Synthesis of azaaromatic?￡?borane intramolecular complexes by palladium-catalyzed reaction of azaaromatic halides with alkynyl(triaryl)borates

Article abstract of DOI:10.1002/hlca.201200554

A diversity-oriented method to synthesize (E)-azastilbenes having an intramolecular B?￡?N coordination bond from alkynyl(triaryl)borates and azaaromatic halides is described. The obtained π-conjugated compounds exhibit an intense blue fluorescence and a high electron affinity, indicating their potential to be used as n-type light-emitting materials. Copyright

Full text of DOI:10.1002/hlca.201200554

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Helvetica Chimica Acta – Vol. 95 (2012)
Synthesis of AzaaromaticꢀBorane Intramolecular Complexes by Palladium-
Catalyzed Reaction of Azaaromatic Halides with Alkynyl(triaryl)borates
by Naoki Ishida, Mizuna Narumi, and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura,
Kyoto 615-8510, Japan
(phone: þ 81-75-383-2747; fax: þ 81-75-383-2748; e-mail: murakami@sbchem.kyoto-u.ac.jp)
Dedicated to Professor Dieter Seebach on the occasion of his 75th birthday
A diversity-oriented method to synthesize (E)-azastilbenes having an intramolecular BꢀN
coordination bond from alkynyl(triaryl)borates and azaaromatic halides is described. The obtained p-
conjugated compounds exhibit an intense blue fluorescence and a high electron affinity, indicating their
potential to be used as n-type light-emitting materials.
Introduction. – Aza-p-conjugated compounds equipped with an intramolecular
BꢀN coordination bond have attracted growing attention because of their interesting
properties such as a high electron affinity [1], an intense fluorescence [2], and
photochromism [3][4]. The conventional methods for their synthesis typically consist
of initial lithiation of a parent N-containing p-conjugated framework and the following
nucleophilic substitution reaction with haloboranes. However, electron-deficient
azaaromatics such as isoquinolines and pyrazines are prone to undergo undesired side
reactions upon treatment with lithiating agents [5]. A new method for the synthesis of a
wide variety of azaaromaticꢀborane complexes is yet to be developed [6].
We previously developed the Pd-catalyzed reaction of alkynyl(aryl)borates (aryl ¼
Ar1) with aryl halides (aryl ¼ Ar2) [7]. Two aryl groups, Ar1 and Ar2, were incorporated
across the CꢁC bond to produce (trisubstituted alkenyl)boranes. This protocol was
successfully applied to the synthesis of amineꢀborane complexes [8] and pyridine N-
oxideꢀborane complexes [9]. Herein, we describe a diversity-oriented method for the
synthesis of (E)-azastilbene derivatives having an intramolecular BꢀN coordination
bond from alkynyl(triaryl)borates and azaaromatic halides. The obtained p-conjugated
compounds exhibit an intense blue fluorescence and a high electron affinity,
demonstrating their potential to be used as n-type light-emitting materials.
Results and Discussion. – 2-Bromoquinoline (1a) and alkynylborate 2a were
reacted under the slightly modified conditions of the previously reported Pd-catalyzed
reaction [7b][9]1). A toluene solution (1 ml) of 1a (0.20 mmol), 2a (0.20 mmol), and
(DPEPhos)Pd(p-allyl)Cl (DPEPhos ¼ bis[2-(diphenylphosphino)phenyl]ether¼ (ox-
ydi-2,1-phenylene)bis(diphenylphosphine); 5 mol-%) was heated at 608 for 5 h
1
)
The preliminary screening of ligands revealed that DPEPhos gave a result superior to other ligands
such as XANTPhos, BINAP, and DPPF.
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

Helvetica Chimica Acta – Vol. 95 (2012)
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(Scheme 1). The reaction efficiently took place, and the following chromatography
afforded analytically pure quinolineꢀborane complex 3 in 88% yield.
