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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.
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.
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. [16]
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. In one example, intramolecular participation of an aryl group leads to the formation of a polycyclic ring system with complete diastereoselectivity ...
Examples of intramolecular reactions with a tethered alkyne [11] and intermolecular reactions with a nontethered alkyne [12] both exist with use of a nickel or rhodium catalyst. With the six-membered alkyl metal enolate intermediate, dimerization [ 13 ] [ 14 ] or reaction with an added alpha-beta unsaturated ketone [ 15 ] yields a 1,3 ...
One theory invokes σ-aromaticity: the stabilization afforded by delocalization of the six electrons of cyclopropane's three C-C σ bonds to explain why the strain of cyclopropane is "only" 27.6 kcal/mol as compared to cyclobutane (26.2 kcal/mol) with cyclohexane as reference with E str =0 kcal/mol, [18] [19] [20] in contrast to the usual π ...
An example of the former, cyclopropyl cyanide is prepared by the reaction of 4-chlorobutyronitrile with a strong base. [1] Phenylcyclopropane is produced analogously from the 1,3-dibromide. [2] A second major route to cyclopropanes entails addition of methylene (or its substituted derivatives) to an alkene, a process called cyclopropanation. [3]
The Charette modification replaces the CH 2 I 2 normally found in the Simmons–Smith reaction with aryldiazo compounds, such as phenyldiazomethane, in Pathway A. [30] Upon treatment with stoichiometric amounts of zinc halide, an organozinc compound similar to the carbenoid discussed above is produced. This can react with almost all alkenes and ...