Borylation of arenes

The borylation of arenes provides direct access to arylboronate esters that can be used in a number of subsequent metal-catalyzed processes. The first example of such a reaction is shown in Equation 18.68. Although the initial system underwent only a few turnovers over long times, two subsequent types of catalyst are particularly effective for these 

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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 was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

Similarly, recent computational studies suggested that the iridium-catalyzed borylation of arenes also proceeded via the insertion mechanism. H. Tamura, H. Yamazaki, H. Sato, S. Sakaki,/. Am. Chem. Soc. 2003, 325, 16114-16126and references therein. 

Iridium-Catalyzed Borylation of Arenes 126 Tatsuo Ishiyama and Norio Miyaura... 

A highly regioselective borylation of arenes and heteroarenes (such as benzo[ ]furan 82) was achieved by the iridium-catalyzed C-H activation reaction, as shown in Equation (94) <2003CC2924>. 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

Although direct borylation of arenes in specific cases is possible and metal-catalyzed C H and C-X activation reactions provide new possibihties, most arylboranes are prepared from organometalhc precursors. In particular, monoborylated... 

Tris(boryl) complexes have been shown to have a central role in the iridium-catalyzed borylation of arenes [136,137]. The syntheses of the half-sandwich iridium(III) systems ( 6-toluene)Ir(Bcat)3, 9.34, and rf-mesitylene)Ir(Bcat)3, 9.35, were reported by Marder and co-workers in 1993, by exploiting the reaction of ( 5-Ind)Ir(cod) with excess HBcat using the arene as a solvent. The organic products of the reactions were found not only to include species resulting from the hydroboration or hydrogenation of the cod or indenyl ligands, but also compounds derived from borylation... 

Murphy JM, Tzschucke CC, Hartwig JF (2007) One-pot synthesis of arylboronic acids and aryl trifluoroborates by Ir-catalyzed borylation of arenes. Org Lett 9 757... 

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). 

Although this is not a Pd-catalyzed reaction, direct borylation of arenes 15 with pinacolborane 10 to give arylboronate 16 can be achieved using several transition metal complexes as catalysts. Some Rh and Ir catalysts are known to be active [21,22],... 

Use of iron(iii) oxide has been reported for a number of other organic transformations, most notably a 3-component aldehyde, alkyne and amine coupling, and for the C(sp )-H borylation of arenes using bis(pinacolato)diboron. ... 

Table 10. Iridium-catalysed borylation of arenes with pinacol ester of diboron (350)...

In contrast to the rare occurrence of the oxidative addition of C-C bonds, the oxidative addition of the nonpolar bonds between two main group atoms, such as boron and silicon, can be facile. The oxidative addition of Si-Si bonds in disilanes and the B-B bond in diborane(4) reagents is likely to be a step in a variety of catalytic reactions, including the additions of disilanes, - diborane(4) reagents, and silaboranes - across olefins and alkynes, the silylation and borylation of arene C-H bonds, - - the borylation of alkane C-H bonds, and the conversion of aryl halides to arylsilanes and arylbo-ronate esters. " ... 

Figure 18.2. Products from the borylation of arenes with B pin as reagent.

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... 

Rhodium- and Ir-catalyzed C-H borylation of arenes are nowadays important pathways. The regioselectivity of the first successful reactions were chiefly under steric control (to the least hindered positions), often offering a simple access to positions (for instance position 5 of 1,3-dimethoxy-benzene) where installation of a halide group for Pd-catalyzed borylation is difficult. These successes prompted a promising development of strategies for directed C-H borylations. ... 

The Ir catalyst [Ir(COD)OMe]2 was successfully exploited for the borylation of arenes under microwave conditions, this was then followed by the in situ copper-catalyzed sulfenylation of the intermediate aryl boronate to the aryl thioether product using an arylthiol, with microwave conditions [118]. 

Under conditions similar to those used for aliphatic C-H borylation, rhodium complexes catalyze the borylation of arenes by pin2B2 [53], Cp Rh( n -QMeJ (19), which in situ generates a coordinatively unsaturated rhodium species (20) active for oxidative addition, was found to be the best catalyst, giving a 92% yield after 2.5 h with 5 mol% catalyst loading and an 82% yield (328 turnover number of the catalyst, TON) with 0.5 mol% loading at 150 °C. 

The ready availability of arylboronates by an aromatic C-H borylation provides a synthetic link to the well-established palladium-catalyzed cross-coupling reactions, rhodium-catalyzed 1,4-addition to a,p-unsaturated carbonyl compounds, and other bond forming reactions using arylboronic esters (Scheme 2.12). Borylation of 1,3-dichlorobenzene with pinacolborane is followed directly by a cross-coupling reaction with methyl p-bromobenzoate for the synthesis of a biaryl product in 91% yield [60]. Pinacol esters of arylboronic acids react much slower than the free acids [62], but both derivatives achieve high isolated yields and comparable enantioselectivities (91% ee) in asymmetric 1,4-addition to N-benzyl crotonamides [63]. Borylation of arenes followed by oxidation of the C-B bond is synthetically equivalent to an aromatic C-H oxidation to phenols [64]. Oxidation of the resulting arylboronates with Oxone in a 1 1 acetone-water solution is completed within 10 min at room temperature. 

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