Arenes carbon

The infrared and proton NMR spectra of the vanadium carbonyl cations have been reported (28, 29, 32). The investigation of the reactivity of these cations with nucleophiles is limited to a study of the hydridic reduction of the tetracarbonyl species (31). A mechanism is proposed which involves primary coordination of hydride to vanadium the hydrogen then transfers to an arene carbon atom to form a methylene group, with the hydridic hydrogen in the endo position, viz., The product is 77-cyclohexadienyl-vanadium tetracarbonyl. 

In the literature there are reactions of DMC with what would be considered ambident nucleophiles. For instance, indole is an ambident nucleophile. Using various organic chemistry procedures, reactions can either occur at the nitrogen or the conjugated arene carbon." In the field of DMC chemistry, Shieh and co-workers were able to selectively methylate the nitrogen using DABCO as a catalyst in contrast to other systems that effected carboxymethylation at the same nucleophilic centre. " Also cyclic ketones have been successfully reacted witli DMC to produce acyclic diesters via carboxymethylation at the alpha carbon. In these examples, however, the ambidenticity of these systems toward DMC has not been shown (Scheme 12). Only activation of one of tlie two centres has occurred. 

Jones WD, Feher FJ (1984) The mechanism and thermodynamics of alkane and arene carbon-hydrogen bond activation in (C3Me3)Rh(PMe3)(R)H. J Am Chem Soc 106 1650-1663... 

Aoyama Y, Yoshida T, Sakurai K-i, Ogoshi H (1983) Activation of arene carbon-hydrogen bonds. Direct electrophilic aromatic metalation with a rhodium-porphyrin complex. J Chem Soc Chem Commun 478-479... 

Arencarbonsaure-Schwefelsaure-fluorid-Anhydride entstehen aus Aren-carbon-saure-fluoriden mit Schwefeltrioxid133 ... 

Dean et al. (65) have prepared organometallic mercury (2+) compounds of the type Hg2(AsF6)a Arene. The NMR spectra of these complexes exhibit averaged arene carbon signals due to rapid free-complexed arene exchange at NMR probe temperatures. On the basis of the variation of the aryl carbon chemical shift with the arene/Hg22 ratio for the system Hg2(AsFe)2-hexa-methylbenzene (HMB) the existence of the following two equilibria was determined ... 

Several research groups have also used theoretical methods in an effort to understand the activating and deactivating effects of the substituents in S Ar reactions. For example, Galabov and coworkers have developed a computational approach for determining electrophile affinity, Ea, as a measure to determine arene reactivity and positional selectivity in S Ar reactions [36]. Other recent approaches to this problem include the development of reactive hybrid orbital analysis [37], the topological analysis of electron localization function [38], the calculations of electrostatic potentials at the arene carbons [39], and several other methods. A comprehensive summary of this area is beyond the scope of this chapter however, the interested reader may consult one of the recent reviews of this topic [40]. 

Phenols can be viewed as stable forms of enol tautomers, and phenolate anions display ambident nucleophilicity at oxygen as weU as C2/C6 and C4 (ortholpara positions). Consequently, phenolate anions are susceptible to C—C bond formation upon reaction with appropriate organic electrophiles (e.g., alkyl halides and sulfonates). When bond formation occurs at a substituted arene carbon, a quatonaty centCT is generated, which may lead to isolation of stable cyclohexadienone products and complete a net alkylative dearomatization (Scheme 15.1) [2]. 

Sevaal metal-catalyzed dearomatization sequences have been developed that rely upon the electro-phUicity of jt-aUyl metal complexes to initiate carbon-carbon bond formation with arene nucleophiles. When alkylation of jt-aUyl metal intemediates takes place at a substituted arene carbon, then a quatonaiy centCT is genCTated, thCTcby preventing subsequent rearomatization. Ranote electron-donating substituents (such as phenohc residues) help to increase and direct the reactivity of arene substrates in these transformations. 

The synthesis of a-aryl substituted ketones has been the subject of intense research over the past 35 years. Several old methodologies emerged in the past to perform the synthesis of these useful compounds, including the reaction of an enolate with a derivative of benzyne affording an arene carbon bond at the a-position of a carbonyl compound [7], A number of specific main group aryl reagents have also been reported for the synthesis of a-arylated ketones. Among them were included... 

Rose et a/. have examined the regioselectivity of addition of the carbanion CH(CH3)CN to the ort/io-disubstituted arene 34 when coordinated to Cr(CO)3 or FeCP units. In the former case addition is fairly regioselective. The major product (74%-79%) after oxidative removal of the metal corresponded to attack at an arene carbon atom which is eclipsed with respect to a carbonyl group of the Cr(CO)3 unit of the most stable conformer. " In contrast, carbanion addition to the arene when attached... 

Extended Hiickel M.O. calculations suggest that the regioselectivity of electrophilic attack upon the arene ligand in [Cr(CO)3(T]-arene)] complexes is controlled not only by the ring substituent, but also by the conformation of the Cr(CO)3 unit. Electrophilic substitution is preferred at arene carbons which are staggered with respect to the carbonyl groups. 

The 19-electron complexes [Fe Cp(T] -arene)] have a modest stability (up to -10°C) if the arene is not CsRe (R = Me or Et) or l,3,5-C6H3t-Bu3. For instance, with C6Me6.nHn (n < 6), they dimerize in pentane around 0°C via an unsubstituted arene carbon, which is taken into account by the mesomer 18-electron structure below. As often in organotransition-metal chemistry, the frontier between mesomerism and tautomerism is fuzzy. From the point of view of the strict organic definition, the different forms should be tautomers, because carbon atoms are changing place. In reality, however, there may be only one potential well for the three forms with a too low conversion barrier, and the three forms are probably best viewed as mesomers since they are not observable as distinct species at the infrared time scale (10 /s) ... 

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