Iron catalysis, coupling

Scheme 28 Domino iron catalysis for cross-coupling of alkyl and aryl halides [90]...

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

Even alkyl Grignard reagents can be coupled with alkyl halides 1 using iron catalysis with the xantphos ligand 11 (entry 16) [56]. The yields are acceptable for primary alkyl bromides. Secondary alkyl halides reacted only in low yield. 

Low-Valent Iron Catalysis 1,5-Hydrogen Transfer/Kumada Coupling... 

B. Iron Catalysis in the Cross Coupling of Alkenyl Halides and Grignard Reagents Grignard reagents are cross coupled stereospecifically with alkenyl halides such as 1-bromopropene in the presence of catalytic amounts of iron complexes.(27)... 

Iron catalysis is experiencing a renaissance. In 2007, Bolm described a selective Nl-arylation using FeCls as the catalyst and K3PO4 as the base [218]. The coupling reaction was facilitated by the addition of 20 mol% of DMEDA as a chelating agent. 

Scheme 13.20 Atyl-allq l coupling reactions between allq l halides and Grignard reagents under iron catalysis.

Nakamura s group reported the first efficient cross-coupling reaction between primary and secondary allq l sulfonates using arylzinc reagents achieved under iron catalysis (Scheme 13.23). This contribution displays... 

A large number of cross-dehydrogenative couplings using iron catalysis under oxidative conditions have been reported [89,90]. The coupling of sp -sp, sp -sp, and sp -sp bonds has been achieved. These reactions proceed through iron-mediated electron-transfer processes and are outside the scope of this review. 

The use of palladium, and to a minor extent nickel, catalysts (the Negishi cross-coupling cf. Chapter 3 of this book), is prevalent and stiU attracts most of the research interest. In recent years, however, some other variants involving, for instance, cobalt or iron catalysis have emerged. 

To date, catalytic applications of NHC-Fe systems are still scarce and do not reflect the full potential of this cheap, abundant and non-toxic metal that shows great promise for the formation of carbon-carbon bonds via cross-coupling reactions. Recourse to ill-defined species generated in situ and difficulties in understanding the intimate nature of reaction mechanisms do not ease the development of organo iron catalysis. Support from the related field of iron biocatalysis and recent discoveries in organometallic chemistry are expected to provide help and inspiration for further advances. 

Cross-coupling between alkyl halides and alkenyl Grignard reagents has also been succeeded using iron catalysis. Catalytically active systems are tris(acetylacetonato)iron with TMEDA and hexamethylenetetramine (HMTAy or iron(IIl) chloride as a precatalyst in combination with TMEDA as a ligand (Scheme 4—233). ... 

Easy-to-handle arylboronic compounds can also be reacted in a Suzuki-Miyaura-like fashion with nonactivated alkyl halides using iron catalysis (Scheme 4-239). Two novel iron complexes with sterically hindered diphosphane ligands have been developed for this transformation. Additionally, a magnesium cocatalyst is required. For the mechanism, action of the redox couple Fe(III)/Fe(II) is discussed. This requires the intermediate formation of an alkyl radical species as displayed in Scheme 4-238. " ... 

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