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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). Carbenes derived ...
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.
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.
Concerted addition of the metal carbene to the olefin (without direct coordination of the olefin to the metal) generates the observed cyclopropane product. [5] The configuration of the olefin is retained throughout the process; [ 6 ] however, metal carbenes with heterotopic faces may generate a mixture of diastereomers, as shown at the right of Eq.
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]
The Buchner ring expansion reaction was first used in 1885 by Eduard Buchner and Theodor Curtius [1] [2] who prepared a carbene from ethyl diazoacetate for addition to benzene using both thermal and photochemical pathways in the synthesis of cycloheptatriene derivatives.
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 and [5+2] cycloadditions for preparing seven-membered rings. [7]