Carbanions arenes

We examined the possibility of a direct formation of two C-C bonds by reaction of a carbanion with [Fe(arene)2]2+ in which the arene bears methyl groups. We could indeed repeat Hellings s experiments but found that mesitylene was the only aromatic allowing this possibility in reasonable yields. With p-xylene, a low yield of an unstable complex was obtained corresponding to double nucleophilic attack of phenyllithium on the same ring in spite of the bulk of the methyl groups [23]. Eq. (4) ... 

The nucleophilic substitution of the nitro group in nitro-arene complexes works almost as well as that of Cl" and such substitutions were achieved by Chowdhurry et al. with O, S, and N nucleophiles and with stabilized carbanions [97,98] Eq. (28) and Table 8. 

When the nucleophile is a stabilized carbanion such as the enolate of acetylacetone, 1-benzoylacetophenone, diethylmalonate, or ethyl acetatoacetone, the reaction proceeds similarly. The monosubstituted complex is isolated as long as it contains an acidic hydrogen in the benzylic position. In addition, for the case of diketones CH2(COR)2 (R = Me, Ph, OEt), a deacetylation is observed in an acidic medium [92,93]. These features are the same as described above in the case of the substitution of Cl by stabilized carbanions in monochloroaromatics (the second chlorine being an inert arene substituent [99] Scheme XVII, Eq. (31) and Tables 10 and 11. 

The chemistries of the benzyiic and allylic positions are very similar. Intermediate carbocations, free radicals and carbanions formed at these positions are stabilized by delocalization with the adjacent ir system, the benzene ring in the case of the benzyiic position. Another aspect of arene chemistry is the enhanced stability of unsaturated arenes having double bonds conjugated with the benzene ring. This property is akin to the stability of conjugated di- and polyenes. 

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

Soft and polarizable nucleophiles, e.g., azide, thiol anions, and phenoxides, add readily to arene oxides, whereas harder anions like carbanions and... 

The synthetic application of vicarious nucleophilic substitution, whereby hydrogen of an electrophilic arene is replaced by an a-functionalized alkyl substituent, has been reviewed 177 the sequence usually involves attack on a nitroalkene by a carbanion containing a leaving group X at the carbanionic centre, /i-elimination of HX from the er-adduct, and rearomatization on subsequent protonation. 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

For other polysubstituted arenes or heteroarenes, deprotonation at different sites can compete and yield product mixtures. The first reaction in Scheme 5.46 is an example of kinetically controlled carbanion formation, which shows that for some substrates regioselective metalations might be achieved by careful control of the reaction conditions. 

The second important use of superbases is side-chain alkylation of aromatic compounds [22, 34]. In these reactions a benzyl anion generated by the superbase catalyst subsequently attacks olefins such as ethene or propene as a nucleophile. The result of such a nucleophilic addition of a carbanion is side-chain alkylation of the arene by ethene. The reaction was commercialized by Sumitomo for the side-chain alkylation of cumene (Scheme 5, a) [34]. 

Synthesis of the closely related acyclic (19) and macrocyclic (20) polyradicals has recently been reported (Figure 5.1).1231 The -conjugated carbanions (e.g., the calix[4]-arene-based tetraanion and the related calix[3]arene-based trianion) were synthesized and studied.1241 Oxidation of these tetra- and tri-anions gave the corresponding tetra- and tri-radicals, respectively. It has been shown in closely related systems that it is not the shape or overall geometric symmetry of the molecules, but rather it is the juxtaposition of the carbenic centers within the jt-cross-conjugated structure, that is most important in determining the spin multiplicity of the alternant hydrocarbon molecule.1251... 

The addition of two nucleophiles to a coordinated //6-arene is a synthetically important goal. A one-pot synthesis of 1,3-disubstituted cyclohexadienes involving an initial ipso addition to fluoroarene complexes is possible. Indeed, p-fluorotoluenetricarbonylchromium complex 22c reacts with isobutyronitrile carbanion (2 equiv.) in THF to give, after 5 days at —30 °C and acidic treatment under CO atmosphere, the cyclohexadiene 66 in 47 % yield (Scheme 31) [53] the yield can reach 75 % after several weeks Carbon monoxide was used in order to decoordinate the // -cyclohexadiene intermediate and to recover Cr(CO)6 needed for the preparation of the starting material [54]. 

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