Carbenoids nucleophilic character

As mentioned earlier, the carbene ligand in our complexes shows nucleophilic character with respect to the metal fragment. Therefore, we decided to combine it with an electrophilic carbene. For this purpose we treated pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with phenyl(tri-chloromethyl) mercury (85). Compounds of this kind have been studied intensively by Seyferth et al. (86) and are known as a source of dihalogeno-carbenes. The carbene complex reacted with the carbenoid compound at... 

Ttie nucleophilic character of transition metal carbenoids has been demonstrated... 

Interestingly, cobalt porphyrin catalysts tend to prevent carbene dimerization reactions, and allow cyclopropanation reactions with electron-deficient alkenes. This feature illustrates the more nucleophilic behavior of the carbenoid species formed as compared to typical electrophilic Fischer carbenes. The enhanced nucleophilic character of the carbene reduces its tendency to dimerize and allows reactions with more electron-deficient olefins. 

We add an investigation of Chinese chemists to this section, although it is not related to carbenoid reagents that we have discussed above. Zhou et al. (1993) studied reactions of dimethyl diazomalonate and ethyl diazoacetate with carbonyl compounds mediated by diorganyl tellurides and catalytic amounts of cuprous iodide (8-69). Dibutyl telluride (8.155) yields the dimethyl l-arylethene-2,2-dicarboxylate 8.157 with 4-chlorobenzaldehyde in 95<7b yield at 100 °C. It is assumed that the reaction passes the telluronium ylide 8.156 as intermediate. If so, the process is clearly different from the carbenoid transformations discussed in this section. The originally diazo-substituted C-atom has nucleophilic character in 8.156 and is not electrophilic, as in 8.104. 

The electrophilic reactivity of lithium carbenoids (reaction b) becomes evident from their reaction with alkyl lithium compounds. A, probably metal-supported, nucleophilic substitution occurs, and the leaving group X is replaced by the alkyl group R with inversion of the configuration . This reaction, typical of metal carbenoids, is not restricted to the vinylidene substitution pattern, but occurs in alkyl and cycloalkyl lithium carbenoids as well ". With respect to the a-heteroatom X, the carbenoid character is... 

Whereas carbenoid character is definitely present in metalated alkyl vinyl ethers, lithiated alkyl and aryl vinyl sulfides and thioesters, which are easily available by hydrogen-lithium exchange, do not display carbenoid-typical reactions . They rather behave like nucleophilic reagents, so that their discussion is beyond the scope of this overview despite their utility in synthesis The same appiies to various derivatives of enamines, deprotonated in the vinyiic a-nitrogen position - . 

A similar type of substitution, which clearly shows the electrophilic character, occurs in vinylidene carbenoids. In an early example of this reaction, Kobrich and AnsarP observed that the aUcene 70 results when the fi-configurated vinyl lithium compound 68 is treated with an excess of butyllithium and the fithioafkene 69 formed thereby is protonated (equation 41). Obviously, the nucleophilic attack of the butyl residue on the carbenoid takes place with inversion of the configuration. 

The electron-deficient double bond in 2-phenylsulfonyI-l,3-dienes underwent regioselective cyclopropanation with sulfur ylides in very good yield, e.g. cyclopropanation of 13, 14 and 15 with sulfur ylides.On the other hand, the use of zinc-copper couple and diiodomethane afforded a 83 17 mixture of vinylcyclopropanes 16 and 17 from 2-(phenylsulfonyl)cyclohexa-1,3-diene (15) in moderate yield. The reaction at the electron-deficient double bond rather than at the expected electron-rich double bond shows that the Simmons-Smith carbenoid is fairly nucleophilic in character. 

Before we leave this section on cyclopropanes, we want you to take a step back from simply thinking about carbenes, and consider the types of reagents that form three-membered rings generally. They all have something in common, which we could call -enoid character. Cyciopropanes form when a carbene (which, in the singlet state, has an empty, electrophilic p orbital and a full, nominally nucleophilic sp orbital) attacks alkenes. The Simmons-Smith carbenoid is not a carbene, but nonetheless has a carbon atom with joint nucleophilic (alkyl zinc) and electrophilic (alkyl iodide) character. When... 

The carbenes or carbenoids can be generated in a variety of ways. It is not always clear whether the reaction is concerted or stepwise and whether the carbene behaves as an electrophilic, nucleophilic or radical species. For instance, a carbenoid generated from bismuthonium ylide (19) in the presence of copper(I) chloride would behave as a triplet and add as a radical to a terminal alkyne" (equation 17). The dicarbonyl structure and the absence of reaction with methyl propionate to a furane might well indicate electrophilic character of this carbene. 

The intramolecular carbolithiation of a series of chloro-substituted alkynes led to exocyclic alkylidene carbenoids, of which both nucleophilic and electrophilic characters could be derived (14CEJ10249). 

The enhanced nucleophilicity of coordinated CO2 in complex [Fe(C02)(depe)2] allowed reaction with organotin chlorides. In the resulting carboxylate complexes 31, the CO2 unit links the two metals via a fi-ij C) if(P,0 ) bonding mode. Although the X-ray data of 31b exclude the contribution of a carbenoid form in the solid state, chemical shifts of IX-CO2 in the NMR spectra indicate a borderline case between oxycarbonyl and dioxycarbene character... 

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