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A singlet carbene contains an empty p orbital and a roughly sp 2 hybrid orbital that has two electrons. Singlet carbenes add stereospecifically to alkenes, and alkene stereochemistry is retained in the cyclopropane product. [1] The mechanism for addition of a carbene to an alkene is a concerted [2+1] cycloaddition (see figure).
Carbene radicals, in which the carbene is bonded to an open-shell metal with the carbene carbon possessing a radical character. Carbene radicals have features of both Fischer and Schrock carbenes, but are typically long-lived reaction intermediates. The "second generation" of the Grubbs catalysts for alkene metathesis features an NHC ligand.
Dichlorocarbene reacts with alkenes in a formal [1+2]cycloaddition to form geminal dichlorocyclopropanes. These can be reduced to cyclopropanes or hydrolysed to give cyclopropanones by a geminal halide hydrolysis. Dichlorocyclopropanes may also be converted to allenes in the Skattebøl rearrangement.
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.
A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. [1] Carbene complexes have been synthesized from most transition metals and f-block metals , [ 2 ] using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. [ 1 ]
Hérisson and Chauvin first proposed the widely accepted mechanism of transition metal alkene metathesis. [12] 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 ...
Bonding scheme of carbene radical complexes as compared to Schrock and Fischer-type carbene complexes. Carbene radicals are a special class of organometallic carbenes.The carbene radical can be formed by one-electron reduction of Fischer-type carbenes using an external reducing agent, or directly upon carbene formation at an open-shell transition metal complex (in particular low-spin cobalt(II ...
Bidentate CAACs featuring alkene, ether, amine, imine, and phosphine donors have been reported. [16] Bidentate CAAC-Au(I) complexes have been used for C-C oxidative addition of biphenylene. [16] Bidentate CAAC-Cu(I) compelxes have shown promise as catalysts for hydroarylation and anti-Markovnikov hydrohydrazination reactions. [16]