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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.
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 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] The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. [2]
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 ...
Bonding between carbene and the metal centre involves a strong σ donation from sp 2 orbital to an empty d orbital on metal centre and a weak π back donation from the metal centre to the empty p z orbital. Because the π donation is weak, the carbene carbon is electrophilic in nature. [1] Because of this bonding property, Fischer carbenes ...
The net reaction gives a transition metal carbene complex: L n M≡CR + HX → L n (X)M=CHR. These complexes can also undergo photochemical reactions. In some carbyne complexes, coupling of the carbyne ligand to a carbonyl is observed. Protonation of the carbyne carbon and conversion of the carbyne ligand into a π-allyl. [13]
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The carbene compound may include a bioorthogonal tag or handle by which the protein of interest can be isolated. The protein can then be digested and sequenced by mass spectrometry in order to identify which residues the carbene containing ligand is bound to, and hence the identity of the binding site in the receptor.