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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 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.
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.
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.
This can react with almost all alkenes and alkynes, including styrenes and alcohols. This is especially useful, as the unmodified Simmons-Smith is known to deprotonate alcohols. Unfortunately, as in Pathway B shown the intermediate can also react with the starting diazo compound, giving cis - or trans - 1,2-diphenylethene.
The disadvantages of the reaction involve side reactions of the carbene moiety. The choice of solvent for the reaction needs to be considered. In addition to the potential for carbon-hydrogen bond insertion reactions, carbon-halogen carbene insertion is possible when dichloromethane is used as the solvent. [20] C-Cl bond insertion
Carbene dimerization is a type of organic reaction in which two carbene or carbenoid precursors react in a formal dimerization to an alkene.This reaction is often considered an unwanted side-reaction but it is also investigated as a synthetic tool.