Transition metals iron cross-coupling reactions

Transition metal-catalyzed cross-coupling reactions between vinyl organometallic compounds and unactivated alkyl halides that can be usually performed with palladium, nickel and cobalt are of particular synthetic interest [37-39]. Recently, the groups of Cahiez [48] and Cossy [49] concurrently reported the first iron-catalyzed reaction of alkenyl Grignard compounds with primary and secondary alkyl halides (X=Br, I) (Scheme 5.15). The two protocols basically differ in the iron source... 

During the past few decades, transition metal-catalyzed cross-coupling reactions have become a powerful tool for the construction of C—C and C-heteroatom bonds [1]. This strategy allows the conceptually simple and yet powerful and reliable approach for synthesizing structurally complex pharmaceuticals and biologically active molecules. The two most vastly used transition metal catalysts in carbon-heteroatom bond formation are palladium (mainly depends on its ancillary ligands) and copper (depends on the optimization of the catalytic system as a whole copper source, solvent, base, concentrations, etc.). Besides, considerable developments have also been made with other transition metal catalysts such as nickel, iron, etc. 

Selective cross-coupling reactions between C(sp ) and C(5p ) centers had been one of the most difficult tasks in carbon-carbon bond synthesis until the early 1970s, when it was first reported that iron, nickel, palladium and copper - catalysts are extremely effective for cross-coupling of Grignard reagents with organic halides. Now, nearly 20 years later, transition metal catalyz cross-coupling has become the reaction of first choice for this purpose. 

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). 

Other transition metals also proved to be useful for cross-coupling reactions. Here can be briefly mentioned the Ni-catalyzed cross-coupling of functionalized aryl iron intermediates obtained directly by a directed ferration. Thus, the reaction of 4-fluorobenzonitrile with TMP2pe (0.75 equiv., 25 °C, 18 h) provides a functionalized iron(II)-intermediate that undergoes a smooth cross-coupling catalyzed by Ni-impurities... 

Iron, cobalt and manganese are effective catalysts for cross coupling and other reactions. They were studied in the very early days of transition-metal catalysis and are now being resurrected." ... 

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). 

These transition elements are Fe, Co, Ni, Pd, Cu and Ag. These metal catalysts have respective characteristics. For example, the representative catalytic actions are as follows silver is for alkyl-alkyl homo-coupling, copper is for cross-coupling, nickel and iron are for cross-coupling containing unsaturated halides such alkenyl and aryl halides [56]. These basic reaction examples are shown in eqs. (4.28)-(4.32) [55,56]. Kumada el al. synthesized many compounds as ligands which have... 

In biatyl synthesis, although transition metal catalyzed aryl-aryl crosscoupling reactions have been widely exemplified using Suzuki-type reactions, CDC reactions between sp C-H bonds leading to sp C-C bonds are scarce. In 2010, Katsuki reported an enantioselective CDC reaction between two naphthol moieties catalyzed by a chiral iron(salan) complex under aerobic oxidative conditions (Scheme 4.23). Using 4 mol% of the iron(salan) complex 23-A as the catalyst in toluene at 60 °C for 48 h, two different 2-naphthols were coupled and the cross-coupled derivatives were isolated with moderate yields (44-70%) and good ee (87-95%). It must be pointed out that the homocoupling derivatives are also obtained in low yields. 

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