Iridium Catalyst Borylation

Keywords Alkane metathesis Borylation C-H bond activation Dehydrogenation Hydroarylation Iridium catalyst Silylation... 

Perhaps the most interesting development of these types of reactions, the formation of a single product from terminal borylation of linear alkanes by a transition metal catalyzed reaction, comes from Hartwig. Cp Rh(jj -C6Me6) catalyzes the high-yield formation of linear alkylboranes from commercially available borane reagents under thermal conditions. Iridium catalysts are also effective. ... 

Smith and Marder reported the dehydrogenative borylation of arenes, yielding arylboronates, with pinacolborane in the presence of rhodium and iridium catalysts such as Cp Rh(7/ -C6Me6), CpIrPMes, and [RhClP( Pr)3]2N2 (eq 22). Toluene and other methyl substituted arenes react with pinacolborane in the presence of [RhClP(Tr)3]2N2 and furnish benzylboronates via benzylic C-H activation and dehydrogenative borylation (eq 23). 

An alternative method for the direct arylation of thiophenes via C-H bond functionalization is a one-pot C-H borylation/Suzuki-Miyaura reaction sequence (Scheme 16) [66]. The first step is the generation of the organoboron intermediates 33 and 36 from the thiophenes 32 and 35 by means of an iridium catalyst. The second step, the Suzuki-Miyaura cross-coupling, can be accomplished subsequently by adding an aryl bromide to the solution of crude boronate esters without the necessity to remove the spent iridium catalyst beforehand. 

Beyond arylation, pyrroles can also be borylated with iridium catalysts using either pinacol borane or bis(pinacolato)diboron (eq 14). These products can then be carried on for further functionalization. In this case, a-ketoesters are also thought to coordinate to zirconium in a bidentate fashion. 

In addition to arylboronic acids, arylboronates have also been successfully used in fluorination reactions (Scheme 7.56) [93]. These boron compounds are attractive substrates since they are typically more robust than other boron species and can often be stored for long periods of time. The catalyst system for this reaction was a copper(I) triflate species along with 2 equiv of silver fluoride. While several sources of electrophilic fluorine generated the aryl fluorides, N-fluoro-2,4,6-trimethylpyridinium hexafluorophosphate was the most effective. The chemistry displayed broad functional group tolerance with the lowest yields obtained with heteroarylboronates. The authors were also able to devise a one-pot borylation-fluorination reaction starting from the parent arene. The arene was converted into an arylboronate through an iridium-catalyzed borylation reaction in the first step of the reaction, while fluorination was achieved during the second step. This is particularly attractive since it facilitates the conversion of unfunctionalized substrates into aryl fluorides. 

A sequential borylation/bromination one-pot reaction selectively converted arenes into aryl bromides (Example 7.23) [131], Borylation was the first step in the process and was achieved using an iridium catalyst and B Pin. Once this step was complete, the solvent was removed under vacuum and replaced with a methanol/water mixture, and an excess of copper(II) bromide was added. The chemistry displayed broad substrate scope and effectively converted a range of functionalized arenes into aryl bromides. The chemistry was extended to aryl chlorides, and over 10 arenes were successfully chlorinated in moderate to good yields. 

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 direct borylation of arenes is an attractive strategy for accessing synthetically useM arylboron reagents. Iridium complexes have emerged as the catalyst of choice for the selective borylation of arenes using HB(pin) or Bj(pin)j [67], Extensive studies by the Hartwig, Ishiyama, and Miyanra groups have led to the identification of the Ir OMe(cod)j/dtbpy as the optimal catalyst system for these transformations [68-70], As illustrated in Scheme 24.59, electronically diverse arenes are borylated at room temperature to afford the products in excellent yields. The site-selectivity of these... 

Iridium(I)-catalyzed aromatic C-H bond borylation widi pinacolborane (HBpin) and its mechanism have been studied extensively by Smith [57]. Iridium complexes (27, 28) themselves are inefficient, but addition of a small electron-donating phosphine such as PMej or chelating dmpe [l,2-bis(dimethylphosphino)ethanej to give an iridi-um(I)-phosphine complex (29, Scheme 2.8) substantially increases catalyst activity and turnover number [57aj. The maximum turnover number achieved in the borylation of benzene with HBpin at 150 °C in a sealed ampoule is 4500 [57aj. 

A class of iridium(I) complex (30) possessing 2,2 -bipyridine (bpy) or 4,4 -di-fe/t-butyl-2,2 -bipyridine (dtbpy) ligands exhibits excellent activity and selectivity for aromatic C-H borylation with Bzpinj [59] or HBpin [60], An Ir catalyst prepared from %[IrCl(COD)2l2 (COD = 1,5-cyclooctadiene) and dtbpy achieves a maximum turnover number (8000) with 0.02 mol% catalyst loading at 100 °C. The reaction was first demonstrated at 80-100 °C using an Ir-Cl complex, but was foimd to proceed smoothly even at room temperature when the catalyst is prepared from... 

Silylene-based pincer ligands offer exciting reactivities in terms of transition metal complex formation and their applications in catalytic systems. The pincer complex [SiCSi)Ni(II) can be synthesized by oxidative addition of C—H bond of the corresponding [SiC(H)Si] ligand. [SiCSi]Ni(II) complex has been employed as catalyst for Ni-catalyzed Sonogashira reactions (8). Moreover bis(silylene) pincer complexes of iridium and rhodium reveal strong 5-donating ability of divalent silicon and have demonstrated selectivity in catalytic C—H borylation reactions with arenes (9). 

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