Pyrazole carbene insertion

There are several reports in the literature dealing with the bimolecular [3 + 2] cycloaddition reactions of alkynyl-substituted diazo compounds. Propargyl diazoacetate 212, when stored for 2 weeks at 0 °C, was transformed into an oligomer to which the constitution 213 was assigned (273) (Scheme 8.50). The alkynyl-diazoketone 214 requires a much higher temperature and is transformed into pyrazole 215, which probably arises from intermolecular cycloaddition, pyrazole tautomerization, and carbenic N/H insertion (274). The inter-intramolecular... 

In general, photolytic azo-extrusion from 4,5-dihydro-3/f-pyrazoles is superior to thermolysis. Photolysis was introduced as a method for synthesis by Jeger s group (Kocsis et al., 1960). The configurations of the products in the thermolysis, in the direct photolysis, and in the sensitized photolysis are often quite different. Cyclopropanation for the synthesis of alkyl cyclopropanes via dihydropyrazoles is preferred to the direct route via carbenes, because, in the latter, the C - H insertion of the carbene into the alkyl group is faster than the cyclopropanation. The dihydro-pyrazole pathway was used in particular for the formation of highly strained bicyclo[1.1.0]butanes (Franck-Neumann, 1967 Komendantov and Bekmukhametov, 1971 6.89) and bicyclo[2.1.0]pentanes (Vogelbacher et al., 1984 6.90). 

Another example of fragmenting carbenes is provided by the 3 Jf-pyrazolylidenes. The 3/f-diazo-pyrazole (234), on pyrolysis, yielded the carbene 235 probably stabilized as shown below. Although the carbene can undergo typical carbene reactions, such as insertion into a C—H bond, ultimately yielding 236 with cyclohexane, it can also decompose to the 2 -azirine 237. The methyl cyanocarbene subsequently yields acrylonitrile. 

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