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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).
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.
Carbene addition to alkenes. Singlet and triplet carbenes exhibit divergent reactivity. [11] [page needed] [12] Triplet carbenes are diradicals, and participate in stepwise radical additions. Triplet carbene addition necessarily involves (at least one) intermediate with two unpaired electrons.
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 ]
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.
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.
Strained alkenes like trans-cycloheptene derivatives have also been reported to react in an antarafacial manner in [2 + 2]-cycloaddition reactions. Doering (in a personal communication to Woodward) reported that heptafulvalene and tetracyanoethylene can react in a suprafacial-antarafacial [14 + 2]-cycloaddition. However, this reaction was later ...
An enyne metathesis is an organic reaction taking place between an alkyne and an alkene with a metal carbene catalyst forming a butadiene. This reaction is a variation of olefin metathesis. [1] The general scheme is given by scheme 1: When the reaction is intramolecular (in an enyne) it is called ring-closing enyne metathesis or RCEYM (scheme 2):