Radical iron-catalyzed oxidative coupling

Shirakawa and Hayashi reported the iron-catalyzed oxidative coupling of arylboronic acids with arenes and heteroarenes (Eq. 28) [66]. They used iron(lll) triflate, a bipyridine-type ligand, and a peroxide as an oxidant. For substituted arenes, a mixture of ortho-, meta-, and para-substituted compounds was obtained, with modest selectivity for the ortho-isomer. The authors propose that Fe(lll) mediates generation of t-BuO radical from the peroxide, which oxidizes the arylboronic acid to generate an aryl radical that adds to the arene substrate. 

The most prominent reactions catalyzed by low-valent iron species involving radical intermediates are cross-coupling reactions of alkyl halides (recent reviews [32-35]) and atom transfer radical reactions. In cross-coupling reactions the oxidation state of the catalytically active species can vary significantly depending on the reaction conditions very often it is not known exactly. To facilitate a summary, all iron-catalyzed cross-coupling reactions are treated together and involved oxidation states, where known, are mentioned at the example. In contrast, iron-catalyzed Kharasch reactions will be treated at the oxidation state of the iron precursors. 

It has been established that Fe(II) complexes such as 74 and 75 are active catalysts in iron-catalyzed cross-couplings of alkyl halides (Figure 1.3) [347, 348]. The couplings probably involve a Fe(I)-Fe(III) cycle, in which radical intermediates in the coupling step can be excluded, although they might be involved in the oxidative addition step [349, 350]. 

Termination of the autoxidation process occurs as peroxyl radicals couple to produce nonradical products. Additional sources of free radicals to initiate the free radical chain process include ultraviolet (UV) light and heavy metals (copper, iron, cobalt, manganese, and nickel) which catalyze oxidation by shortening the induction period and promoting free radical formation. 

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. 

Fig. 20 Oxidative radical hydroxycarboxylation reactions catalyzed by (phthalocyanine)iron (The depiction of the hydroperoxyl radical is formal to account for the correct proton and electron balance. Another formal hydroperoxyl radical (not shown for clarity) results from coupling of the initially generated superoxide and the proton resulting from formation of the azo radical)...

---