Catalytic C-H bond activation, which was an elusive subject of chemical research until the 1990s, has now become a standard synthetic method for the formation of new C-C and C-heteroatom bonds. The synthetic potential of C-H activation was first described for ruthenium catalysis and is now widely exploited by the use of various precious metals. Driven by the increasing interest in chemical utilization of ubiquitous metals that are abundant and nontoxic, iron catalysis has become a rapidly growing area of research, and iron-catalyzed C-H activation has been most actively explored in recent years. In this review, we summarize the development of stoichiometric C-H activation, which has a long history, and catalytic C-H functionalization, which emerged about 10 years ago. We focus in this review on reactions that take place via reactive organoiron intermediates, and we excluded those that use iron as a Lewis acid or radical initiator. The contents of this review are categorized by the type of C-H bond cleaved and the type of bond formed thereafter, and it covers the reactions of simple substrates and substrates possessing a directing group that anchors the catalyst to the substrate, providing an overview of iron-mediated and iron-catalyzed C-H activation reported in the literature by October 2016.

A 2,2-disubstituted propionamide bearing an 8-aminoquinolinyl group as the amide moiety can be arylated at the β-methyl position with an organozinc reagent in the presence of an organic oxidant, a catalytic amount of an iron salt, and a biphosphine ligand at 50 °C. Various features of selectivity and reactivity suggest the formation of an organometallic intermediate via rate-determining C-H bond cleavage rather than a free-radical-type reaction pathway.

Arylamine derivatives have been used as the hole-transporting material (HTM) in multilayer organic light-emitting diodes (OLEDs) for 20 years. 1,2 R-NPD, 3 which is a diarylamino-substituted biphenyl, is the current standard, 4 giving us the impression that the arylamino group is indispensable for high-performance HTMs. Indeed, there have been scarcely any attempts to find new HTMs that do not contain any arylamino groups. 5 We describe herein a new method for the synthesis of a series of 2,3,6,7-tetraarylbenzo-[1,2-b:4,5-b′]difurans (BDFs), and report that they function as HTMs in layered OLEDs. The BDF skeleton by itself serves as an excellent HTM, and improvement of the physical properties can be achieved by suitable functionalization.

Conversion of a C(sp(2))-H or C(sp(3))-H bond to the corresponding C-Me bond can be achieved by using AlMe3 or its air-stable diamine complex in the presence of catalytic amounts of an inorganic iron(III) salt and a diphosphine along with 2,3-dichlorobutane as a stoichiometric oxidant. The reaction is applicable to a variety of amide substrates bearing a picolinoyl or 8-aminoquinolyl directing group, enabling methylation of a variety of (hetero)aryl, alkenyl, and alkyl amides. The use of the mild aluminum reagent prevents undesired reduction of iron and allows the reaction to proceed with catalyst turnover numbers as high as 6500.