-methylation. Consequently, a mixture of the three Wmethyltriazoles is formed when methyl cyanoformate (71) (216) or trichloroacetonitrile (276) (217) is treated with excess diazomethane (Scheme 8.51). Huisgen and co-workers found that methyl diazoacetate reacts with TCNE by a 1,3-dipolar cycloaddition at the C=C bond and not, as published earlier by other authors, at one of the nitrile functions (72). 

Regio- and Diastereoselectivity of Dipolar Cycloadditions with Diazoalkanes... 

Di, tri-, and tetrahalocyclopropenes undergo cycloaddition with diazoalkanes to give unstable pyrazolines, which readily rearrange to pyridazines with loss of hydrogen halide. Tetrachlorocyclopropene is commercially available but many of the bromo compounds are easily prepared in two steps from vinyl bromides by addition of dibromocarbene, followed by reaction with methyllithium, the last step being carried out in situ for the cycloaddition step. ... 

SCHEME 12.18 [3+2] Cycloadditions with diazoalkanes and nitrile imines. 

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