Borylation, arenes

Scheme 11.9 Arene borylation in the synthesis of taiwaniaquinol B by Haitwig...

In the realm of C-H bond transformations applied toward the synthesis of fine chemicals, iridium has not achieved the prominence attained in recent years by the second-row platinum group metals, particularly palladium [10]. A notable exception, however, has been the leading role of iridium in the valuable chemistry of arene borylation [11]. 

Subsequent reports showed that a wide range of arene substrates could be borylated, with regioselectivity predominantly dictated by steric factors [77]. In the case of monosubstituted arenes, borylation generally occurs predominantly at the meta and para positions, with a near-statistical ratio while substitution at the ortho position is minor (Scheme 4). In the case of 1,3-disubstituted arenes, borylation occurs exclusively at 5-position. The selectivity was reversed in favor of the ortho position in the case of diethylbenzamide, presumably due to chelation [77]. 

One of the most active and well-studied catalytic borylation systems is that generated from iridium(l) precursors such as [lr(COD)Cl]2 or [lr(COD)(OMe)]2 and bipyridine type ligands such as 2,2 -bipyridine or 4,4 -di-ferf-butyl-2,2 -bipyridine (dtbpy). In 2002, Ishiyama, Miyaura, and Hartwig et al. reported that the combination of [lr(COD)Cl]2 and 2,2 -bipyridine catalyzes arene borylation in the presence of excess arene under mild conditions (80°C). When the catalyst is generated from [lr(COE)2Cl]2 and dtbpy, the reaction proceeds even at room temperature [78, 79]. The same groups optimized conditions and found that the combination of [Ir(COD) (OMe)]2 and dtbpy (10) is a highly effective catalyst in the borylation of arenes so that reactions can be successfully performed with equimolar ratio of arenes and... 

The structure of a further iridium bis(boryl) complex, the cationic species [(tbbpy)(cod)Ir(Bpin)2]+[OTf] (9.33), together with that of a related neutral tris(boryl) system (vide infra), has been reported as part of a study to elucidate the mechanism and intermediates of arene borylation chemistry [136]. 9.33 features an approximately octahedral coordination geometry with mutually cis pairs of bipyridyl, diene and Bpin ligands. The two Ir-B bond lengths [2.06(2) and 2.12(1) A] are effectively identical within the standard 3a limit. 

SCHEME 24S9 Representative example of arene borylation. 

LiuT, Shao X, Wu Y, ShenQ (2012) Highly selective trifluoromethylation of 1,3-disubstituted arenes through iridium-catalyzed arene borylation. Angew Chem Int Ed 51 540-543. doi 10. 1002/anie.201106673... 

The authors also investigated the C—H borylation of a series of substituted pyridine 332-333 and arene 334-336 derivatives. Careful examination of the reaction conditions revealed that high yields could be obtained in the presence of 3 mol% catalyst and the diborane B2Pin2 (HBPin is formed in situ) in 0.55 M THE In these cases, THF was favored over running the reaction neat since the pyridine and arene derivatives were often solids. Through their studies the authors determined that borylation occurred at the most sterically accessible C—H bonds, with the selectivity arising from the C—H oxidative addition step. In addition, electron-poor arenes borylated much more readily than the electron-rich examples due to the acidity of the C—H bond. 

Conclusion. [Ir(OMe)(cod)]2 is a very useful catalyst for C-H activation. With 4,4 -di-rm-butyl-2,2 -bipyridine as ligand, it can catalyze arene borylation to form arylboronates. The borylated arene can then be further converted to other products such as phenols, aryl halides, aryl silanes, arylboronic acids and aryl trifluo-roborates, in a single pot. 

This result represents the first example of a synthetic cycle for arene borylation facihtated by an f-element. The authors investigated the reaction mechanism and proposed a concerted direct B—H attack on an aromatic C—H bond, which resulted in the formation of the B—C bond and the release of one molecule of H2. This type of reactivity resembles a-bond metathesis if boron is considered a metal center. Albeit uranium was not directly involved in the borylation of the arene C—H bond, the formation of the diuranium arene inverse-sandwich complex plays an important role. The arene, i.e., benzene or naphthalene, is partially reduced upon coordination to uranium,which makes it more susceptible to attack by an electrophile such as borane. Therefore, although uranium is not direcdy involved in the C—H bond activation step, this example illustrates that f-elements can render arenes reactive in nonmetal-mediated transformations by forming activated arene metal complexes. 

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