C-H borylation

The beneficial effects of chelating ligands were also demonstrated by Hartwig, Ishiyama and Miyaura [62]. This group isolated the iridium(I) complex [lr(Bpin)3(COE)(DTBPY) modified with simple 2,2 -bipyridine ligands (such as 4,4 -di-tert-butyl-2,2 -bipyridine DTBPY), which seemed to be responsible for the first catalytic C—H borylation at room temperature (Scheme 7.30). An extension... 

Ishiyama, T., Takagi, J., Hartwig, J.F. and Miyaura, N., A stoichiometric aromatic C—H borylation catalysed by iridium(I)/2,2 -bipyridine complexes at room temperature, Angew. Chem., Int. Ed. Engl, 2002, 41, 3056-3058. 

Aromatic C-H borylation is catalyzed by an Ir complex with a small and electron-donating ligand, for example PMe3, l,2-bis(dimethylphosphino)ethane (dmpe), 2,2 -bipyridine (bpy), and 4,4,-di-tert-butyl-2,2 -bipyridine (dtbpy) (Table 1)... 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

Scheme 2. Proposed mechanism of photochemical C-H borylation catalyzed by Cp Re(CO)3.

Direct borylation of hydrocarbons catalyzed by a transition metal complex has been extensively studied by several groups and has become an economical, efficient, elegant, and environmentally benign protocol for the synthesis of a variety of organoboron compounds. The Rh-, Ir-, Re-, and Pd-catalyzed C-/H borylation of alkanes, arenes, and benzylic positions... 

The catalytic C-H borylation of hydrocarbons with B2pin2 or HBpin has been reviewed.2,201,335... 

Table 3 Aromatic C-H borylation with B2pin2 and [lr(OMe)(COD)]2-2dtbpy at room temperature...

The reaction of HBpin in toluene in the presence of RhCl P(/-Pr)3 2(N2) (1 mol%) at 140 °C resulted in a mixture of (borylmethyl)benzene (69%) and bis(boryl)methyl benzene (7%), along with several products arising from aromatic C-H borylation (ca. 15%).345 Rhodium-bpy complexes catalyzed the borylation at the benzylic C-H bond.351 Pd/C was found to be a unique catalyst for selective benzylic C-H borylation of alkylbenzenes by B2pin2 or HBpin (Equation (70)).360 Toluene, xylenes, and mesitylene were all viable substrates however, the reaction can be strongly retarded by the presence of heteroatom functionalities such as MeO and F. Ethylbenzene resulted in a 3 1 mixture of pinacol 1-phenylethylboron and 2-phenylethylboron derivatives. 

Initial reports on the borylation of alkanes using isolated transition-metal-boryl complexes date back to 1995, when Hartwig showed that Cp Re(CO)2(Bpin)2 converts pentane to 1-borylpentane with high regioselectivity. " The catalytic C-H borylation of alkanes with Cp Re(CO)3 using photochemical activation was demonstrated soon thereafter (equation 25). Also, an efficient thermal process that involves the use of rhodium catalysts has since been developed (equation 26). It is interesting to note that this methodology is not restricted to small molecules, but has recently been exploited for the direct side-chain functionalization of polyolefins. ... 

V. A. Kallepalli, S. M. Preshlock, P. C. Roosen, R. E. Maleczka, Jr., and M. R. Smith, III, Iridium-Catalyzed C-H Borylation Recent Synthetic Advances, Abstracts of Papers, 237th ACS National Meeting, Salt Lake City, UT, March 22-26, 2009. See also G. A. Chotana, V. A. Kallepalli, R. E. Malezcka Jr., and M. R. Smith, Tetrahedron, 2008, 64, 6103. Malezcka and Smith were honored in 2008 as winners of the Presidential Green Chemistry Challenge Award. 

Although not a palladium-catalyzed reaction, the Ir(I)-catalyzed C-H borylation reaction developed independently by Smith and Malezcka [58] and Hartwig and Miyaura [59] deserves some mention in the context of indole and pyrrole functionalization. Based on the original studies, indoles and pyrroles can be borylated (and hence cross coupled under Suzuki conditions) to form either the C2 or C3 functionalised products (Scheme 35) [60, 61]. Free (NH)-indoles and pyrroles react exclusively at the C2, whereas /V-TIPS indole and pyrroles are borylated at the C3 positions. Interestingly, Smith, Maleczka and co-workers also demonstrated that when the C2 position of indole is blocked, then the borylation reaction takes place at the C7-position of the indole nucleus [62]. They propose that an A-chelation to Ir (or B) is responsible for the observed selectivity. 

Scheme 35 Regioselective Ir(I)-catalyzed C-H borylation of indoles and pyrroles...

They found that combination of the Ir(T) catalyzed C-H borylation and Suzuki coupling sequence led to a two-step, one-pot C-H Suzuki arylation that enabled direct transformation of the N-Boc pyrrole to the C3 arylated intermediate in 78% yield. Following installation of the required acyl group, an application of their oxidative Pd-catalyzed C-H alkenylation reaction enabled formation of the key structural architecture of the natural deliver the natural product. The orthogonal selectivity characteristics displayed by these C-H functionalization processes makes possible iterative functionalization of the heteroaromatic pyrrole core. Utilization of the highly versatile C-H borylation - Suzuki coupling to install the aromatic functionality opens up possibilities of facile analogue synthesis via this route. 

Intensive studies also showed that many transition metal complexes are able to catalyze aromatic C-H borylation of various arenes (Scheme 7), e.g., Cp Ir(H)(Bpin)(PMe3) [64,65], Cp Rh(Ti4-C6Me6) [65,66], ( 75-Ind)Ir(COD) [67], (776-mesitylene)Ir(Bpin)3 [67], [IrX(COD)]2/bpy (X = Cl, OH, OMe, OPh) [68-70]. A very recent study by Marder and his coworkers showed that [Ir(OMe)(COD)]2 can also catalyze borylation of C-H bonds in N-containing heterocycles [71]. For the Ir-catalyzed borylation reactions, it is now believed that tris(boryl)iridium(III) complexes [67,69], 40c, [72] are likely the reactive intermediates and a mechanism involving an Ir(III)-Ir(V) catalytic cycle is operative [67,69]. A recent theoretical study [73] provided further support for this hypothesis. A mechanism, shown in Scheme 8, was proposed. Interestingly, there are no cr-borane complexes involved in the Ir-catalyzed reactions. The very electropositive boryl and hydride ligands may play important roles in stabilizing the iridium(V) intermediates. 

Density functional theory (DFT) modebng studies [56] pointed out the relatively high activation energy of the oxidative addition. Since carbopaUadation of 24 is probably fast, complex 24 could not be detected. However, in the acetoxylation and the C-H borylation studies, the more strongly oxidizing iodine was used instead of 21, and detection of the Pd(IV) intermediate could be reabzed (Sections 4.4.2 and 4.4.3). 

C-H borylation is a widely used methodology for the synthesis of organoboronates [63-65]. Most of the applications have been presented for the synthesis of aryl-boronates. However, functionalization of alkenes has also attracted much interest [66, 67]. In most applications, iridium catalysis was used. However, in case of alkenes, borohydride forms as a side product of the C-H borylation, which undergoes hydroboration with alkenes. This side reaction can be avoided using palladium catalysis under oxidative conditions. In a practically useful implementation of this reaction, pincer-complex catalysis (Ig) was appHed (Figure 4.17) [51]. The reaction can be carried out under mild reaction conditions at room temperature using the neat aUcene 34 as solvent. In this reaction, hypervalent iodine 36, the TFA analog of 29, was employed. In the absence of 36, borylation reaction did not occur. 

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