Iron Catalyst Arene coupling

Intermolecular dehydrogenative oxidative homocouplings of (hetero)arenes turned out to be among the most important methods for the synthesis of symmetrically substituted biaryls [122]. A recent illustrative example is oxidative coupling reactions of 2-naphthols, which were accomplished in an asymmetric fashion employing an inexpensive iron catalyst (Scheme 9.47) [123]. 

Various transition metals (Pd, Rh, Ru, Cu, Fe, etc.) have been shown to be effective for cross-coupling reactions involving C-H bond activation in recent years (for copper, see [33-38] for iron, see [39, 40]). Pd(II) salts have emerged as the preferred catalysts to promote the C-H bond cleavage in catalytic direct arylation reactions. This process can be classified into two parts, the organometallic reagent is replaced by a simple arene in one case, and the aryl halide is replaced by a simple arene in another (Scheme 4). 

In 2010, the groups of Charette and Lei independently reported an iron/diamine-catalyzed reaction of aryl iodides or bromides with a solvent amount of arene at 80-90°C (Eqs. 24 and 25) [60, 61]. A mixture of ortho-, meta-, and para-isomers was obtained when substituted arenes were used as the substrate, the ort/jo-isomer being the major product. The Charette group reported a KIE value of 1.04, while Lei group measured a KIE of 1.7. Based also on reaction inhibition by a radical scavenger, Charette suggested that radical processes are involved. Recent studies have revealed that cross-coupling of an aryl halide with an arene can proceed in the absence of a transition metal catalyst ([62] and references therein). 

The photolysis of Os(CO)5 in benzene has led to (n -C6H6)Os(CO)4 which is said to be similar to, but more reactive than, the iron analogue.The dimeric halide complexes [(arene)MX2]2 (M= Ru, Os) remain a useful entry into arene complex chemistry. The development of the ligand chemistry of these complexes continues.These substrates have also been used to make metalloborane complexes, used as catalysts for the coupling of furans and thiophenes and simple derivatives such as [ (p-cymene)20s2(0H)3] used for the catalytic oxidation of aldehydes to carboxylic acids. ... 

Cross coupling of EtsAl with aryl phosphates or alkynyl bromides proceeds with a Ni catalyst to provide alkylated arenes or aUrynes (eqs 5 and 6). While Pd or Cu complexes are used as the catalyst for the coupling of EtsAl with carboxylic acid chlorides or thioesters (eq 7), Iron(HI) Chloride is used for reactions of propargyl acetates to give substituted allenes (eq 8)."... 

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