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Addition of a carbene to an alkene to form a cyclopropane. One of the most synthetically important cheletropic reactions is the addition of a singlet carbene to an alkene to make a cyclopropane (see figure at left). [1] A carbene is a neutral molecule containing a divalent carbon with six electrons in its valence shell.
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]
Carbene intramolecular reaction Carbene intermolecular reaction. The 1,2-rearrangement produced from intramolecular insertion into a bond adjacent to the carbene center is a nuisance in some reaction schemes, as it consumes the carbene to yield the same effect as a traditional elimination reaction. [16]
In this reaction type either the two carbenic intermediates react or a carbenic intermediate reacts with a carbene precursor. [1] An early pioneer was Christoph Grundmann reporting on a carbene dimerisation in 1938. [2] In the domain of persistent carbenes the Wanzlick equilibrium describes an equilibrium between a carbene and its alkene.
Several synthetic routes to cyclopropyl and cyclopropenyl compounds involve the cycloaddition of a metal carbene to an alkene or alkyne. [ 4 ] [ 5 ] [ 6 ] Metal-stabilized allyl and pentadienyl complexes are used in [4+3] and [5+2] cycloadditions for preparing seven-membered rings.
The direct [2+2] cycloaddition of two alkenes is formally symmetry forbidden and thus has a high activation energy. The Chauvin mechanism involves the [2+2] cycloaddition of an alkene double bond to a transition metal alkylidene to form a metallacyclobutane intermediate.
The configuration of the product is determined by the trajectory of approach of the olefin to the metal carbene. In reactions of monosubstituted metal carbenes with terminal olefins, the olefin likely approaches "end-on" (with the carbon-carbon double bond of the olefin nearly parallel to the metal-carbon double bond of the carbene) with the olefin R group pointed away from the substituent of ...
Enders carbene would become the first commercially available carbene. [6] These carbenes, however, were still expensive, as noted by Färber et al. [5] Following this commercialization and dissemination, many analogues of the 1,2,4-triazol-5-ylidene system have been reported and utilized, most often as transition metal coordination compounds.