Scheme 1
The broad scope of this Pd-catalyzed reaction was compiled in Table 1. Not only 2-
bromoquinoline (1a), but also 8-bromoquinoline (1b) participated in the reaction to
furnish six-membered azaboracycle 4 in 81% yield (Entry 1). Isoquinoline, pyrimidine,
and pyrazine moieties were all suitably and underwent the present reaction (Entries 2 –
5), whereas these are prone to decompose upon lithiation [5]. The alkynylborates 2d –
2f, which are equipped with various aryl groups, undergoing the Pd-catalyzed reaction,
afforded the corresponding azaaromaticꢀborane complexes 10 – 12 (Entries 7 – 9).
Excellent functional-group compatibility was demonstrated by the reaction of 2-
bromopyridines bearing functional groups which were potentially reactive towards Pd
catalysts. The pinacolatoboryl moiety was tolerated on the pyridine ring, yielding
pinacolatoboryl-substituted pyridineꢀborane complex 13 in 82% yield (Scheme 2).
When 2 equiv. of 2,5-dibromopyridine 1i was used, the 2-Br group reacted in
preference to the 5-Br group, as was the case with the Pd-catalyzed cross-coupling
reactions (Scheme 3) [10]. The 5-bromopyridineꢀborane complex 14 was obtained in
78% yield. These boryl and Br groups remaining in the products served as footholds for
the subsequent SuzukiꢀMiyaura cross-coupling reaction. Treatment of 13 and 14 with a
catalytic amount of Pd[P(t-Bu)3]2 in the presence of NaOH in THF/H2O provided
bipyridne 15 in 84% yield (Scheme 4).
Scheme 2
Thus, a wide variety of azaaromaticꢀborane intramolecular complexes were
successfully synthesized through the Pd-catalyzed reaction of alkynylborates with
azaaromatic halides. We next examined the electrochemical properties of the
quinolineꢀborane complex 3 to demonstrate a high electron affinity of the products.
Table 2 contains its characteristic properties in comparison with Alq (¼tris(8-
hydroxyquinolinato)aluminum), the most conventional fluorescent material with a
high electron affinity. The cyclic voltammogram of quinolineꢀborane complex 3
showed a reversible reduction wave with the potential peak Vpc of ꢀ 1.80 V, which was

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Helvetica Chimica Acta – Vol. 95 (2012)
Table 1. Aza-p-Conjugated Compounds Having BꢀN Coordination Bonda)
Entry Azaaromatic
halides 1
Alkynylborates 2
Products
Yield
[%]
1
1b
2a
4
84
2
1c
2a
5
81
3
4
1d
1e
2a
2b
6
7
80
67
5
1f
2b
8
43
6
7
1g
1a
2c
9
87
89
2d
10

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2477
Table 1 (cont.)
Entry Azaaromatic
halides 1
Alkynylborates 2
Products
Yield
[%]
8
1a
2e
11
66
9
1a
2f
12
84
a) Reaction conditions: 1.0 equiv. of aryl halide, 1.0 equiv. of alkynylborate, 5 mol-% (DPEPhos)Pd(p-allyl)Cl,
toluene (0.2m), 608, 5 h.
Scheme 3
Scheme 4
less negative than that of Alq (ꢀ2.14 V under the same conditions). The LUMO level
of 3 was calculated as 3.8 eV, which was significantly higher than that of Alq (3.0 eV).
These results indicate that 3 has an even higher electron affinity than Alq. The
photophysical properties are also compiled in Table 2. The quinolineꢀborane complex
3 showed a sky blue fluorescence (lmax 473 nm in CH2Cl2) with the quantum yield of
0.26, whereas Alq exhibited a green fluorescence (lmax 526 nm, F 0.17) [11]. These
electrochemical and photochemical properties demonstrated the potential usefulness
of this class of molecules as the n-type blue light-emitting materials.

