Aromatic borylation

Iridium-catalyzed borylation has also proved to develop the first general approach to functionalized unprotected indoles at the 7-position [67]. This selectivity can be explained by the nitrogen-directed aromatic borylation pathway in the mechanistic steps (Scheme 7.34). 

Marder reported that the alkyl C-H bonds in benzylic compounds are selectively borylated with a rhodium catalyst. Aromatic borylation prodncts were only observed as minor by-products. Similar results were obtained by Miyaura with Pd/C (equation 28). ... 

Paul S, Chotana GA, Holmes D, Reichle RC, Maleczka RE Jr, Smith MR III (2006) Ir-catalyzed functionalization of 2-substituted indoles at the 7-position Nitrogen-directed aromatic borylation. J Am Chem Soc 128 15552... 

In an interesting footnote to these alkane and aromatic borylation reactions, photolysis of Cp M(CO)3Me, M = Mo or W, and HsBPMe3 resulted in the formation of methane and Cp M(CO)3BH2PMe3.i ... 

Table 6 Catalytic aromatic borylation/amidation/oxidation of 3-substituted halobenzenes...

Cho J-Y, Iverson CN, Smith MR. Steiic and chelate directing efFects in aromatic borylation. J Am Chem Soc. 2000 122 12868-12869. 

Commercially available and inexpensive y-Fe203 magnetic nanoparticles (particle size 58 nm) also efficiently activate B2pin2 and promote a direct borylation of alkenes. " The mechanism of this unusual nano-Fc203-catalyzed aromatic borylation reaction is not clear. The kinetic isotope effect was measured to be 1.3, indicating that a C—H bond activation by oxidative addition to the iron catalyst is not likely. An electrophUic metalation by Fe—B species, followed by reductive ehmination, seems conceivable. 

In 2011, Hartwig and coworkers reported the total synthesis of taiwaniaquinol B (55, Scheme 11.9), a member of a family of diterpenoids that are derived from the abietane skeleton [36]. A key aspect of the Hartwig synthesis of taiwaniaquinol B was the use of the iridium-catalyzed borylation reaction to accomplish the C(5) functionalization of resorcinol derivative 53. This regioselectivity for the overall bromination is complementary to that which would be obtained using a standard electrophilic aromatic substitution (EAS) reaction. In the transformation of 53 to 54, a sterically controlled borylation was first accomplished, which was then followed by treatment of the boronic ester intermediate with cupric bromide to... 

In summary, boryl enolate 38 can be obtained via in situ O-borylation of N-propionylsultam 37 and converted to aldol product 40 upon treatment with aliphatic, aromatic, or a,/l-unsaturatcd aldehdyes at - 78°C in the presence of TiCU- As aldol product 40 can normally be obtained in crystalline form, in most cases diastereomerically pure anti- Ao 40 can also be obtained after the recrystallization. 

For each case we will also present catalytic analogues, namely (1) the activation of methane to form methanol with platinum, the reaction of certain aromatics with palladium to give alkene-substituted aromatics, and (2) the alkylation of aromatics with ruthenium catalysts, and the borylation of alkanes and arenes with a variety of metal complexes. 

A large number of studies have addressed the condensation of cyclic ketones with both aliphatic and aromatic aldehydes under conditions that reflect both thermodynamic (cf. Table 2) and kinetic control of stereochemistry. The data for cyclohexanone enolates are summarized in Table 8. Except for the boryl enolates cited (6), the outcome of the kinetic aldol process for these enolates... 

Iridium-Catalyzed Silylation and Borylation of Aromatic C—H Bonds... 

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 carbon—hydrogen bonds, catalzyed borylation, 9, 172 Aromatic carbonyl compounds, in samarium pinacol couplings,... 

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)... 

Functional group tolerance of the borylation is quite high. The reaction occurs selectively at the C-H bond for 1 bearing MeO, Me, I, Br, Cl, F3C, Me02C, and NC groups (Scheme 2) [4—10], In particular, it is interesting to note that aromatic C-H bonds are selectively borylated even in the presence of weaker benzylic C-H bonds or C-halogen bonds. 

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],... 

Table 2. Stoichiometric borylation of aromatic compounds 1 by HBpin 2 or B2pin2 3 catalyzed by 1 /2[lr(OMe)(cod)]2-dtbpy in hexane at room temperature a 5equiv. of to 2, b lOequiv. of 1 to 3, c 0.45 equiv. of to 2, d 1 equiv. of to 3.

The C-H coupling of aromatic heterocycles with bis(pinacolato)di-borane(4) was carried out in octane at 80-100 °C in the presence of a half equivalent [IrCl(COD)]2-(4,4 -di-tert-butyl-2,2 -bipyridine) catalyst (3 mol%). The reactions of five-membered substrates such as thiophene, furan, pyrrole, and their benzo-fused derivatives exclusively produced 2-borylated... 

The scope and value of the benzannulation reaction is further increased by the substitution pattern of the arene ring, which can be modified by the incorporation of allcynes bearing additional functional groups such as silyl, stannyl, or boryl substituents. These functional groups have been used in various palladium-catalyzed (cross)-coupling reactions [63, 64]. Further structural elaboration may be based on benzannulation followed by nucleophilic aromatic addition [63b]. 

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