Oxidative addition of arenes

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). 

S.8.2.6.5.1] (r-Aryl-Metal Complexes by Oxidative Addition of Arenes. 

T -Arene complexes containing late metals have been known for many years, but the scope and utility of these complexes have increased in recent years. Copper(I) and silver form labile arene complexes of various stoichiometries that are apparently T -arene complexes. A few of these complexes have been structurally characterized. More recently, a large number of V-arene and heteroarene complexes of osmium, rhenium, molybdenum, and tungsten have been prepared for the purpose of dearomatization of the arene or heteroarenes. Two examples are shown in Figure 2.33. This dearomatization creates a diene or vinyl unit that imdergoes the organic chemistry of ttiese isolated units, instead of the chemistry of an arene. n -Arene complexes of rhodium and platinum have been characterized structurally and studied in the context of their likely intermediacy in the oxidative addition of arene C-H bonds. ... 

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]. 

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... 

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 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. 

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. 

Multiple arylations of polybromobenzenes have been conducted to generate electron-rich arylamines. Tribromotriphenylamine and 1,3,5-tribromobenzene all react cleanly with A-aryl piperazines using either P(o-tolyl)3 or BINAP-ligated catalysts to form hexamine products [107]. Reactions of other polyhalogenated arenes have also been reported [108]. Competition between aryl bromides and iodides or aryl bromides and chlorides has been investigated for the formation of aryl ethers [109], and presumably similar selectivity is observed for the amination. In this case bro-mo, chloroarenes reacted preferentially at the aryl bromide position. This selectivity results from the faster oxidative addition of aryl bromides and is a common selectivity observed in cross-coupling. Sowa showed complete selectivity for amination of the aryl chloro, bromo, or iodo over aryl-fluoro linkages [110]. This chemistry produces fluoroanilines, whereas the uncatalyzed chemistry typically leads to substitution for fluoride. 

On the basis of these findings, a combination of this intramolecular crosscoupling with an initial intermolecular Michael addition was reported by Singer in order to afford cyanobenzofulvene acetal 85 which was an intermediate of the synthesis of a benzazepine [81]. Thus, Michael addition of 2-halophenylacetonitrile derivatives of 86 to ethoxy acrylate performed in the presence of a large excess of base leads to the corresponding conjugated allylic anion 87. The crucial issue in this process is the oxidative addition of the palladium to the electron-rich arene. This problem was solved... 

The gas-phase reactions of the cationic Irm complexes follow a previously unreported mechanism for their observed a-bond metathesis reactions. Previous discussions had considered a two-step mechanism involving intermolecular oxidative addition of either [Cp Ir(PMe3)(CH3)]+ or [CpIr(PMe3)(CH3)]+ to the C-H bond of an alkane or arene producing an Irv intermediate, followed by reductive elimination of methane, or a concerted a-bond metathesis reaction sim-... 

The corresponding hydrido/alkyl (and aryl) complexes v-[RuHR(L-L), ] (L-L = dppe, dppm, dmpe R = Me, Et, Ph) are readily prepared from m-[RuClR(L-L)2] and Li[AlH4]1659 whereas treatment of cis- or tvans-[RuCl2 (dmpe)2 ] with arene radical anions affords d.v-[RuH(f 1-aryl)(dmpe)2] (aryl = phenyl, 2-naphthyl, anthryl, phenanthryl).1389 In solution, these compounds are in tautomeric equilibria with significant concentrations of Ru° complexes (e.g. equation 148) although X-ray analysis for aryl = 2-naphthyl confirms the presence of the six-coordinate Ru" species (373) in the solid state.2459 Some reactions of (373) with various substrates to produce other hydrido complexes are shown in Scheme 74.44>24m Note that the compound of empirical formula [ Ru(dmpe)2 ] obtained by pyrolysis of [RuH(2-np)(dmpe)2] (reaction (iv) Scheme 74) is a binuclear Ru" hydrido complex, resulting from intermolecular oxidative addition of methyl groups to ruthenium.1390... 

The facility of arene reductive elimination underpins numerous C-C, C-O and C-N bond-forming reactions, which may be catalysed by late transition metals, in particular palladium (Figure 4.10). Although there are many variants, the general reaction scheme involves introduction of the aryl in electrophilic form via oxidative addition of an aryl halide (or sulfonate), substitution of the palladium halide by a nucleophile (which may also be carbon based) followed by reductive elimination. It is noteworthy that nucleophilic aromatic substitution in the absence of such catalysts can be difficult. 

Oxidative-addition of CH bonds of arenes and alkanes is discussed in Section 21-4. 

The enantioselective step is the oxidative addition of H2 to the square diastereomeric substrate complexes that are in rapid dissociative equilibrium. The major enantiomer of the product arises from the minor substrate-catalyst diastereomer, this isomer cannot always be detected since it reacts much more rapidly with H2 than the major diastereomer. Molecular modelling suggests that the principal enantiodifferentiating interactions are between the enamide ester function and the nearest arene substituent of the chiral diphosphine.12 The large increase in reaction rate for the minor diastereomer arises from the increased stability of the corresponding dihydro intermediate, that is, the enantioselective step is under product control. 

More recently, cationic intermediates have been observed in the Heck reactions of arene diazonium salts catalyzed by triolefinic macrocycle Pd(0) complexes [17,59], o-iodophenols and enoates to form new lactones [60], and o-iodophenols with olefins (the oxa-Heck reaction) [61 ]. In the first case ions were formed by oxidation of the analyte at the capillary, or by association of [NH4] or Na". In the two other cases ionization occurred through the more typical loss of a halide ligand. The oxa-Heck reaction provides a good example of how these experiments are typically performed and the type of information that can be obtained. The oxyarylations of olefins were performed in acetone, catalyzed by palladium, and required the presence of sodium carbonate as base. Samples from the reaction mixtures were diluted with acetonitrile and analyzed by ESI(+)-MS. Loss of iodide after oxidative addition of o-iodophenol to palladium afforded positively-charged intermediates. Species consistent with oxidative addition, such as [Pd(PPh3)2(C6H50)], and the formation of palladacycles of the type seen in Scheme 8 were observed. Based on this, a mechanism for the reaction was proposed (Scheme 8). 

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