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The mechanism for addition of a carbene to an alkene is a concerted [2+1] cycloaddition (see figure). Carbenes derived from chloroform or bromoform can be used to add CX 2 to an alkene to give a dihalocyclopropane, while the Simmons–Smith reagent adds CH 2 .
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 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.
Alkylidenes and other carbene analogs participate readily in cycloaddition reactions. Cycloaddition reactions of Ruthenium phosphinidenes with alkenes and alkynes is an active area research and has promise as catalytic cycle for hydrophosphination. [8] [9] Cycloaddition of a Ruthenium phosphinidene with an alkyne
Metal-catalyzed cyclopropanations are chemical reactions that result in the formation of a cyclopropane ring from a metal carbenoid species and an alkene. [1] In the Simmons–Smith reaction the metal involved is zinc. Metal carbenoid species can be generated through the reaction of a diazo compound with a transition metal).
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]
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.
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.