2,7-Diborylanthracene as a useful building block for extended φ-conjugated aromatics

Article abstract of DOI:10.1246/cl.2011.941

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 φ-conjugated 2,7-disubstitut

Full text of DOI:10.1246/cl.2011.941

doi:10.1246/cl.2011.941
Published on the web September 5, 2011
941
2,7-Diborylanthracene as a Useful Building Block for Extended ³-Conjugated Aromatics
Ryota Ozawa,1 Kenji Yoza,2 and Kenji Kobayashi*1
1Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529
2Bruker axs, 3-9-B Moriya, Kanagawa-ku, Yokohama, Kanagawa 221-0022
(Received May 11, 2011; CL-110401; E-mail: skkobay@ipc.shizuoka.ac.jp)
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 Suzuki-Miyaura cross-
coupling using 2,7-diborylanthracene gave extended ³-conju-
gated 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
Scheme 1. Synthesis of diborylanthracenes 1 and 2.
of arylboronic acid pinacol esters,1 which are reagents for the
Suzuki-Miyaura cross-coupling reaction.2 Marder et al. first
observed the synthesis of 2,6- and 2,7-diborylnaphthalenes.3
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 semi-
conductors 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 macrocycles5 and self-assembled macro-
cycles.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,
Scheme 2. Synthesis of 3-7. (a) Method A: [Pd(PPh3)4]
(4 mol %), Na2CO3 (10 equiv), toluene-EtOH-H2O (4:2:1 for
3 and 4 or 10:5:1 for 7), 80 °C, 42 h for 3 and 4 and 13 h for 7.
(b) Method B: [Pd(OAc)2] (4 mol %), SPhos (4 mol %), K3PO4
(4 equiv), THF-H2O (9:1), 60 °C, 42 h. (c) 2 M HCl-i-PrOH-
THF (1:2:2), 40 °C, 22 h.
we report on: (1) the Ir-catalyzed direct diborylation of an-
thracene to give a 1:1 mixture of 2,6- and 2,7-diborylanthracenes
1 and 2, (2) the Suzuki-Miyaura 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(pina-
colato)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
1H NMR spectra, 1 and 2 showed two and three singlet aromatic
signals, respectively, and in the 13C NMR spectra, 1 and 2
exhibited six and seven aromatic signals, respectively (the
carbon atom attached to the boron atom was not observed).11
The 2,6- and 2,7-diborylanthracenes 1 and 2 are very useful
building blocks for the regiospecific synthesis of extended
³-conjugated anthracenes. The Suzuki-Miyaura cross-coupling
reaction of 1 and 2 with 2.2 equiv of 4-bromopyridine
hydrochloride catalyzed by [Pd(PPh3)4] (4 mol %) in the pres-
ence of Na2CO3 (10 equiv) in toluene-EtOH-H2O at 80 °C
(Method A) for 42 h gave 2,6- and 2,7-di(4-pyridyl)anthracenes
3 and 4 in 80% and 91% yields, respectively (Schemes 2a and
2b). The reaction of 2 with 4-bromopyridine catalyzed by
Pd(OAc)2 (4 mol %), SPhos (4 mol %),12 and K3PO4 (4 equiv) in
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Scheme 3. Synthesis of 7-10. (a) Oxone (4 equiv), THF-
acetone-H2O (10:2:1), rt, 3 h. (b) Tf2O (3 equiv), Et3N (4 equiv),
CH2Cl2, 0 °C to rt, 18 h. (c) [PdCl2(PPh3)2] (8 mol %), CuI
(8 mol %), PPh3 (16 mol %), toluene-Et3N (1:1), rt, 2 h to 60 °C,
2 h to 80 °C, 12 h.
Scheme 4. Self-assembly of 4 and [Pd(dppp)(OTf)2] into 11.
THF-H2O at 60 °C (Method B) for 42 h produced 4 in 41%
yield. On the other hand, the coupling reaction of 2 with 3 equiv
of 1-iodo-4-(methoxymethoxy)benzene based on Methods A and
B gave 5 in 38% and 60% yields, respectively (Scheme 2b).
Compound 5 was hydrolyzed by HCl (aq) to produce 2,7-bis(4-
hydroxyphenyl)anthracene (6) in 92% yield. The Suzuki-
Miyaura cross-coupling reaction can be applied to 1-bromo-
alkynes,13 which is an alternative method to the Sonogashira
cross-coupling reaction. Thus, the coupling reaction of 2 with 3
