Thiophenes borylations

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.

A convenient direct route has recently been described for obtaining regioregular polyalkylthiophenes using a tandem iridium-catalyzed borylation to produce the monomer, and a palladium-mediated coupling to produce the polymer [68]. The treatment of substituted thiophenes with B2pin2 in the presence of [lrCl2(COD)]2/ 4,4 -di-tert-butyl-2,2 -bipyridine (DTBPY) provided the expected monomer in 97% yield (Scheme 7.35). 

Although not fitting exactly into the scope of this book, the iridium catalyzed borylation of five membered heterocycles through C-H bond activation also deserves mentioning. A recent report by Miyaura disclosed the reaction of bis(pinacolato)diboron with heteroaromatic systems, where thiophene, fiirane and pyrrole were converted to their 2-boryl derivatives with good selectivity (6.86.), The yields presented refer to the diboron compound since the heterocycles were used in excess in all cases. Indole, benzofurane and benzothiophene were monoborylated with similar efficiency.116... 

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 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 ... 

Borylation. Arenes (including thiophene ) are borylated by pinacolatoborane using [(cod)IrOMe]2 as catalyst. Tbe remarkable feature of this reaction is m-substitution, through such unusually patterned aromatic compounds become available. " ... 

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 reactive boronium ion (88) has been prepared from l,8-bis(dimethylamino) naphthalene and has been shown to be effective in the borylation of pyrrole and indole derivatives. Catechol-ligated borenium cations, such as (89), have also been used in the borylation of a range of anilines, thiophenes, and (V-heterocycles. ... 

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. 

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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