Aryl C-H activation

Rh(TMP)- under these conditions, and in fact the selective activation of methane in benzene solution is a distinctive and unusual feature of this system, given that aryl C—H activation ought to be thermodynamically favored over alkyl C—H activation. The proposed linear transition state proposed in Fig. 8 is the key to this different reactivity. The corresponding trimolecular transition state for an arene would be expected to be bent, and this would be precluded by the bulky TMP... 

The meta-selectivity for toluene activation, observed for both systems, is very unusual (Fig. 5). Also remarkable is the switch in selectivity from aryl C-H activation to benzylic activation inp-xylene, just by changing the chelate ligand from the diimine equipped with trifluoromethyl substitutents in the meta-positions of the phenyl residue to the diimine bearing methyl substituents in the ortho-positions (Fig. 5). The authors suggested that the C-H bond activation is reversible and the isomeric a-methane complexes are in equilibrium prior to the substitution of... 

An Ir(l)-catalyzed asymmetric intermolecular hydroarylation of norbornene with benzamide was reported in good to excellent enantiomeric excess, albeit in low yields, via the aryl C—H activation (Scheme 5.14). In some cases, the hydroami-nation products of norbornene were also formed in high enantioselectivities. 

The kinetics of this C-H activation process have been studied in a series of hydrocarbons, " establishing that (i) linear hydrocarbons are activated faster than their cyclic congeners, a fact attributed to preferential activation of primary, over secondary C H functions, and (ii) aryl C H activation is significantly slower, due to the intermediate being a strongly bound re-arene complex. 

A Pd(II)-catalysed aerobic oxidative approach to 2,3-dihydro-l//-pyrrolo[l,2-u]indoles from Ai-alkynyl anilines by tandem intramolecular hydroamination and aryl C-H activation process has been reported. Molecular oxygen was the oxidant to recycle the Pd catalysis (Scheme 142). ... 

Zhu et al. report a domino copper-catalyzed synthesis of benzimidazoles from boronic acids and amidines." This involved a Chan-Lam-Evans Al-arylation, C—H activation, and C—N bond formation (Scheme 7.18). These steps could be carried out in a one-pot reaction to give both primary and secondary benzimidazoles in good yields under mild reaction conditions (Schemes 7.19 and 7.20). 

Employing NHCs functionalized at the ortho positions of the iV-aryl substituents i.e. IMes or IPr) can increase stability, but in place of iV-aryl C-H activation, the C-H bonds of the methyl or even isopropyl groups might react. Sometimes, the C-H activation is reversible and thus goes undetected, however the presence of a molecule that can accept hydrogen from the metal is often used to drive this reaction to completion. ... 

More complex mechanisms may involve the deprotonation of a substrate that is bound to the transition metal. The bidentate nature of the acetate anion is critical in some palladium-catalyzed direct arylation/C-H activation reactions. In these reactions, the acetate moiety participates in a concerted metalation-deprotonation (CMD) step wherein a transition metal-bound acetate group removes a proton from an aryl C(sp2)-H bond as the metal-C(sp2) bond is being formed (eq 32). Many acetate bases have been used (e.g., KOAc, NaOAc, CsOAc, and CsPiv), and the optimal base used in these reactions is generally determined during the optimization process. 

Remarkably, both alkyl and aryl C-H activations have been described in the formation of the metallacyclic osmium(iv)-hydride complex 25, generated as the minor component (12%) of a mixture with the deprotonated methylene-amido complex [Os(=N=CH2)Gp(GH2SiMe3)2], from the reaction of the imido complex [Os(=NMe)Cp(CH2SiMe3)2][OTf] with PPhs. ... 

Iridium complex-catalyzed cyclization of an Af-arylcarbamoyl chloride with an alkyne has been reported by Tsuji and coworkers [153]. In a typical example, Af-methyl-Af-phenylcarbamoyl was reacted with 5-decyne and a catalytic amount of [IrCl(cod)]2 (2.5mol%) and additional cod (30mol%) in refluxing o-xylene for 20 h to give 3,4-dimethyl-l-methyl-2-quinolone in 92% yield (Scheme 11.5). During this reaction, no indole product formed by decarbonylation was observed. This reaction is proposed to proceed by oxidative addition of Af-arylcarbamoyl chloride to Ir(I), giving a carbamoyl chloroiridium(III) species. Subsequently, the formation of a five-membered iridacycle by ortho-aryl C-H activation followed by insertion of the alkene and reductive elimination produces the 2-quinolone derivative. 

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