Iron cross-coupling reactions

Our group has recently developed an alternative method for alkyl-(hetero)aryl- as well as aryl-heteroaryl cross coupling reactions catalyzed by iron salts.3 4 This methodology was inspired by early reports of Kochi et al.5>6 on iron-catalyzed cross coupling of vinyl halides and is distinguished by several notable advantages. 

TABLE IRON CATALYZED CROSS COUPLING REACTIONS OF GRIGNARD REAGENTS WITH (HETEROJARYL CHLORIDES OR -TRIFLATES... 

Scheme 29 Iron-catalyzed cross coupling reaction of aryl Grignard reagents with alkyl halides...

Iron-catalyzed C(sp )-C(sp ) bond-forming cross-coupling reactions of alcohols with alkenes has been reported by Tu and coworkers in 2009 [109]. Reactions of primary alcohols with various alkenes in the presence of a catalytic amount of FeCls in 1,2-dichloroethane afford the desired secondary alcohols as the crosscoupling products in moderate to good yields (Scheme 33). Iron sources such as... 

As shown above, iron salts are able to catalyze cross-coupling reactions. While most of the known procedures nowadays start with commercially available iron... 

As complex 67 outperforms the Fe(0)-ate complexes in rate and yield, shown in the reaction of cyclooctenyl bromide with PhMgBr (full conversion complex 67 <20 min, 81% yield, 38 18 h, 39% yield), it was shown that both Fe(0)-ate and Fe (—2)-ate complexes should be intermediates in cross-coupling reactions, but the major contribution should be made by the route emanating from Fe(—2)-com-plexes. The superiority of Fe(—2)-ate complexes was also shown in the stoichiometric insertion of 67 into allylic halides, which proceeded much faster (<5 min) than with any higher valent iron complex (hours or days). 

Phosphine ligands based on the ferrocene backbone are very efficient in many palladium-catalyzed reactions, e.g., cross-coupling reactions,248 Heck reaction,249 amination reaction,250 and enantioselective synthesis.251 A particularly interesting example of an unusual coordination mode of the l,l -bis(diphenylphosphino)ferrocene (dppf) ligand has been reported. Dicationic palladium(II) complexes, such as [(dppf)Pd(PPh3)]2+[BF4 ]2, were shown to contain a palladium-iron bond.252,253 Palladium-iron bonds occur also in monocationic methyl and acylpalladium(II) complexes.254 A palladium-iron interaction is favored by bulky alkyl substituents on phosphorus and a lower electron density at palladium. 

Iron-catalyzed cross-coupling reactions of various acyl chlorides or thioesters with Grignard reagents have been pioneered by Marchese et al. and other research groups.322 These transformations provide general and convenient access to a wide range of ketones and have been further extended to the use of a supported iron(lll) complex.323... 

Recent notable improvements by Knochel and co-workers include iron-catalyzed cross-coupling reactions of various acid chlorides 148 with dialkylzinc reagents (Equation (24))324 as well as the iron-catalyzed arylation of aroyl cyanides 149 with Grignard reagents (Equation (25)).3 5 In the first case Knochel s reaction conditions tolerate ester groups on the organozinc compounds, while in the latter case ester, aryl alkyl ether, cyano, and chloro functionalities on the aromatic moieties are compatibles with the reaction conditions. 

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

Review on advances in iron-catalyzed cross-coupling reactions A. Ftirstner and R. Martin, Chem. Lett., 34, 624 (2005). 

IV. IRON-CATALYZED CROSS-COUPLING REACTIONS A. Iron-catalyzed Alkenylation of Grignard Reagents... 

In 1971, a year before the groups of Corriu and Kumada [5] independently reported the groundbreaking work on the topic of nickel-catalyzed cross-coupling reactions of aryl and vinyl halides with Grignard reagents, Tamura and Kochi described an iron-catalyzed vinylation reaction of Grignard reagents with vinyl halides (Scheme 5.1) [6]. 

Scheme 5.1 Stereospecific iron-catalyzed cross-coupling reaction reported by Kochi and Tamura in 1971.

Table 5.3 Iron catalyzed cross-coupling reactions of alkenyl sulfonates.

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