C -H coupling reactions

Takagi, J., Sato, K., Hartwig, J. F., Ishiyama, T., Miyaura, N. Iridium-catalyzed C-H coupling reaction of heteroaromatic compounds with bis(pinacolato)diboron regioselective synthesis of heteroarylboronates. Tetrahedron Lett. 2002,43, 5649-5651. 

DeBoef and co-workers have reported a similar reaction, wherein direct C-H to C-H indole-arene cross-coupling can be controlled through the use of a particular oxidant (Scheme 34) [53,54], The basis of their selectivity concept is the formation of different polyvalent clusters between the Pd(OAc)2 and the AgOAc or Cu(0 Ac)2 oxidants respectively, and the subsequent reactivity of these complex in the aryla-tion reaction. The same group also demonstrated the utility of an intermolecular C-H to C-H coupling reaction. 

In the intermediate of higher oxidation state, the metal is harder, and a hard-soft dissymmetry of the hgands favors the irreversible elimination of the product. In this way the key role of oxidative addition and its reverse reaction in the homogeneous catalysis of C-C and C-H coupling reactions can be understood in the simplest case, the interplay of oxidative addition and reduetive elimination ean be represented by a redox reaction (Eq. 2-83). 

The difference between metal-catalyzed and metal-free C-H/C-H coupling reactions is nicely illustrated by new approaches to modify the structure of imidazolines which have recently been developed in our laboratory (Scheme 69) [212],... 

Oxidative C-H coupling reactions are frequently carried out in the presence of Ag(l) additives [57-64]. As described below, due to (1) the demonstrated availability of one-electron oxidation processes for compounds such as 7 [56], (2) the propensity of Ag(l) to facilitate one-electron oxidative cleavage of Pd-C bonds [65] (vide infra), and (3) the frequency with which Ag(l) additives are employed in Pd-catalyzed cross-coupling chemistry [57-64], the reaction chemistry of Ag(l) salts with organometallic Pd(ll) complexes has received experimental scrutiny. 

The chemistry of complex 11 with AgPFg was evaluated because Ag(I) is a common additive in Pd-catalyzed oxidative C-H coupling reactions [57-64] and a potential one-electron oxidant, similar to Fc". Treatment of dimethyl Pd(II) complex 11 with AgPFg resulted in the immediate formation of an intermediate (16), as observed by NMR spectroscopy, which subsequently generated 13, ethane, and Ag mirror (Fig. 9). The authors proposed that Ag(I) acts as an inner-sphere one-electron oxidant. Initial coordination of Ag(I) to Pd to generate 16, followed by electron transfer, would furnish proposed Pd(III) intermediate 12. Disproportionation of Pd(III) intermediate 12 to Pd(II) complex 13 and Pd(IV) intermediate 15... 

Reflecting on their pioneering work in formation of aryl radicals from aryl iodides mediated by potassium t-butoxide, the Itami group noticed cyclohexane-incorporated byproducts when the reaction was run in the presence of cyclohexane. They postulated a radical transfer from the aryl radical to form the alkyl carbon-centered radical. This hypothesis caused them to wonder if a more direct generation of alkyl radicals could affect a direct C—H/C—H coupling reaction. 

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

Blaser, H.-U., Indolese, A.,Naud, F. etal. (2004) Industrial R D on catalytic C—C and C—N coupling reactions apersonal account on goals, approaches and results. Advanced Synthesis and Catalysis, 346 (13-15), 1583-1598. 

Metal complexes as catalysts for oxygen, nitrogen and carbon-atom transfer reactions (Tsutomu Katsuki) Metal complexes as catalysts for H-X (X = B,CN, Si, N, P) addition to CC multiple bonds (M. Whittlesey) Metal complexes as catalysts for C-C cross-coupling reactions (I. Beletskaya, A.V. Cheprakov)... 

Protonation reactions of the related dimethyl(hydrido)platinum(IV) complex TpMe2PtMe2H (58) leading to rapid methane reductive elimination have also been reported (86). This protonation was shown to occur exclusively at the pyrazole nitrogen, presumably forming a five-coordinate Pt(IV) intermediate. This species should undergo C-H coupling, and while a Pt(II) methane complex is not observed, trapping with... 

The C-H coupling of aromatic heterocycles with bis(pinacolato)di-borane(4) was carried out in octane at 80-100 °C in the presence of a half equivalent [IrCl(COD)]2-(4,4 -di-tert-butyl-2,2 -bipyridine) catalyst (3 mol%). The reactions of five-membered substrates such as thiophene, furan, pyrrole, and their benzo-fused derivatives exclusively produced 2-borylated... 

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