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Helvetica Chimica Acta – Vol. 95 (2012)
Table 2. Photophysical and Electrochemical Properties of 3
Compounds
Vpc/Va)
HOMO/eVb)
LUMO/eVc)
lem/nm
F
3
Alq
ꢀ 1.80
ꢀ 2.14
6.6
5.7
3.8
3.0
473d)
526e)
0.26d)
0.17e)
a) In g-butyrolactone with Bu4NClO4 at a scan rate of 100 mVsꢀ1. Potentials vs. Fc/Fcþ. b) Determined by
c
e
UPS. ) Calculated from the HOMO and the UVabsorption edge. d) Taken from [8]. ) Taken from [11].
In summary, we have synthesized (E)-azastilbenes having an intramolecular BꢀN
coordination by the Pd-catalyzed reaction of azaaromatic halides with alkynyl(tri-
aryl)borates. This method is versatile enough to incorporate a wide variety of aza-
aromatics including those vulnerable to the conventional lithiating conditions. The
obtained p-conjugated compounds exhibit a strong fluorescence and a high electron
affinity, i.e., with a potential to be used as the n-type light-emitting materials.
This work was supported in part by NEDO and a Grant-in-Aid for Scientific Research from MEXT.
Experimental Part
General. Unless otherwise noted, all chemicals and anh. solvents were obtained from commercial
suppliers. Toluene was dried over sodium benzophenone ketyl. (DPEPhos)Pd(p-allyl)Cl [12] and
alkynyl borates 2 [9] were prepared according to the reported procedures. Column chromatography
(CC): silica gel 60 N (Kanto). Prep. TLC: Silica gel 60 PF254 (Merck). Gel permeation chromatography
(GPC): Japan Analytical Industry LC-908 or LC-9204. NMR Spectra: Varian Gemini 2000 (1H: 300 and
13C: 75 MHz), Varian Mercury vx (1H: 400 and 13C: 100 MHz), JEOL JNM-A500 (1H: 500 and 13C:
150 MHz), or Varian 400-MR Auto Tune X5 (11B: 128 MHz) spectrometers; unless otherwise noted,
CDCl3 was used as a solvent; chemical Shifts in d ppm referenced to a residual CDCl3 (d 7.26 for 1H, d
1
77.0 for 13C), CD3CN (d 1.94 for H, d 1.32 for 13C), and BF3 · OEt2 (d 0.00 for 11B). HR-MS: Applied
Biosystems Voyager Elite or JEOL JMS-HX110A spectrometer.
Alkynylborate 2b. 1H-NMR (CD3CN): 2.22 (s, 1 H); 3.00 (s, 12 H); 7.24 – 7.30 (m, 3 H); 7.37 – 7.43 (m,
12 H); 7.53 (d, J ¼ 7.2, 6 H); 7.62 (d, J ¼ 7.8, 6 H). 13C-NMR (CD3CN): 56.0; 125.5; 127.1; 127.4; 129.5;
135.9; 136.1; 143.4. 11B-NMR (CD3CN): ꢀ 12.6. HR-FAB-MS: 495.2293 ([M ꢀ (Me4N)]ꢀ, C38H28Bꢀ;
calc. 495.2284).
Alkynylborate 2f. 1H-NMR (CD3CN): 2.21 (s, 1 H); 2.92 (s, 12 H); 6.90 – 6.91 (m, 6 H); 7.13 – 7.14 (m,
6 H). 13C-NMR (CD3CN): 56.0; 124.5; 127.2; 128.6. 11B-NMR (CD3CN): ꢀ 18.0. HR-FAB-MS: 285.0031
([M ꢀ (Me4N)]ꢀ, C14H10BSꢀ3 ; calc.285.0038).
Pd-Catalyzed Reaction of 2-Bromoquinoline (1a) with Alkynylborate 2a. A Typical Procedure. In an
oven-dried flask was placed (DPEPhos)Pd(p-allyl)Cl (3.6 mg, 5 mmol) and 2a (34.7 mg, 0.10 mmol).
The flask was then evacuated and purged by Ar three times. A toluene soln. (0.5 ml) of 2-
bromoquinoline (21.2 mg, 0.10 mmol) was added to the flask, and then the mixture was stirred at 608.