equiv of bromo(triisopropylsilyl)acetylene13b using Method A
gave 2,7-bis[(triisopropylsilyl)ethynyl]anthracene (7) in 94%
yield (Scheme 2c).
Figure 1. 1H NMR spectra of (a) [Pd(dppp)(OTf)2], (b) 4, (c) a
1:1 mixture of 4 and [Pd(dppp)(OTf)2] (formation of 11), and
(d) a 2:1 mixture of 4 and [Pd(dppp)(OTf)2] in CD2Cl2 at rt.
The transformation of 2,7-diborylanthracene 2 into 2,7-bis-
triflate 9 would further enhance the utility of 2 as a synthetic
building block, because 9 can be used in the Sonogashira cross-
coupling (Scheme 3). Thus, the reaction of 2 with Oxoneμ
produced crude 2,7-dihydroxyanthracene (8),14a which was
treated with Tf2O to give 9 (63% yield in two steps).14b The
Sonogashira cross-coupling reaction of 9 with 3 equiv of
4-ethynylpyridine and triisopropylsilylacetylene gave 2,7-bis-
(4-pyridylethynyl)anthracene (10) and 7 in 62% and 85% yields,
respectively.
2,7-Di(4-pyridyl)anthracene (4) serves as a building block
for self-assembled macrocycles.6,15 A 1:1 mixture of 4 and
square-planar [Pd(dppp)(OTf)2] self-assembled into a [3 + 3]
macrocycle, cyclo-[4¢Pd(dppp)]3¢(OTf)6 (11), via Pd-pyridyl
coordination bonds (Scheme 4). The 1H NMR spectrum of a 1:1
mixture of 4 and [Pd(dppp)(OTf)2] in CD2Cl2 at room temper-
ature after 10 min showed a highly symmetric new species,
along with the disappearance of 4 and [Pd(dppp)(OTf)2],
suggesting the formation of 11 (Figure 1). The pyridyl ¡-proton
was shifted downfield by 0.33 ppm relative to that of free 4,
indicative of Pd-pyridyl coordination bonds being formed. The
UV-vis spectrum also supported the formation of a Pd-pyridyl
bond, where the longest wavelength -max of this mixture in
CH2Cl2 was red-shifted by 6 nm relative to that of free 4.11 A 2:1
mixture of 4 and [Pd(dppp)(OTf)2] gave a mixture of 11, free 4,
and unknown species (Figure 1d), indicating that 11 is not
stable. Finally, the molecular structure of 11 was confirmed from
single-crystal X-ray diffraction analysis, although the final R
indices were poor because the triflate ions and inclusion solvents
were highly disordered.11,16
Figure 2. X-ray crystal structure of 11: (a) front and (b) side
views. Triflate ions are omitted for clarity.
conformations for both the 4 unit (ant-A and ant-B) and the
[Pd(dppp)] unit (Pd1 and Pd2) to form a triangular structure. The
lengths of sides of the triangular cavity are Pd1£Pd2 = 18.8 ¡
and Pd1£Pd1 = 18.4 ¡. The distances between the anthracene
rings are C10£C10 (between the ant-A rings) = 15.9 ¡ and
C10£C10¤ (between the ant-A and ant-B rings) = 15.6 ¡ for the
upper rim and C9£C9 = 11.8 ¡ and C9£C9¤ = 12.0 ¡ for the
lower rim.
In summary, we have demonstrated the Ir-catalyzed direct
diborylation of anthracene to form 2,6- and 2,7-diborylanthra-
cenes 1 and 2. The Suzuki-Miyaura cross-coupling using 2 gave
extended ³-conjugated 2,7-disubstituted anthracene derivatives
as building blocks for macrocycles. A study on the synthesis of
Single crystals of cyclo-[4¢Pd(dppp)]3¢(OTf)6 (11) suitable
for X-ray diffraction analysis were obtained by slow evaporation
of a CHCl3 solution of 11. Figure 2 shows the molecular
structure of 11, wherein there are two types of slightly different
Chem. Lett. 2011, 40, 941-943
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an extended ³-conjugated anthracene macrocycle based on 2, 7,
and 9 is currently in progress.
Lett. 2011, 40, 12.
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This work was supported in part by the Asahi Glass
Foundation.
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
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Chem. Lett. 2011, 40, 941-943
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/