Bonding of Carbenes

The bonding of a triplet carbene is less closely related to the metal-ligand bonds discussed thus far. The carbenes in these complexes are considered dianionic. With this assignment of electrons and charges, the a- and ir-orbitals of this dianionic fragment can be considered to bind the metal by donation of two electron pairs, one into each of the unoccupied orbitals of the metal of a- and ir-symmetry. These carbene ligands have ir-bonds that are stronger than those in Fischer carbenes, but much weaker than those in alkenes. The barrier to rotation in these complexes is t5q)ically 19 kcal/mol. -  

Relative to phosphines, however, the carbenes are stronger a-donors, have stronger M-Lbond strengths, and have steric properties that are distinct from those of a ligand with 

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One of the characteristic reactions of free carbenes is their insertion into the C —H bond. So far, examples of insertions into the C — H bonds of carbenes which are coordinated to the metal are not known. However, such carbenes may insert into more reactive M —H bonds where M = Si, Ge, or Sn. ... 

Hydrolysis of the Cr=C bonds of carbenes Cr(CO)s =C(OMe)R (R = alkenyl, Ar) with hexamethylenetetramine (urotropine) results in the amine complex Cr(CO)5(urotropine) and the corresponding aldehydes RCHO. The alkynyl derivative Cr(CO)s =C(OEt)CsCPh behaves differently in its reaction with urotropine and the result is complex 107 arising from the fragmentation of the urotropine skeleton. 3... 

Another common set of ligands are alkylidenes and alkylidynes. These are defined as carbon ligands that make double and triple bonds to metals, respectively. They can be viewed as involving the bonding of carbenes (ICR2) and methines ( CR), respectively, to metals. An alkylidene contributes two electrons to the metal count, one each from the a and TT bonds. An alkylidyne contributes three electrons to the metal count. 

The reactions of carbenes, which are apparently unique in displaying electrophilic character in strongly basic solutions, include substitution, addition to multiple bonds, and co-ordination with lone pairs of electrons to form unstable ylides. This last reaction is of obvious relevance to a consideration of the reactions of heterocyclic compounds with carbenes and will be summarized. 

Cyclic Si-, Ge-, Sn- and Pb-analogs of carbenes and alkenes 99EJI373. Germylenes and germanium double-bonded species in heterocyclic organoger-manium chemistry 99KGS1155. 

By application of the Simmons-Smith reaction it is possible to synthesize a cyclopropane from an alkene by formal addition of carbene to the carbon-carbon double bond, without a free carbene being present in the reaction mixture the... 

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

Photolytically generated carbene, as mentioned above, undergoes a variety of undiscriminated addition and insertion reactions and is therefore of limited synthetic utility. The discovery (3) of the generation of carbenes by the zinc-copper couple, however, makes carbene addition to double bonds synthetically useful. The iodo-methylzinc iodide complex is believed to function by electrophilic addition to the double bond in a three-center transition state giving essentially cis addition. Use of the... 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

In a similar process, tertiary enaminones react with alkynylcarbene complexes to give the corresponding pyranylidene complexes following a reaction pathway analogous to that described above. First, a [2+2] cycloaddition reaction between the alkynyl moiety of the carbene complex and the C=C double bond of the enamine generates a cyclobutene intermediate, which evolves by a conrotatory cyclobutene ring opening followed by a cyclisation process [94] (Scheme 49). 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Woodworth, based on the common reaction of addition of carbenes to double bonds to form cyclopropane derivatives (15-50). If the singlet species adds to cis-2-butene, the resulting cyclopropane should be the cis isomer since the movements of the two pairs of... 

An unusual reaction of carbenes is that of insertion into C—H bonds (12-19). Thus. CH2 reacts with... 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide. The reaction has been carried out with R = aryl, cyano, EtOOC, and RSO2, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (15-62). Reaction of NsONHC02Et/ CuO [Ns = A(/7-toluenesulfonyl-inimo)] and a conjugated ketone, for example, leads to the A-carboethoxy aziridine derivative.Calcium oxide has also been used to generate the nitrene.Other specialized reagents have also been used." ... 

The Addition of Carbenes and Carbenoids to Double and Tripie Bonds... 

Carbenes and substituted carbenes add to double bonds to give cyclopropane derivatives ([1 -f 2]-cycloaddition). Many derivatives of carbene (e.g., PhCH, ROCH) ° and Me2C=C, and C(CN)2, have been added to double bonds, but the reaction is most often performed with CH2 itself, with halo and dihalocarbenes, " and with carbalkoxycarbenes (generated from diazoacetic esters). Alkylcarbenes (HCR) have been added to alkenes, but more often these rearrange to give alkenes (p. 252). The carbene can be generated in any of the ways normally used (p. 249). However, most reactions in which a cyclopropane is formed by treatment of an alkene with a carbene precursor do not actually involve free carbene... 

Carbenes are so reactive that they add to the double bonds of aromatic rings. The products are usually not stable and rearrange to give ring expansion. Carbene reacts with benzene to give cycloheptatriene ... 

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