After 5 h, H2O was added. The resulting mixture was extracted with CH2Cl2 (3 ꢂ ), washed with H2O
(once), brine (once), dried (MgSO4), and concentrated. The residue was purified by prep. TLC to give
quinolineꢀborane complex 3 (34.8 mg, 0.088 mmol, 88% yield). The spectra of the obtained 3 were
identical to the reported data [8].
QuinolineꢀBorane Complex 4. 1H-NMR (CDCl3): 7.03 – 7.19 (m, 12 H); 7.32 – 7.35 (m, 4 H); 7.43 (dd,
J ¼ 8.4, 6.0, 1 H); 7.66 – 7.68 (m, 3 H); 8.33 (dd, J ¼ 8.1, 1.5, 1 H); 8.79 (dd, J ¼ 8.4, 2.1, 1 H). 13C-NMR
(CDCl3): 121.0; 124.88; 124.94; 125.2; 125.9; 127.1; 127.3; 127.9; 129.1; 129.4; 130.0; 133.6; 134.0; 138.7;
141.0; 146.0; 149.5. Two kinds of C-atoms bound to B-atom were not detected due to quadrupolar
relaxation. 11B-NMR (CDCl3): 1.7. HR-EI-MS: 395.1848 (Mþ, C29H22BNþ; calc. 395.1845).

Helvetica Chimica Acta – Vol. 95 (2012)
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IsoquinolineꢀBorane Complex 5. 1H-NMR (CDCl3): 7.18 – 7.42 (m, 14 H); 7.73 – 7.81 (m, 3 H); 7.84 –
7.87 (m, 2 H); 7.92 (s, 1 H); 8.10 (d, J ¼ 6.6, 1 H); 8.51 (d, J ¼ 7.5, 1 H). 13C-NMR (CDCl3): 117.8; 118.4;
123.8; 125.8; 126.8; 127.1; 127.5; 128.19; 128.21; 128.5; 128.8; 132.5; 133.8; 134.7; 137.0; 138.7; 160.8. Two
kinds of C-atoms bound to B-atom were not detected due to quadrupolar relaxation. 11B-NMR (CDCl3):
3.7. HR-APCI-MS: 396.1914 ([M þ H]þ, C29H23BNþ; calc. 396.1918).
1
IsoquinolineꢀBorane Complex 6. H-NMR (CDCl3): 7.09 – 7.22 (m, 10 H); 7.34 – 7.37 (m, 4 H); 7.43
(td, J ¼ 8.4, 0.9, 1 H); 7.58 – 7.61 (m, 2 H); 7.65 – 7.70 (m, 2 H); 7.75 – 7.79 (m, 2 H); 8.93 (s, 1 H). 13C-NMR
(CDCl3): 114.6; 121.3; 125.3; 125.8; 126.5; 127.0; 127.4; 127.8; 128.0; 128.2; 129.1; 133.2; 133.8; 138.3;
138.9; 147.2; 153.9. Two kinds of C-atoms bound to B-atom were not detected due to quadrupolar
relaxation. 11B-NMR (CDCl3): 3.8. HR-EI-MS: (C29H22BN (M)þ 395.1845) 395.1843.
PyrimidineꢀBorane Complex 7. 1H-NMR (CDCl3): 7.07 (dd, J ¼ 5.4, 4.8, 1 H); 7.29 – 7.63 (m, 26 H);
7.84 – 7.87 (m, 2 H); 8.52 (dd, J ¼ 6.6, 5.7, 1 H); 8.88 (dd, J ¼ 4.5, 2.1, 1 H). 13C-NMR (CDCl3): 115.1;
121.5; 126.4; 126.79; 126.88; 126.92; 127.0; 127.4; 128.6; 128.7; 129.4; 134.2; 136.6; 138.9; 140.4; 141.3;
142.1; 150.0; 161.2; 168.4. Two kinds of C-atoms bound to B-atom were not detected due to quadrupolar
relaxation. 11B-NMR (CDCl3): 2.9. HR-EI-MS: 574.2578 (Mþ, C42H31BNþ2 ; calc. 574.2583).
