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The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate , which cyclopropanates an aromatic ring.
Carbenes had first been postulated by Eduard Buchner in 1903 in cyclopropanation studies of ethyl diazoacetate with toluene. [29] In 1912 Hermann Staudinger [30] also converted alkenes to cyclopropanes with diazomethane and CH 2 as an intermediate. Doering in 1954 demonstrated their synthetic utility with dichlorocarbene. [31]
The strategy can start with a Simmons-Smith-like cyclopropanation of a cyclic alkene. [8] A related cyclopropane-based ring expansion is the Buchner ring expansion. The Buchner ring expansion is used to convert arenes to cycloheptatrienes. The Buchner ring expansion is encouraged to open to the desired product by placing electron withdrawing ...
Cyclopropanation is also stereospecific as the addition of carbene and carbenoids to alkenes is a form of a cheletropic reaction, with the addition taking place in a syn manner. For example, dibromocarbene and cis -2-butene yield cis -2,3-dimethyl-1,1-dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane.
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Cyclopropanation of olefins with diazocarbonyl compounds is commonly accomplished using rhodium carboxylate complexes, although copper was originally used. [11] The scope of the olefin is generally quite broad—electron-rich, [ 12 ] neutral, [ 13 ] and electron-poor [ 14 ] olefins have all been cyclopropanated efficiently using rhodium-based ...