Furans borylations

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... 

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>. 

The formation of borylated heterocyclic componnds has attracted mnch interest with regard to their use as starting materials for cross-conpling reactions, the possible formation of interesting macrocyclic species, and their applications in optoelectronic materials. Several synthetic routes are available for the borylation of the five-membered heterocycles thiophene, furan, pyrrole, and related componnds (101-103 ... 

As is obvious from the B-NMR spectral data of borylated thiophene, /V-methylpyrrole, and furan in Fig. 5, "B signals appear at higher field in comparison with phenylborane. This suggests that B-C (pn-p ) interaction is much more significant in five-membered heteroaromatic rings than in a phenyl ring (76CB1075). 

Fig. 5. "B NMR data of phenylborane, as well as borylated thiophene, A-methylpyrrole, and furan.

The reaction involves the formation of carbanions that are stabilized by boryl groups . Similar carbanions are prepared from 9-methyl-9-borabicyclo[3.3.1]nonane (B-methyl-9-BBN), 9-alkenyl-9-borabicyclo[3.3. l]nonanes(B-alkenyl-9-BBN) , or alkenyldisiamylboranes with 2,2,6,6-tetramethylpiperidine (Li-TMP) in tetrahydro-furan (THF) ... 

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C-2- and C-3-borylated furans was developed. It was found that the copper catalyst as well as the gold catalyst with a relatively more basic triflate counterion favored boryl migration toward C-3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate afforded C-2-borylated furans via a formal 1,2-hydrogen shift (14JA13146). 

C—H borylation of indoles has been reported by a number of groups. Chirik and coworkers utilized pincer-ligated cobalt complexes with N-methyhndole and recorded the C-2 borylated indole (133) as the major product (2014JAC4133). The catalysts employed demonstrated high catalysis turnover and low catalytic loading and also demonstrated efficacy with other electron-rich heteroarenes (furan, thiophene, benzofuran) as well as electron-deficient pyridines. More recendy, platinum-NHC complexes have been used in the selective C—H borylation of indoles (2015JAC12211). The authors reported higher isolated yields with this... 

The first Cu-catalyzed asymmetric borylative cyclization reaction between cyclohexadienone-containing 1,6-enynes and B2pin2 was established to afford an optically pure d5-hydrobenzo[l ]furan framework bearing alkenylboronate and enone substructures through a tandem process of selective P-borylation of the propargylic ether and subsequent enantioselective conjugate addition to cyclohexadienone (13JA11700). 

Like the other five-membered heterocycles, furan can also be regioselectively bor-ylated in high yield to afford the C2-borylated product using the Hartwig-Ishiyama protocol. ... 

Interestingly, [IrCl(cod)]2 did not catalyze the reaction. The [Ir(OMe)(cod)]2]/dtbpy catalytic system in nonpolar solvents such as hexane was found to be very effective for the synthesis of arylboronates, using stoichiometric amounts of arene and pin2fi2 at room temperature (Table 1). The borylation t) ically occurs at the para or meta position with respect to the functional groups on the arene. The ortho C-H positions are less active due to the steric hindrance. In addition, the electronic effect plays a minor role, where the electron-poor carbon seems to be more active. This allows the reaction to occur regiospecifically (Table 1). For example, the borylation of 1,3-disubstituted arenes selectively occurs at the common meta position, while the borylation of heterocycles such as benzo[l)]thiophene, benzo[fc]furan, or indole occurs selectively at the 2-position (Table 1). 

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