1
PyrazineꢀBorane Complex 8. H-NMR (CDCl3): 7.31 – 7.64 (m, 26 H); 7.83 (d, J ¼ 8.4), 8.27 (br. s,
1 H); 8.44 (br. s, 1 H); 9.01 (br. s, 1 H). 13C-NMR (CDCl3): 117.8; 126.5; 126.8; 126.91; 126.92; 127.0;
127.5; 128.6; 128.7; 129.1; 134.1; 135.8; 136.8; 139.1; 140.2; 140.4; 141.3; 141.9; 143.3; 146.4; 154.0; 185.3.
11B-NMR (CD3CN): 3.4. HR-ACPI-MS: 575.2650 ([M þ H]þ, C42H32BNþ2 ; calc. 575.2653).
1
PyridineꢀBorane Complex 9. H-NMR (CDCl3): 2.17 (s, 3 H); 7.03 – 7.05 (m, 1 H); 7.11 – 7.31 (m,
13 H); 7.48 (dt, J ¼ 8.4, 1.5, 1 H); 7.53 (d, J ¼ 8.0, 2 H); 7.93 (t, J ¼ 8.0, 1 H); 8.23 (d, J ¼ 5.6, 1 H).
13C-NMR (CDCl3): 11.8; 117.7; 119.5; 125.5; 126.1; 127.2; 127.5; 127.9; 128.3; 133.3; 140.3; 140.9; 143.1;
161.8. Two kinds of C-atoms bound to B-atom were not detected due to quadrupolar relaxation. 11B-
NMR (CDCl3): 3.4. HR-EI-MS: 359.1854 (Mþ, C26H22BNþ; calc. 359.1845).
QuinolineꢀBorane Complex 10. 1H-NMR (CDCl3): 3.73 (s, 6 H); 3.76 (s, 3 H); 6.73 – 6.79 (m, 6 H);
7.08 (s, 1 H); 7.27 (d, J ¼ 8.4, 4 H); 7.34 – 7.44 (m, 2 H); 7.46 – 7.50 (m, 2 H); 7.62 (d, J ¼ 8.8, 1 H); 7.79 (dd,
J ¼ 7.8, 1.0, 1 H); 7.92 (d, J ¼ 8.8, 1 H); 8.26 (d, J ¼ 8.8, 1 H). 13C-NMR (CDCl3): 54.8; 55.1; 113.0; 113.5;
118.1; 120.0; 122.8; 125.3; 126.1; 128.5; 126.1; 128.5; 129.9; 131.1; 131.8; 134.4; 140.8; 141.5; 157.5; 160.0;
162.1. Two kinds of C-atoms bound to B-atom were not detected due to quadrupolar relaxation. 11B-
NMR (CDCl3): 4.0. HR-EI-MS: 485.2164 (Mþ, C32H28BNO3þ ; calc. 485.2162).
QuinolineꢀBorane Complex 11. 1H-NMR (CDCl3): 6.81 – 6.95 (m, 6 H); 7.08 (s, 1 H); 7.18 – 7.23 (m,
4 H); 7.32 – 7.37 (m, 2 H); 7.42 – 7.49 (m, 2 H); 7.71 (d, J ¼ 8.7, 1 H); 7.76 – 7.79 (m, 1 H); 7.86 – 7.89 (m,
1 H); 8.39 (d, J ¼ 8.7, 1 H). 13C-NMR (CDCl3): 114.3 (d, J(C,F) ¼ 19.1); 115.2 (d, J(C,F) ¼ 21.3); 118.2;
121.9; 122.7; 126.0; 126.4; 128.8; 129.8 (d, J(C,F) ¼ 8.1); 131.6; 134.6 (d, J(C,F) ¼ 6.5); 135.2; 141.4; 141.6;
161.6 (d, J(C;F) ¼ 241.5); 162.0; 163.0 (d, J(C,F) ¼ 247.5). Two kinds of C-atoms bound to B-atom were
not detected due to quadrupolar relaxation. 11B-NMR (CDCl3): 3.6. HR-EI-MS: 449.1562 (Mþ,
C32H19BNFþ3 ; calc. 449.1563).
