Arenes nucleophiles

Triethyl orthoformate or chloroform can react with arene nucleophiles to give triphenylmethanes with three identical aryl groups.5,52,67 In addition, dialkylarylamines, when treated with dialkoxycarbenium tetrafluoro-borates under thermodynamic conditions or with triethyl orthoformate/zinc chloride in ether under anhydrous conditions, give triarylmethanes.68 For example, 4-methoxycarbazole and triethyl orthoformate in the presence of acid catalyst give 44 in 66% yield69 (Scheme 7). In general, phenolic or... 

Perfluoroarenes were also found to be highly reactive coupling partners in intermolecular direct arylation [68, 69]. A wide range of aryl halides can be employed, including heterocycles such as pyridines, thiophenes, and quinolines. A fluorinated pyridine substrate may also be cross-coupled in high yield and it was also found that the site of arylation preferentially occurs adjacent to fluorine substituents when fewer fluorine atoms are present. Interestingly, the relative rates established from competition studies reveal that the rate of the direct arylation increases with the amount of fluorine substituents on the aromatic ring. In this way, it is inversely proportional to the arene nucleophilicity and therefore cannot arise from an electrophilic aromatic substitution type process (Scheme 7). 

In 2007, the Fagnou group achieved a much more practical and selective Ar-H/ Ar-H cross-coupling [50]. Electron deficient palladium(II) complexes can react via an electrophilic C-H activation mechanism with good selectivity for electron rich arenes. In contrast, Fagnou [51] recently showed that complimentary reactivity to this is displayed by some ArPd(II) complexes that react through a proton-transfer-palladation mechanism, and that they depend on arene C-H acidity rather than arene nucleophilicity (Scheme 31). 

According to this scheme the step 2 is realized by means of electrochemical oxidation of intermediate o-complexes through the loss of one electron (Sn reaction) or two electrons (Sn reaction). These electrochemical oxidations proved to occur at different potentials (depending on the starting arene, nucleophile, and type of the intermediate o-complex). It is worth to note that the oxidation peak potentials for o -complexes are higher than those for the corresponding o -com-plexes, and by using the correct potential one can obtain the desired substitution product. 

Purpose To demonstrate alkylation by electrophilic aromatic substitution and to assess the relationship between the extent of carbocation rearrangement and arene nucleophilicity. 

As described earlier, the S Ar involves the reaction of an electrophilic species with an arene nucleophile. There are several types of arenes common to the S Ar reactions substituted benzenes, polycyclic aromatic compounds, and heterocyclic compounds. Substituent effects largely control the chemistry of substituted benzenes and related compounds. This includes both activating and directing effects of substituents on the S Ar reaction. 

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. 

A highly electrophilic phosphite gold(I) catalyst was applied to the intramolecular allene hydroarylation reaction to give a vinyl-substituted carbocycle [17], This cyclization proceeds effectively in cases where the aryl groups are electron-rich arene nucleophiles. A dimethoxybenzene bearing an allene group furnished a vinyl-substituted tetralin derivative in 85% yield (Scheme 18.18). From the mechanistic... 

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