2,7-Diborylanthracene as a Useful Building Block for Extended π-Conjugated Aromatics

Abstract

Ir-catalyzed direct diborylation of anthracene produced a 1:1 mixture of 2,6-and 2,7-diborylanthracenes, which could be separated by recrystallization. The SuzukiMiyaura crosscoupling using 2,7-diborylanthracene gave extended ³-conjugated 2,7-disubstituted anthracene derivatives as building blocks for macrocycles. A 1:1 mixture of 2,7-di(4-pyridyl)anthracene and [Pd(dppp)(OTf) 2 ] self-assembled into a [3 + 3] macrocycle.Ir-catalyzed direct borylation of aromatic compounds using bis(pinacolato)diboron is a very useful reaction for the synthesis of arylboronic acid pinacol esters, 1 which are reagents for the SuzukiMiyaura cross-coupling reaction.2 Marder et al. first observed the synthesis of 2,6-and 2,7-diborylnaphthalenes. Recently, using this reaction, we have reported on the selective synthesis of 2,8-and 2,9-diboryltetracenes, which are very useful building blocks for the regiospecific synthesis of extended ³-conjugated tetracene derivatives of organic semiconductors used in organic field-effect transistors. 4 It is well known that the meta-phenylene unit and its derivatives are useful building blocks for the construction of extended ³-conjugated macrocycles 5 and self-assembled macrocycles. 6 An extended version of the meta-phenylene unit is the 2,7-anthracene unit. 7,8 Anthracene derivatives are materials for use in optoelectronics. 9 In addition, 2,7-dibromoanthracene should be useful as a building block for the synthesis of extended ³-conjugated anthracene macrocycles. However, the synthesis of 2,7-dibromoanthracene is difficult, because it requires a six-step synthetic procedure from anthrone and only gives a 9% yield in total.10 Our work was concerned with the efficient synthesis of 2,7-difunctionalized anthracenes. Herein, we report on: (1) the Ir-catalyzed direct diborylation of anthracene to give a 1:1 mixture of 2,6-and 2,7-diborylanthracenes 1 and 2, (2) the SuzukiMiyaura cross-coupling using 2 to give extended ³-conjugated 2,7-disubstituted anthracene derivatives, and (3) the self-assembly of 2,7-dipyridylanthracene and [Pd(dppp)(OTf) 2 ] into a [3 + 3] macrocycle as an application of this reaction.The reaction of anthracene with 2.2 equiv of bis(pinacolato)diboron in the presence of [Ir(OMe)(COD)] 2 (5 mol %) and 4,4¤-di-tert-butyl-2,2¤-bipyridine (dtbpy, 10 mol %) 1,3,4 in cyclohexane at 80°C for 18 h under Ar in the dark gave a 1:1 mixture of 2,6-and 2,7-bis[(pinacolato)boryl]anthracenes 1 and 2 in 88% yield (Scheme 1). Recrystallization of an initial 1:1 mixture of 1 and 2 followed by repeated recrystallizations of the 2-rich mixture from hot toluene gave pure 2 (41% yield in total from anthracene). Similarly, repeated recrystallizations of the 1-rich mixture from hot benzene yielded pure 1 (39% yield in total from anthracene).11 The NMR spectra of both products revealed the positions of the diborylation of anthracene. In the 1 H NMR spectra, 1 and 2 showed two and three singlet aromatic signals, respectively, and in the 13 C NMR spectra, 1 and 2 exhibited six and seven aromatic signals, r...

Synthesis of Extended π‐Systems through C–H Activation

“…Zhang and co-workers reported the C-H/C-H cross-coupling of thiophenes and electrondeficient arenes [44] under previously reported reaction conditions. [45] This reaction was applied to bithiophene (49) and terthiophene (50) in order to introduce pentafluorophenyl groups at the a-positions, providing 51 and 52 in good yields (Scheme 21). The p-extended thiophenes 51 and 52 are known as n-type organic semiconductors.…”

“…The C-H borylation of PAHs such as pyrene, perylene, [49] and anthracene [50] has been reported. For pyrene and perylene, 2,7-diborylpyrene and 2,5,8,11-tetraborylperylene were obtained as the sole products, respectively, while anthracene was converted into a 1:1 mixture of 2,6-and 2,7diborylanthracene.…”

Macrocyclic 2,7‐Anthrylene Oligomers

“…[9] The next large acene unit suitable for hexagonal design is 2,7-anthrylene, and such frameworks as [6]cyclo-2,7-anthrylene ( [6]CANT, r = 1.97 nm) have been unknown to the best of our knowledge. [10][11][12] The studies of [6]CNAPi ndicated that in order to synthesize such al arge macrocycle, its structure should be modified so that reasonable solubility is maintained. In the courseo fo ur studies of anthracene-acetylene oligomers, [13] we found that bulky mesityl (Mes) groups on an anthracene unit reasonably improved the solubility and stability because those groups protected the aromatic cores from intermolecular stacking and undesired reactions.…”

Selective Borylation of [4]Helicene

“…Based on preceding studies on the iridium-catalyzed borylation of arenes [10][11][12][13][14][15] and fused polyarenes, [9,[16][17][18][19][20][21][22][23][24][25][26] we subjected [4]helicene 1 to mild borylation conditions. Equimolar quantities of the substrate and B 2 pin 2 (pin = pinacolato) and a catalytic amount of [Ir(OMe)(cod)] 2 (5 mol-% Ir) (cod = 1,5-cyclooctadiene) and 4,4′-di-tert-butyl-2,2′-bipyridine (dtbpy) (10 mol-%) were reacted in cyclohexane at 23°C for 18 h. Removal of the volatiles and subsequent column chromatography on silica gel (hexane/DCM from 100:0 to 0:100) resulted in three colorless fractions: unreacted 1 (39 %), a mixture of monoborylated [4]helicenes (51 %), and the last small fraction containing a mixture of bisborylated [4]helicenes (< 8 %).…”