Arenes oxidative addition

Other sources of Ru(0) can also be used for this synthesis. For example, it was recently demonstrated that [Ru(arene)(diene)] complexes such as 39 undergo double oxidative addition of heterosubstituted dihalo compounds 40 in the presence of phosphine ligands (Eq. 5) [21]. 

Arene oxides and diol epoxides are generally unstable in aqueous, especially acidic media (103-105). and in addition, several groups have noted that DNA has a marked catalytic effect upon diol epoxide hydrolysis (106.107). However, in cells there appears to be sites, probably lipid in nature, in which these compounds can have much longer half-lives. 

The discussion has focused so far on activation of alkanes, where formation of the a-complex seems to precede oxidative addition. For arenes, formation of the analogous a(cH)-arene complex is thought to occur before oxidative addition to form an aryl hydride. These a-com-plexes have never been observed, presumably because they are unstable with respect to the 71-complexes. Both types of arene complexes are, for the case of benzene, shown in Scheme 25 the a(CH)-arene complex as A and... 

FIGURE 4.78 Mechanistic pathways for aromatic hydroxylation by concerted addition of oxene, pathway 1, or by stepwise addition of oxene, pathway 2. Pathways 2, 3, and 4 describe the formation of phenol that bypasses the arene oxide intermediate. 

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

Many related complexes of iridium and rhodium undergo the oxidative addition reaction of alkanes and arenes [1]. Alkane C-H bond oxidative addition and the reverse reaction is supposed to proceed via the intermediacy of c-alkane metal complexes [4], which might involve several bonding modes, as shown in Figure 19.5 (for an arene the favoured bonding mode is r 2 via the K-electrons). 

The overall reaction catalyzed by epoxide hydrolases is the addition of a H20 molecule to an epoxide. Alkene oxides, thus, yield diols (Fig. 10.5), whereas arene oxides yield dihydrodiols (cf. Fig. 10.8). In earlier studies, it had been postulated that epoxide hydrolases act by enhancing the nucleo-philicity of a H20 molecule and directing it to attack an epoxide, as pictured in Fig. 10.5, a [59] [60], Further evidence such as the lack of incorporation of 180 from H2180 into the substrate, the isolation of an ester intermediate, and the effects of group-selective reagents and carefully designed inhibitors led to a more-elaborate model [59][61 - 67]. As pictured in Fig. 10.5,b, nucleophilic attack of the substrate is mediated by a carboxylate group in the catalytic site to form an ester intermediate. In a second step, an activated H20... 

G. H. Loew, A. T. Pudzianowski, A. Czerwinski, J. E. Ferrell Jr., Mechanistic Studies of Addition of Nucleophiles to Arene Oxides and Diol Epoxides Candidate Ultimate Carcinogens , Int. J. Quantum Chem. 1980, 7, 223 - 244. 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

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 authors propose that the influence of the phenyl group is not an electronic effect, as no product formation is observed with palladium catalysts known to be highly active in disilane systems substituted with electronegative elements. Rather, the phenyl group may allow for precoordination via a w-arene complex, which would accelerate the oxidative addition of an Si-Si bond to platinum, a key step in the proposed catalytic cycle. 

Isoquinolines have been prepared on insoluble supports by radical-mediated cycli-zations and by intramolecular Heck reaction (Table 15.25). Entry 1 in Table 15.25 is a rare example of the formation of a biaryl by intramolecular addition of an aryl radical to an arene. Oxidative aromatization was achieved by using a large excess of AIBN. 

Early attempts to utilize I as a donor for a chromium sandwich complex did not meet with success268, but recent studies have now provided several /6-chromium(0) complexes 64- 99-200. Whereas unsubstituted cycloproparenes undergo oxidative addition of the ring to the metal followed by carbon monoxide insertion (Section V.B.4), the l,l-bis(trimethylsilyl) derivatives do not. Instead, reactivity is transferred to the arene and, with tris(acetoni-trile)tricarbonylchromium, -complexes are formed at the ring remote from the cyclo-proparene moiety (equation 28). However, the 1,1 -disilyl derivative of 1 does not react and... 

Three methods are commonly employed for the in situ preparation of organopalladium derivatives (i) direct metallation of an arene or heterocyclic compound with a palladium(II) salt (ii) exchange of the organic group from a main group organometallic to a palladium(II) compound and (iii) oxidative addition of an organic halide, triflate or aryldiazonium salt to palladium(O) or a palladium(O) complex. 

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