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In with an empty p-orbital extending above and below a plane containing R1 and R2 and the Free Electron Pair . Typically these molecules are very short lived, although Persistent Carbene s are now known. The parent carbene is H2C: also called which can be generated In Situ from Chloroform and a strong Base . STRUCTURE Generally there are two types of carbenes; Singlet or Triplet carbenes. Singlet carbenes have a pair of electrons and sp2 Hybrid Structure . Triplet carbenes have two unpaired electrons. They may be either sp2 hybrid or linear sp hybrid. Most carbenes have nonlinear triplet ground state with the exception of carbenes with nitrogen, oxygen, sulfur atoms, and dihalocarbenes. Singlet and triplet carbenes are named so because of the electronic ; 1983; 105; 6833-6845. Triplet carbenes are generally stable in gaseous state while singlet carbenes are often found in aqueous media. REACTIVITY Singlet and triplet carbenes do not demonstrate the same reactivity. Singlet carbenes generally participate in Cheletropic Reaction s as either Electrophile s or Nucleophile s. Singlet carbene with its unfilled p-orbital should be electrophilic. Triplet carbenes should be considered to be Diradicals , and participate in stepwise radical additions. Triplet carbenes have to go through an Intermediate with two unpaired electrons whereas singlet carbene can react in a single Concerted step. Addition of singlet carbenes to Olefin ic double bonds is more stereoselective than that of triplet carbenes. Addition reactions with alkenes can be used to determine whether the singlet or triplet carbene is involved. Reactions of singlet methylene are stereospecific while those of triplet methylene are not. For instance the reaction of methylene generated from Photolysis of Diazomethane with cis-2-butene and trans-2-butene is Stereospecific which proves that in this reaction methylene is a singlet.''Structure of Carbene CH2'' Philip S. Skell, Robert C. Woodworth; J. Am. Chem. Soc. ; 1956; 78(17); 4496-4497. Abstract Reactivity of a particular carbene depends on the Substituent groups, preparation method, reaction conditions such as presence or absence of Metal s. Some of the reactions carbenes can do are insertions into C-H bonds, skeletal rearrangements, and additions to double bonds. Carbenes can be classified as nucleophilic, electrophilic, or ambiphilic. Reactivity is especially strongly influenced by substituents. For example, if a substituent is able to donate a pair of electrons, most likely carbene will not be electrophilic. Alkyl carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds. Carbenes add to double bonds to form Cyclopropanes . A concerted mechanism is available for singlet carbenes. Triplet carbenes do not retain Stereochemistry in the product molecule. Addition reactions are commonly very fast and Exothermic . The slow step in most instances is generation of carbene. A well-known reagent employed for alkene-to-cyclopropane reactions is Simmons-Smith Reagent . This reagents is a system of Copper , Zinc , and Iodine , where the active reagent is believed to be iodomethylzinc iodide. Reagent is complexed by Hydroxy groups such that addition commonly happens Syn to such group. Insertions are another common type of carbene reactions. The carbene basically interposes itself into an existing bond. The order of preference is commonly: 1. X-H bonds where X is not carbon 2. C-H bond 3. C-C bond. Insertions may or may not occur in single step. Intramolecular insertion reactions present new synthetic solutions. Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no Intermolecular insertions are seen. In flexible structures, five-membered ring formation is preferred to six-membered ring formation. Both inter- and intramolecular insertions are amendable to asymmetric induction by choosing chiral ligands on metal centers. Alkylidene carbenes are alluring in that they offer formation of Cyclopentene moieties. To generate an alkylidene carbene a ketone can be exposed to Trimethylsilyl Diazomethane . CARBENES AND CARBENE LIGANDS IN ORGANOMETALLIC CHEMISTRY Carbenes can be stabilized as Organometallic species. These Transition Metal Carbene Complex es fall into three categories, with the first two being the most clearly defined:
GENERATION OF CARBENES
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SEE ALSO Transition Metal Carbene Complex also known as Carbenoids REFERENCES GALLERY |
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