Coupling of heteroarenes

The literature on homo-coupling in arenes suggests that a number of metals and metal complexes will be applicable for homo-couplings of heteroarenes. In the heterocyclic reactions described in the previous sections, variable amounts of homo-coupling accompany reactions in which cross-coupling was intended. When the cross-coupling is relatively slow, homo-coupling may become a major pathway. 

New trends in bismuth-catalyzed synthetic transformations 13OBC2740. Nickel-catalyzed direct coupling of heteroarenes 13YGK576. 

Although the majority of direct arylations have been catalyzed by palladium, rhodium, and ruthenium, some additional studies have also focused on direct arylations catalyzed by first-row metals, such as iron and copper. For example, an iron-catalyzed direct arylation reaction between arylzinc reagents and 2-arylpyridine derivatives has been reported (Equation 19.146). Several direct couplings of heteroarenes with aryl halides (Equation 19.147) or hypervalent iodine reagents ° catalyzed by copper halides have also been reported. 

In 2006, Itami [32] reported a rhodium-catalyzed C-H arylation. The newly developed catalyst, RhCl(CO) P[OCH(CF3)2]3 2, is readily prepared, air stable, and able to promote the C-H coupling of heteroarenes and iodoarenes. The C-H/C-X coupHng of simple arenes can also be accompHshed. For example, the coupling of 1,3-dimethoxybenzene and iodobenzene proceeded in the presence of the aforementioned rhodium catalyst, Ag2COj, and 1,2-dimethoxyethane in m-xylene at 200 °C (microwave) to afford the corresponding couphng product in 76% yield. 

Scheme 17,46 Oxidative C-H/C-H cross-coupling of heteroarenes and arenes by Fagnou and DeBoef.

SCHEME 3.2 Carbonylative coupling of heteroarenes and aryl iodides via C—H activation. 

Afterwards, Antonchick and coworkers [105] successfully extended this methodology to the oxidative cross coupling of heteroarenes and simple unfunctionalized alkanes (Scheme 40 (1)). The new C(sp )-C(sp ) bond formation occurred selectively at the electron-deficient site of the arene. The reaction was... 

Decarboxylative Coupling of Heteroarenes. (V-Pivaloyl-protected indoles can be C3-arylated using benzoic acids bearing electron-withdrawing groups, catalytic Pd, and Ag2C03 via a decarboxylative coupling process (eq 24). ... 

Dehydrogenative Cross-coupling of Heteroarenes. Phenan-throline derivatives can be directly arylated via an oxidative C-H/C-H cross-coupling process (eq 40)... 

Rossi, R. Beilina, E Lessi, M. Manzini, C. Cross-coupling of heteroarenes by C-H functionalization recent progress towards direct arylation and heteroarylation reactions involving heteroarenes containing one heteroatom. Adv. Synth. Catal. 2014, 356, 17-117. 

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