Iron arylboronic acids

Scheme 13.5 Iron-mediated arylation of benzene with arylboronic acids.

Hu and Yu reported an iron/macrocyclic polyamine-catalyzed reaction of arylboronic acids with a large excess of pyrrole or pyridine at 130°C tmder air (Eqs. 26 and 27) [63], based on their previous studies on iron-mediated reactions (initial report using a stoichiometric amount of iron [64]). Pyrrole derivatives were arylated at 2-position in good yield (Eq. 26), but when pyridine was used as a substrate, the catalyst turnover was poor and 2-arylpyridine was obtained together with a small amount of 3-aryl- and 4-arylpyridine (Eq. 27). Because a catalytic amotmt of a radical scavenger did not inhibit the reaction, the authors proposed an oxoiron complex as the active species to activate the ort/io-hydrogen of the heterocycle via o-bond metathesis and also performed a DPT analysis of the mechanism. A related iron-catalyzed reaction of aryl boronic acids with heteroarenes was reported by Singh and Vishwakarma [65]. 

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. 

Iron was shown to couple pyridines, quinolines, and isoquinolines with boronic acids without the need for an inert atmosphere (Scheme 39) (13JOC2639). Both pyridine and chloropyridines coupled smoothly. The chloropyridines coupled with phenyl boronic acid as well as those with electron-donating groups such as methoxyl or methyl. Isoquinoline coupled with phenylboronic acid in a similar manner (40% yield) quinoline coupled with a handful of arylboronic acids in 30-38% yields. 

A catalytic cycle shown above has been proposed. The reaction begins with the oxidation of the iron complex to form the oxo-iron species by reaction with oxygen. An electrophilic attack of this oxo-iron complex on C-2 position of heteroarenes is aided by the heteroatom. Following a consequential deprotonation, a tertiary complex comprising iron, heterocycle, and MCPA ligand is formed. With the introduction of arylboronic acid, transmetalation and then elimination of the iron complex can yield the desired product. 

Hu and Yu disclosed an iron-mediated direct Suzuki-Miyaura coupling between IH-pyrrole (1) and arylboronic acids. Macrocyclic polyamines (MCPA) were studied in this process under the reaction conditions, a tertiary pyrrole-MCPA-oxoiron complex 4 is formed which undergoes transmetallation with the arylboronic acid. After elimination of the iron complex, 2-phenyl pyrrole (2) was isolated in 66% yield with only trace amounts of phenol. 

Not only Grignard reagents but also boronates may be employed as nucleophiles in iron-catalyzed cross-coupling. Aryl iodides and bromides react in a Suzuki-Miyaura-type coupling with arylboronic acids in the presence of 10 mol% iron(lll) chloride and a stoichiometric amount of potassium fluoride in ethanol at 100 °C under air to give the corresponding biaryl compounds with good to excellent yields (Scheme 4-227). ... 

Scheme 4-249. Iron mediated coupling of arylboronic acids with unactivated arenes to form unsymmetric biaryls.

Arylboronic acids can be added to electron-deficient aryl aldehydes in the presence of iron(III) chloride and 2-(di-/ert-butylphosphano)biphenyl as ligand (Scheme 4-321). The resulting diaryl methanols are obtained in moderate to excellent yield. ... 

Scheme 4-321. Iron-catalyzed addition of arylboronic acids to aromatic aldehydes.

---