Arenes activation

Diarylamides with arenes activated by electron-donating substituents can be converted to azacycles by anodic oxidation through phenolic oxidative coupling reactions that can be a key step in the synthesis of alkaloids (Schemes 16 and 17). According to the nature of substituents and the experimental conditions, either spiro compounds [22] or non-spiro compounds [23, 24] were obtained. 

Arenes activated with an electron-withdrawing group may react with alkyl peroxides directly, by nucleophilic attack at the ortho- or para-position. Nitroarenes can be hydroxylated with alkyl hydroperoxides in strongly basic media in moderate to good yields (Eq. 7) [39]. 

Arene activation by metal complexation is not always necessary for the cyclisation of samarium ketyls on to aromatic systems. Studies carried out by Reissig demonstrated the utility of these reactions for the synthesis of a wide variety of polycyclic systems, although yields and stereoselectivities depend highly on the substrate structure.68 One well-behaved system is represented by the cyclic y-naphthyl ketones 50-54, which cyclise with excellent diaster-eocontrol, thereby incorporating four- to eight-membered rings into the tetracyclic structures 55-59 (Scheme 5.38).68 Tetracyclic product 56 (n = 1) is of particular interest because of its steroid-like structure. 

The study of mercuration and thallation provides a shaip focus on the experimental delineation of stepwise and concerted mechanisms for arene activation. Thus the unequivoc demonstration of arene radical cations as key intermediates in thallation, particularly of durene and pentamethylbenzene, is consistent with a stepwise (electron-transfer) mechanism for arene activation (compare Scheme 6 and equation 39). 

By the same token, the singular absence of any experimental evidence for such intermediates during mercuration is directly accommodated by a concerted (electrophilic) mechanism for arene activation, i.e. equation (40). 

Moreover, the facile bimolecular reactions of the cationic donor RH and/or the anicmic acceptor A, especially with additives diat are present during oxidation, can accomplish the same displacement of the redox equilibria in measure with the competition from back electron transfer. For example, the arene activation with nitrosonium ion merely reaches a low steady-state concentration of the radical pair, which persists indefinitely in equation (13). However, oxygen r idly tnq>s even small amounts of nitric oxide to render back electron transfer ineffective, and successfully effects aromatic nitration (Scheme 3). ... 

Allylation. 4-t-Butylcalix[4]arene activates the Zr-BINOL complex in the allylation... 

Several other recent examples of arene activation have also appeared. Flood reported that a triazacyclononane rhodium complex could activate arenes. These complexes are quite stable, losing methane or ethane only upon heating to 80°C (Eq.25) [108]. 

Another unusual example of arene activation was reported by Diversi. Here, an 18-electron iridium dimethyl complex is oxidized by one electron prior to reaction with benzene, and the resultant 17-electron complex readily exchanges a methyl group for an aryl group. The product oxidizes the starting material, so that the overall reaction is an example of electron transfer catalysis. The proposed mechanism is shown in Scheme 13 [113]. 

Berry has recently reported an example of catalytic arene activation and functionalization. This report involves either Cp Rh(SiEt3)2H2 or (r 6-arene) Ru(SiEt3)2H2 as catalyst and couples triethylsilane with an arene. t-Butylethyl-ene serves as a hydrogen acceptor (Eq. 31). No intermediates are observed, and a carbosilane dimer is also formed as a significant fraction of the total product [118]. 

The reaction of toluene with 1 could conceivably lead to four different trans-isomeric products by attack at the ortho, meta, para, or alpha positions. The carbanion chemistry of toluene would predict attack at the methyl group but, as in other transition metal arene activation studies (8,9,10), this is not found. The NMR spectrum of the toluene adducts displays two singlets attributable to trans-tolyl adducts in an intensity ratio of 1 1.6 in addition to resonances of cis-tolyl adducts. Selective mono-deuteration experiments show that the weaker downfield resonance is attributable to the para isomer while the stronger one is attributable to the meta isomer no ortho isomer is observed. If one corrects for statistical effects, the preference for para, meta, and ortho isomers is 1 0.8 0. We attribute this distribution of isomers primarily to... 

Metal-arene n-complexes show a rich and varied chemistry. The metal adds a third dimension to the planar aromatic compounds and the two faces of an arene with different ortho or raefa-substituents are enantiotopic. Therefore, coordination of a metal to an arene not only alters the reactivity of ring-carbons and substituents as well as groups in benzylic positions but, in addition, also allows reactions with high stereoselectivities to be carried out. The aim of this book is to provide a coherent picture of the state-of-the-art in this field. It covers the entire spectrum of arene activation from the electrophilic activation of a q -bound... 

RuCp" is very similar to FeCp" as an arene activating group, albeit a little less electrophilic. The advantages of Ru compared to Fe are mild complexation and demetallation conditions. Efficient substitution for chloride in (chloro-arene)RuCp by aryloxides in presence of base-sensitive functionality was demonstrated [62]. This initial success lead to the synthesis of a series of vancomycin-related biaryl ethers via intramolecular S Ar cyclization in a preformed dipeptide derivative (e.g., 37) [89,90]. 

Of the handful of transition metal systems that are known to form stable q complexes with aromatic molecules [8-14], only d octahedral metal complexes have been shown to enhance the reactivity of the aromatic ligand toward electrophiles to date [15]. For nearly a decade, despite our best efforts, this mode of arene activation was known only for the pentaammineosmium(II) system. However, in the past few years a new generation of dearomatization agents have been developed based on a careful matching of the d /d reduction potential of rhe-... 

In the case of arene activation, Jones [132] has shown how the initial interaction with the metal can give an tf-wtnt complex prior to CH activation. This is the case for the [Cp Rh(PMe3)] fragment, formed by photoextrusion of H2. 

In Fujiwara s interesting arene activation chemistry, oxidative addition of an arene CH bond to Pd, specially fast for electron-rich arenes, is followed by a Heck-like insertion of an alkene to give a vinylated arene as product. Arenes undergo unexpected selective trans hydroarylation with both terminal and internal C=C double bonds, both inter- and intramolecularly, with turnover numbers up to 4500, giving the thermodynamically less favorable cis-alkenes. The simplicity, generality, and efficiency of this process could be very attractive for possible industrial application (Eq. 2.48) [133]. 

The Gibbs energy pathway of reaction (69) was presented by Jones and Feher, showing that arene activation in the rhodium system is preferred by thermodynamic rather than kinetic reasons (the activation barriers differ by only 2 kJ mol ). The Rh-77 -C6H6 bond dissociation enthalpy is estimated as ca. 70 kJ mol , in line with the above value forZ)//°(Rh-7] -CioH8). 

A substantial body of C-H activation chemistry can be initiated by reductive elimination from Pt(IV) species either of ethane from stable, five-coordinate (p-diketiminate)Pt Me3 or of alkane from six-coordinate TpPt HR2. In some cases, usually involving arene activation, new stable Pt(IV) products are obtained the course of benzene activation by the TpPt system has been examined theoretically [80]. For alkane activation, the final product is often a Pt(II)-olefin hydride... 

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