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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.
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 ...
Diazomethane is an organic chemical compound with the formula CH 2 N 2, discovered by German chemist Hans von Pechmann in 1894. It is the simplest diazo compound.In the pure form at room temperature, it is an extremely sensitive explosive yellow gas; thus, it is almost universally used as a solution in diethyl ether.
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.
Definitive mechanistic studies of rhodium-catalyzed cyclopropanation are lacking. However, the mechanism has been rationalized based on product distribution and stereoselectivity. [4] Attack of the diazo compound on the metal center generates a zwitterionic metal alkyl complex, which expels nitrogen gas to afford a metal carbene intermediate.
Diazoketones containing pendant double bonds undergo cyclopropanation in the presence of copper. The key step in one synthesis of barbaralone is the selective intramolecular cyclopropanation of a cycloheptatriene. [10] (3) α,β-Cyclopropyl ketones may act as masked α,β-unsaturated ketones.
Cyclopropanation with conjugated diazo compounds produces divinylcyclopropanes that then undergo rearrangement. When cyclic starting materials are used, bridged products result. [12] (8) Substrates containing three-membered heterocyclic rings can also undergo the reaction. cis-Divinylepoxides give oxepines at elevated temperatures (100 °C).
A second major route to cyclopropanes entails addition of methylene (or its substituted derivatives) to an alkene, a process called cyclopropanation. [3] Substituted cyclopropanes undergo the reactions associated with the cyclopropyl ring or the substituents. Vinylcyclopropanes are a special case as they undergo vinylcyclopropane rearrangement.