Reactions Involving Carbenes and Related Intermediates

Substituents perturb the relative energies of the singlet and triplet states. In general, alkyl groups resemble hydrogen as a substituent and dialkylcarbenes are ground state 

Reactions Involving Carbocations, Carbenes, and Radicals as Reactive Intermediates 

The presence of more complex substituent groups complicates the description of carbene structure. Furthermore, since carbenes are high-energy species, structural entities that would be unrealistic for more stable species must be considered. As an example, one set of MO calculations109 arrives at structure I as a better description of carbomethoxycarbene than the conventional structure J. 

There are a number of ways of generating carbenes that will be discussed shortly. In some cases, the reactions involve complexes or precursors of carbenes rather than the carbene per se. For example, carbenes can be generated by a-elimination reactions. Under some circumstances the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. 

The stability and reactivity of metallocarbenes depends on the degree of back donation from the metal to the carbene. If this is small, the metallocarbenes are highly reactive and electrophilic in character. If back bonding is substantial, the carbon will be less electrophilic, and the reactions are more likely to involve the metal. 

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Related reactions involving carbene intermediates have been shown in organo-metallic chemistry and are exemplified in Scheme 6. which is self-explanatory. 

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

A related transformation to the previous carbene transfer reaction involves a nitrene ligand bonded to the metal center, in a metallonitrene intermediate in situ generated upon the appropriate selection of the catalyst and the nitrene precursor. As shown in Scheme 17, some transition metal complexes react with such a precursor to generate an unsaturated intermediate, generally electrophilic in nature, which might react with olefins or C—H bonds affording aziridines or amines in a catalytic manner. The most employed nitrene sources are hypervalent I(III) compounds such as PhI=NTs, chloramine-T or organic azides. 

The reaction involves generation of the caibanion of an activated halide (step i) and subsequent addition to an aldehyde or ketone to generate the diastereomeric aldolates (1 and 2 step ii), which cyclize (internal Sn2) to afford the stereoisomeric epoxides (3 and 4), respectively (step iii). The a-halo aldolates (1) and (2), or the related halohydrins, are not normally isolated, although this has been done with a-flu-oro esters as well as with a-chloro esters. The isolation of these products is amongst the evidence which rules out carbene intermediates, which were invoked in earlier mechanistic proposals. 

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