Cyclopropanes from alkyl carbene insertion

Intramolecular insertion reactions are much more selective because the relative proximity of the various C-H bonds to the carbene center plays a major role in determining product distribution. For this reason, the reaction has synthetic utility and is especially valuable for generating derivatives of highly strained ring systems. Cyclopropanes formed by intramolecular insertion reactions are often major products from alkyl carbenes. A competing process for aliphatic carbenes is stabilization... 

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). 

Cyclic a-diazocarbonyl compounds (59) and enynones (61) have been used as Rh-and Zn-carbenoid precursors, respectively. Cyclic derivatives (59) have been found to favour intermolecular Rh-catalysed cyclopropanation reactions, relative to the formation of conjugated alkene (60) by intramolecular -hydride elimination as is usually observed in the case of a-alkyl-a-diazocarbonyl compounds this high level of chemoselectivity is reported for the first time. Rh-carbenoids derived from (59) have also promoted cyclo-propenation reactions as well as diverse X-H insertion reactions (i.e., X = C, N, O, S). In parallel, highly functionalized cyclopropylfiirans (62) have been successfiilly prepared from an alkene and an enynone (61) by a cyclization/cyclopropanation sequence conducted in the presence of catalytic amounts of ZnCl2, which is cheap and of low toxicity computations support the probable participation of intermediate Fisher-type Zn(II) carbene complexes (63). 

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