Arene complexes electrophilicity

In arene complexes, for example in (q6-arene)chromium tricarbonyl derivatives, the arene ring C — H bonds have enhanced acidity. These complexes, similarly to Cp complexes (see above), are treated with BuLi, and the resulting lithio (q6-arene)chromium tricarbonyl complexes react with a variety of electrophiles. For instance, reacting with BuLi, (C6H5Y)Cr(CO)3 (Y = H, OMe, F, Cl) forms (C6H5Li)Cr(CO)3, and further reaction with C02, MeX, PhCHO, MeCOMe, etc. gives o-products (o-C6H4YR)Cr(CO)3 [74c]. 

Desymmetrisation by enantioselective ortholithiation has been achieved with ferrocenylcarboxamides 434,187 and also (with chiral lithium amide bases) a number of chromium-arene complexes.188 The chromium arene complex 435, on treatment with s-BuLi-(-)-sparteine, gives 436 enantioselectively, and reaction with electrophiles leads to 437. However, further treatment with r-BuLi generates the doubly lithiated species 438, in which the new organolithium centre is more reactive than the old, which still carries the (-)-sparteine ligand. Reaction of 438 with an electrophile followed by protonation therefore gives ent-431.m... 

The yellow CpFe1 (//6-arene) salts (most commonly BF4 or PF6 ) are usually stable up to at least 200 °C, are stable in concentrated sulfuric acid, and are very resistant towards oxidation (until recently, it was believed that they could not be oxidized [23] vide infra). They are not easy to reduce either [23] (vide infra). The chloride salts [CpFe+( f -arene) Cl- are water-soluble they are formed upon hydrolysis following ligand-exchange reactions between ferrocene and the arene in the presence of aluminum chloride [21]. Such aqueous solutions may sometimes be directly used for nucleophilic reactions [22] (vide infra). The BF4- salts are also sometimes quite soluble in water, but the PF6- salts are much less so. Electrophilic reactions that are readily undergone by the free arenes, such as Friedel-Crafts reactions, are no longer possible on the CpFe+( /6-arene) complexes [19, 23]. On the other hand, a range of nucleophilic reactions that are impossible or very difficult to carry out with free arenes become possible under ambient or mild conditions with the CpFe+()/6-arene) complexes (Scheme 2) [16-20]. 

Redistribution of electron density in CT complexes results in a modification of the chemical properties of coordinated arenes, and this effect is widely used in organometallic catalysis [2]. To demonstrate the relationship between charge transfer in arene complexes and their reactivity, we focus our attention on carbon-hydrogen bond activation, nucleophilic/ electrophilic umpolung, and the donor/acceptor properties of arenes in a wide variety of organometallic reactions. 

Fig. 12. Li near correlations of the relative reactivity of arenes in electrophilic substitution reactions (log k/kf) with the optical transition energies in the accompanying CT complexes. Data from ref. [62b],...

Arenes are electron donors by virtue of the ease with which they form intermolecular complexes with a variety of electrophiles, cations, acids, and oxidants that are all sufficiently electron-poor to be generally classified as electron acceptors (A). Three structural features are common to all arene complexes that have been isolated and subjected to X-ray crystallo-... 

Kiindig and Quattropani investigated the effect on the stereoselectivity of electrophiles in metallation of the carbamate-containing arene complexes. Modest stereoselectivities were obtained as seen in Scheme 4081. 

Variation of ether alkyl group, arene complex or electrophile resulted in similar high enantioselectivity (Scheme 47). 

A synthetically very powerful property of chromimn tricarbonyl arene complexes is the ability to stabilize a negative charge at both the a-and /3-positions of an alkyl side-chain. Deprotonation of the benzylic position using sodiiun hydride, t-butyl lithium, or potassium t-butoxide followed by addition of an electrophile affords alkylated products. The... 

Vigalok A, Uzan O, Shimon LJW, Ben-David Y, Martin JML, Milstein D (1998) Formation of r]2 C-H agostic rhodium arene complexes and their relevance to electrophilic bond activation. J Am Chem Soc 120 12539-12544... 

Formation of t] -Cyclohexadlenlde Complexes via Electrophilic Addition to Metal Arene Complexes. 

Abstract Methods of synthesis of i -arene complexes of Cr(CO)3, Mo(CO)3, Mn(CO)3, FeCp+, RuCp+ are reviewed. These electrophilic transition metal complex fragments have foimd application in arene transformations. Critical comparison of the routes of access is made and methods of decomplexation and where possible methods of recovery of the activating group are also detailed. Excluded from the overview are methods involving arene transformations in the coordination sphere of the metal. These wiU be contained in subsequent chapters. 

Electrophilic haUde sources include iodine [25-27],diiodoethane (Eq. 4) [28-29], N-bromosuccinimide [30, 31, 25], dibromoethane [32-34], dibromotetra-chloroethane [35],dibromotetrafluoroethane [23,36], andhi-chlorosuccinimide [30,31,25]. Dihaloethane derivatives are typically preferred since any excess iodine and N-halosuccinimides oxidize the chromium which leads to a reduction in yield of the substituted arene complex [ 16,25,31 ]. 

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