QuinolineꢀBorane Complex 12. 1H-NMR (CDCl3): 6.90 – 6.93 (m, 1 H); 7.02 – 7.05 (m, 3 H); 7.23 –
7.32 (m, 6 H); 7.37 (t, J ¼ 7.8, 1 H); 7.46 – 7.54 (m, 2 H); 7.74 (d, J ¼ 7.8, 1 H); 8.16 (d, J ¼ 8.4, 1 H); 8.22 (d,
J ¼ 8.1, 1 H). 13C-NMR (CDCl3): 118.1; 119.8; 122.4; 125.7; 126.3; 126.4; 127.2; 127.7; 128.0; 128.6; 130.4;
130.8; 131.6; 141.2; 141.8; 142.2; 161.4. 11B-NMR (CDCl3): 0.5. HR-EI-MS: 413.0545 (Mþ, C23H16BNSþ3 ;
calc. 413.0538).
PyridineꢀBorane Complex 13. 1H-NMR (CDCl3): 1.33 (s, 12 H); 7.19 – 7.29 (m, 10 H); 7.37 – 7.40 (m,
4 H); 7.51 (d, J ¼ 8.0, 1 H); 7.64 – 7.67 (m, 2 H); 8.23 (d, J ¼ 8.0, 1.2, 1 H); 8.63 (s, 1 H). 13C-NMR
(CDCl3): 24.8; 84.5; 118.7; 121.2; 125.7; 127.4; 128.1; 128.4; 128.5; 134.0; 138.5; 145.9; 148.7; 161.9. Three
kinds of C-atoms bound to B-atom were not detected due to quadrupolar relaxation. 11B-NMR (CDCl3):
3.3; 28.7. HR-ACPI-MS: 472.2611 ([M þ H]þ, C31H32B2NO2þ ; calc. 472.2614).
PyridineꢀBorane Complex 14. 1H-NMR (CDCl3): 7.17 – 7.38 (m, 15 H); 7.62 – 7.64 (m, 2 H); 7.90 (dd,
J ¼ 8.8, 2.0, 1 H); 8.36 (d, J ¼ 2.0, 1 H). 13C-NMR (CDCl3): 114.3; 120.1; 120.2; 126.1; 127.6; 128.2; 128.4;
128.7; 133.8; 138.2; 142.9; 144.2; 148.4; 159.0; 183.3. 11B-NMR (CDCl3): 4.2. HR-ACPI-MS: 424.0857
([M þ H]þ, C25H20BNBrþ; calc. 424.0867).
PyridineꢀBorane Complex 15. In an oven-dried flask was placed 13 (23.6 mg, 0.050 mmol), 14
(21.2 mg, 0.050 mmol), and NaOH (6.5 mg, 1.5 mmol). The flask was then evacuated and purged by Ar

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Helvetica Chimica Acta – Vol. 95 (2012)
three times. A THF soln. (0.5 ml) of Pd[P(t-Bu)3]2 (1.3 mg, 2.5 mmol) and subsequently H2O (5 ml) were
added, and then the mixture was stirred at 608. After 1 h, H2O was added. The mixture was extracted with
CH2Cl2 (3 ꢂ ), washed with H2O (once), brine (once), dried (MgSO4), and concentrated. The residue
1
was purified by prep. TLC, and GPC to gave 15 (28.9 mg, 0.042 mmol, 84% yield). H-NMR (CDCl3):
7.16 – 7.30 (m, 28 H); 7.55 – 7.62 (m, 6 H); 7.85 (dd, J ¼ 8.4, 2.0, 2 H); 8.33 (d, J ¼ 1.6, 2 H). 13C- and 11B-
NMR could not be recorded due to the low solubility. HR-ACPI-MS: 689.3268 ([M þ H]þ, C50H39B2N2þ ;
calc. 689.3294).
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Received October 3, 2012