Iron-catalyzed reactions homocoupling

CO insertion prior to the transmetallation step. The mechanism of nickel-catalyzed coupling reactions is less established. Early studies indicated that homocoupling processes occur by oxidative addition through radical intermediates and possible intermediacy of Ni(I) and Ni(III) complexes. The copper-catalyzed cross-coupling reactions likely occur by transmetallation prior to oxidiative addition of the aryl halide. Iron-catalyzed reactions likely occur by low-valent, even sub-valent, species. 

Nickel-catalyzed electroreductive homocoupling reaction of organic hahdes and coupling reaction between organic halide and activated olefins could be carried out with iron anode in ionic liquid. With addition of small amount of DMF, coupling compounds were obtained at room temperature in good to high... 

In analogy to the reaction described in the previous section, iron-catalyzed homocoupling of aryl Grignard reagents can be achieved (Scheme 4-253). Two variations of this transformation have been described both using iron(III) chloride as catalyst. In the first procedure, the reaction is performed in tetrahydrofuran at low temperatures between -40 °C and 0 °C employing 1,2-diiodoethane as an oxidant in stoichiometric amounts. This method tolerates nitrile, ester, and nitro groups. An intramolecular version has been employed for the total synthesis of the natural product N-... 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

Cook reported the iron/diphosphine-catalyzed alkylation of arene-, heteroarene-, and alkeneamides with primary alkyl halides (Eq. 19) [53] and, shortly after, the alkylation of aromatic amides with benzyl chlorides and secondary alkyl bromides (Eq. 20) [54]. The reaction with primary alkyl bromides proceeded well for aromatic, heteroaromatic, and alkenyl amides despite using phenylmagnesium bromide as a base, the reaction with benzyl chlorides proceeded well under air. As previously observed by Nakamura [28, 34], the slow addition of the Grignard reagent was crucial in order to achieve high yields, presumably because of the competing homocoupling [29]. Secondary alkyl bromides and iodides could be employed in the reaction with benzamides, but acyclic secondary alkyls underwent partial isomerization to the linear alkyl. 

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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