Arenes electrophilic attack

Other metals capable of electrophilic substitution of C-H bonds are salts of palladium and, environmentally unattractive, mercury. Methane conversion to methanol esters have been reported for both of them [29], Electrophilic attack at arenes followed by C-H activation is more facile, for all three metals. The method for making mercury-aryl involves reaction of mercury diacetate and arenes at high temperatures and long reaction times to give aryl-mercury(II) acetate as the product it was described as an electrophilic aromatic substitution rather than a C-H activation [30],... 

The formation of six-membered or larger rings by intramolecular C-H bond insertion normally requires the attacked position to be especially activated towards electrophilic attack [1157,1158]. Electron-rich arenes or heteroarenes [1159-1162] and donor-substituted methylene groups can react intramolecularly with electrophilic carbene complexes to yield six- or seven-membered rings. Representative examples are given in Table 4.8. 

At present, we do not completely understand why only some of these very similar m-xylyl dicopper(I) complexes systems described above undergo ligand oxygenation reactions. However, based on the results outlined above, we can speculate on a number of aspects of this 02-activation process. Our studies implicate the presence of a copper-dioxygen (peroxo dicopper(II)) adduct as an intermediate in the oxygenation reaction and more recent kinetic studies (51) further support this conclusion. This adduct then either directly or via some further intermediate undergoes an electrophilic attack of the arene. The unique nature of this very fast reaction 2->3, and the observed inability to intercept the active... 

Like alcohols, arenes can attack the electrophilic a-position of metal vinylidenes (see Section 9.4.6). Substrate IIS was transformed into tetracycle 117 in high yield, presumably via 6it-electrocyclization and subsequent rearomatization (Equation 9.10). To date, no intermolecular examples of metal alkenylidene-mediated catalysis have come to light. The extension of Lee s alkylative approach to catalysis by other metals may prove fmitfiil in this regard. 

Step (1) is reminiscent of electrophilic addition to an alkene. Aromatic substitution differs in that the intermediate carbocation (a benzenonium ion) loses a cation (most often to give the substitution product, rather than adding a nucleophile to give the addition product. The benzenonium ion is a specific example of an arenonium ion, formed by electrophilic attack on an arene (Section 11.4). It is also called a sigma complex, because it arises by formation of a o-bond between E and the ring. See Fig. 11-1 for a typical enthalpy-reaction curve for the nitration of an arene. 

A bizarre silylene insertion into the I—I bond has been suggested as the initial step in the mechanism for the low-temperature (—90 °C) reaction of dihalosilylenes (SiF2, SiCl2, SiBr2) with solutions of iodine in toluene, as shown in equation 58125. These reactions can be considered to involve electrophilic attack on an arene by SiX2l+126. 

Functionalization of C—H bonds via aromatic substitution is an important means of adding functional groups to all kinds of arenes and as such is of significant relevance to many areas of chemistry. Notably, the key step in several industrially important reactions is an electrophilic attack on aromatic Jt-systems by carbocations or other strong electrophiles. 

The consistent picture being painted for this arene hydroxylation is an electrophilic attack of the bound peroxo ligand. The k2 value observed for [Cu2(H—XYL—D)]2+ (10-D) is within experimental error of that seen for the -H parent compound (10) this lack of deuterium isotope effect is consistent with electrophilic attack on the arene substrate n system, which precludes C—H bond cleavage in the rate-determining step. 

As illustrated in Scheme 3 [la], palladium-catalyzed carboxylation of alkanes may proceed in a fashion similar to the carboxylation of arenes, involving electrophilic attack of cationic [Pd02CCF3]+ species on the C-H bonds of alkanes to give alkyl-Pd(II)-02CCF3 species. 

The process observed is reminiscent of the NIH shift , observed previously in iron hydroxylases, in which a reactive iron-oxy species (with an as yet undetermined identity) is an electrophile, attacking an arene substrate. This results in hydroxylation-induced migrations, due to the formation of carbonium ion intermediates and retention of heavier... 

Perdenteration of the methylene hnker affords a relatively kinetically stable complex, which allows for the monitoring of exogenons snbstrate oxidations. When (7) is exposed to cold (-95 °C) acetone solntions of the lithium salts of para-substituted phenolates, clean conversion to the corresponding o-catechols is observed. Deuterium kinetic isotope effects (KIEs) for these hydroxylation reactions of 1.0 are observed, which is consistent with an electrophilic attack of the peroxo ligand on the arene ring. An electrophilic aromatic substitution is also consistent with the observation that lithium jo-methoxy-phenolate reacts substantially faster with (7) than lithium / -chloro-phenolate. Furthermore, a plot of observed reaction rates vs. / -chloro-phenolate concentration demonstrated that substrate coordination to the metal center is occurring prior to hydroxylation, and thus may be an important feature in these phenolate o-hydroxylation reactions. 

Lead tetraacetate reacts with arenes to lead either to aromatic nucleus substitution or to substitution on the benzylic position of the side chain. Substitution on the nucleus involves electrophilic attack of (AcO)3Pb " to give aryllead tricarboxylates. Subsequently, these aryl species react with acid to afford eventually the corresponding aryl esters. ... 

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