Carbenes


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Carbenes

 

unstable organic compounds containing an electrically neutral divalent carbon atom R’R“C: (the dots denote two electrons); they are intermediate particles in many organic reactions. The simplest carbene, methylene, :CH2, is formed in thermal or photochemical dissociation of diazomethane or ke-tene:

CH2N2→:CH2 + No

Other carbenes may be produced analogously. Dihalocarbenes form during thermal dissociation of alkali salts of trihaloacetic acids:

CCl3COONa → :CC12 + NaCl + CO2

Various methods are used for stabilizing carbenes, depending on the conditions of generation and the nature of the compounds interacting with the carbenes. For example, the following can occur: (1) dimerization of carbenes, :CH2 + :CH 2→CH2═CH2; (2) intrusion of carbenes through the carbon-hydrogen bond (for example, into hydrocarbons), R−H + :CH2 → R− CH3; or (3) combination of carbenes by a multiple bond (for example, ethylene), with the formation of a three-member ring,

The last reaction is widely used in the synthesis of various three-member cyclic compounds.

REFERENCE

Knuniants, I. L., N. P. Gambarian, and E. M. Rokhlin. “Karbeny.” Uspekhi khimii, 1958, vol. 27, issue 12, p. 1361.

B. L. DIATKIN

References in periodicals archive ?
Colmenares, "A two-step reaction scheme leading to singlet carbene species that can be detected under matrix conditions for the reaction of Zr(3F) with either C[H.sub.3]F or C[H.sub.3]CN," Journal of Computational Chemistry, vol.
Herrmann, "N-heterocyclic carbenes: a new concept in organometallic catalysis," Angewandte Chemie International Edition, vol.
Chemists describe the current knowledge about N-heterocyclic carbenes and their applications.
Furstner, "The first crystal structure of a reactive dirhodium carbene complex and a versatile method for the preparation of gold carbenes by rhodium-to-gold transmetalation," Angewandte Chemie International Edition, vol.
With the help of some talented graduate students and postdoctoral fellows, that inquiry made a key distinction between the role of N-heterocyclic carbene (NHC) and phosphine (PH3), which many observers had thought were equivalent participants in the cross-coupling.
ATTEMPTED SYNTHESES OF ARYL-SUBSTITUTED N-HETEROCYCLIC CARBENES: DIFFICULTY IN VEERING OFF THE BEATEN SYNTHETIC PATH.
They made a series of seven new compounds, called caffeine-based gold (I) N-heterocyclic carbenes, in the laboratory and studied them.
Toth, "Formation and stability of N-heterocyclic carbenes in water: the carbon acid [pK.sub.a] of imidazolium cations in aqueous solution," Journal of the American Chemical Society, vol.
In order to get functionalized with these active molecules, CNTs allow side-wall covalent attachment of functional groups by the addition of radicals, nucleophilic carbenes, nitrenes, nucleophilic cyclopropanation, and electrophiles [79, 83, 84].
Diazo-compounds are known to yield carbenes by heating, catalytic decomposition or ultraviolet (UV) irradiation (23), (24).
Thus, the change of ligand increases the catalytic ability in the silver complex, similar to some ligands based on N-heterocyclic carbenes [30-32], phosphines [33, 34], phosphoramidites [35], or phosphinites [36] which stabilize and improve the catalytic power in some copper complexes.
(1996) Main group element analogues of carbenes, olefins, and small rings.