Arene oxides addition reactions

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). 

New nickel-benzyne complexes (143-147) have been prepared by reaction of o-dihaloarenes with Ni(COD)2 in the presence of a trialkylphosphine followed by reduction of the oxidative addition product with either Li or 1% Na/Hg in ether [e.g., Eq. (23)]. The oxidative addition reaction depends on the nature of substituents on the arene and fails to occur when strong electron-donating groups are present. Based on NMR and mass spectrometry (MS) data, the new complexes were formulated as monomeric. It had been... 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

It is therefore not surprising that the reactivities of arenes and alkanes in electrophilic substitution reactions are very different, with the former being much more active. At the same time, the mechanism of the interaction (oxidative addition) of both saturated and aromatic hydrocarbons with complexes of metals in a low oxidation state is in principle the same. The reactivities of arenes and alkanes in oxidative addition reactions with respect to low-valent metal complexes therefore usually differ insignificantly. Furthermore, a metal complex via the oxidative addition mechanism can easily cleave the C-H bond in olefin or acetylene. 

DFT calculations indicate that the oxidative addition reactions of a G-F bond in GeFe at Ni(H2PGH2GH2PH2) and Pt(H2PGH2GH2PH2) proceed initially via exothermic formation of an 77 -arene complex. The G-F oxidative addition reaction is more exothermic at nickel than at platinum. The barrier for exothermic oxidative addition is higher at Pt than at Ni because of strong d-p repulsions in the transition state. Similar repulsive interactions lead to a relatively long Pt-F bond with a considerably lower stretching frequency in the oxidative addition product than for... 

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

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... 

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... 

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. 

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. 

---