Metallic coordinated arene ring complexes

By coordinating to arenes, transition metals can facilitate ring lithiation by decreasing the electron density in the ring and acidifying the ring protons. We shall consider briefly the two most important metal-arene complexes in this regard—arenechromium tricarbonyls and ferrocenes. 

Perutz and coworkers [180] have used 2D EXSY to study the dynamic behavior of RhCp(PMe3)(q -naphthalene), 109, which is thought to be a model intermediate for the oxidative addition of arenes to a metal center. In this complex, there are two processes taking place. The first involves an equilibrium between the -naphthalene complex, 109, and the naphthyl hydride complex, 110. The second process involves an intramolecular [l,3]-shift which moves the coordination site of the naphthalene ring from one side of the ring to the other (Scheme 1.12). 

The intermediate cyclooctene complex appears to be more reactive with respect to CS coordination and more sensitive to oxidation when the arene ring bears electron-withdrawing groups (e.g., C02CH3). Dicarbonyl(methyl rj6-benzoate)-thiocarbonyl)chromium is air stable in the solid state and reasonably stable in solution.9 The infrared spectrum exhibits metal carbonyl absorptions at 1980 and 1935 cm"1 and a metal thiocarbonyl stretch at 1215 cm"1 (Nujol) (these occur at 1978, 1932, and 1912 cm"1 in CH2C12 solution).10 Irradiation of the compound in the presence of phosphite or phosphine leads to slow substitution of CO by these ligands, whereas the CS ligand remains inert to substitution. The crystal structure has been published."... 

Reactions of cobalt and nickel atoms with toluene and other arenes yield condensates in which the metal is in a very reactive state (105). However, none of the products of reaction of these condensates with other ligands has contained the arene coordinated to the metal. It seems possible that the condensates contain ditoluenecobalt and ditoluene-nickel, but in these complexes (unlike the chromium or iron complexes) the two arene rings are probably not symmetrically bonded to the metals. 

This chapter covers reactions in which coordination of a transition metal to the ir-system of an arene ring activates the ring toward addition of nucleophiles, to give V-cyclohexadienyl-metal complexes (1 Scheme 1). If an electronegative atom is present in the ipso position, elimination of that atom (X in 1) leads to nucleophilic aromatic substitution (path a). Reaction of the intermediate with an electrophile (E+) can give disubstituted 1,3-cyclohexadiene derivatives (path b). If a hydrogen occupies the ipso posi-... 

The complexation of an arene to the tricarbonylchromium unit promotes the addition of nucleophiles to the arene ring due to the strong electron-withdrawing ability of the Cr(CO)3 group. Other effects of the coordination of the metal on the reactivity of the arene ligand have been well-documented in the literature [1] and concern (Scheme 1) (i) the stabilization... 

Localization of re-bonds (also termed double-bond fixation ) is an important structural feature frequently observed upon arene coordination to a metal center. To rationalize this effect, the formation of covalent (a) bonds between the metal and particular carbon atoms of the arene ring is commonly invoked. However, this cannot explain all the unusual bond distances observed and is not generally applicable to the analogous findings with organic acceptors. Analysis based on the CT concept allows a comparative treatment of all types of donor/acceptor complexes, and predicts a close relationship between the degree of bond localization and the donor/acceptor strengths of the complexed partners. 

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. 

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