Iron catalysis arylation

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

However, arylmagnesium organometaUics can only be used in a few cases " like an iron-catalyzed homocoupling (equation 92) . Iron catalysis is further used for the dechlorination of electron-rich aryl chlorides by Grignard reagents . ... 

Recently, Taillefer et al. reported an Fe/Cu cooperative catalysis in the assembly of N-aryl heterocycles by C—N bond formation [90]. Similarly, Wakharkar and coworkers described the N-arylation of various amines with aryl halides in the presence of Cu—Fe hydrotalcite [91]. Interestingly, Correa and Bolm developed a novel and promising ligand-assisted iron-catalyzed N-arylation of nitrogen nucleophiles without any Cu co-catalysts (Scheme 6.19) [92]. Differently substituted aryl iodides and bromides react with various amides and N-heterocycles. The new catalyst system consists of a mixture of inexpensive FeCl3 and N,N -dimethylethylenediamine (dmeda). Clearly, this research established a useful starting point for numerous future applications of iron-catalyzed arylation reactions. 

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 1 Selective C2-arylation of IH-pyrroles via rhodium, palladium, and iron catalysis.

Nitrogen-directed C—H activation to form C—C bonds like aryl-aryl is a relatively recent class of reactions. In 2008, Nakamura and coworkers presented the first report of an iron-catalyzed C—H arylation reaction—a homogeneous iron catalysis featuring C—C bond formation via C—H activation. As an overall synthetic transformation, it was a formal nucleophilic displacement of the ort/ o-hydrogen atom by... 

Yamamoto and coworkers studied the substitution of ally lie phosphates by Grignard reagents in the presence of copper or iron salts. Only the Sn2 product is formed under copper catalysis whereas, in the presence of iron(III) acetylacetonate, the Sn2 product is generally obtained with an excellent selectivity (Scheme 49). It should be noted that aryl-, alkenyl-, aUcynyl- and aUcyhnagnesium halides can be used successfully. 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies.659 663 The use of C6F5IO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. 59 The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fev-oxametallacyclobutane intermediate.660 Picket-fence porphyrin (TPiVPP)FeCl-imidazole, 02 and [H2+colloidal Pt supported on polyvinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes.663... 

Iron phthalocyanine catalysis is more effective for alkenes conjugated to an aryl ring as compared with the Fe2+/Fe3+ salts (Scheme 3.34), whereas the Nicholas method worked much better for acyclic non-conjugated alkenes such as li (Scheme 3.35). 

Inspired by the observation of efficient hydrogenation catalysis with iron compounds bearing redox-active tridentate ligands, our laboratory also explored related chemistry with reduced iron compounds with bidentate chelates [92]. Aryl-substituted a-diimines were attractive supporting ligands due to their ease of synthesis, estabUshed one- and two-electron redox-activity [92-95] and precedent in iron-catalyzed C-C bond forming reactions [96-98]. 

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