Syntheses of mono-, di- and triethynylazulenes

Article abstract of DOI:10.1016/S0040-4039(00)00308-7

Simple and productive routes to mono-, di- and triethynylazulenes are described. Azulenes ethynylated in the five-membered ring can be prepared on a multigram scale by Pd-catalyzed cross-coupling of iodo- or bromoazulenes with trimethylsilyl-acetylene and subsequent desilylation. 5,7- Diethynylazulene was synthesized by dibromomethylenation of the corresponding dialdehyde and subsequent dehydrobromination. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00308-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2855–2858
Syntheses of mono-, di- and triethynylazulenes
Kai H. H. Fabian, Ahmed H. M. Elwahy and Klaus Hafner ∗
Institut für Organische Chemie, Technische Universität Darmstadt, Petersenstraße 22, D-64287 Darmstadt, Germany
Received 7 February 2000; accepted 14 February 2000
Abstract
Simple and productive routes to mono-, di- and triethynylazulenes are described. Azulenes ethynylated in
the five-membered ring can be prepared on a multigram scale by Pd-catalyzed cross-coupling of iodo- or
bromoazulenes with trimethylsilyl-acetylene and subsequent desilylation. 5,7-Diethynylazulene was synthesized
by dibromomethylenation of the corresponding dialdehyde and subsequent dehydrobromination. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: ethynylazulenes; Pd-cross-coupling reaction; Corey–Fuchs reaction.
The increasing interest in aromatic compounds with extended π-electron systems because of their
importance as materials with special optical and electrical properties for the design of molecular devices
has focused in the last decades to a large extent on the synthesis of polyethynylated as well as donor
and/or acceptor substituted benzenoid and heteroaromatic compounds.1–3 However, up to now non-
benzenoid aromatic or even antiaromatic π-electron systems were only scarcely employed as precursors
for the synthesis of new materials with potential useful electronic properties. Especially, the azulene
system with its pronounced polarizability and its tendency to form stabilized radical cations as well as
anions should be predestinated as building block for the construction of new structures with interesting
chemical and physical properties. With this respect we recently synthesized 2,4,6,8-tetracyanoazulene
(TCNA) which proved to be a strong acceptor for electrically conductive charge-transfer complexes and
radical anion salts.4
We report here on the synthesis of mono- and polyethynylated azulenes 1a–h utilizing the now availa-
ble versatile methods for new carbon–carbon bond forming reactions of modern acetylene chemistry5 as
well as the classical Corey–Fuchs reaction.6
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00308-7
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So far, only a few ethynylazulenes and some highly functionalized derivatives synthesized by mul-
tistep procedures with moderate yields were described.7,8 In search of an expedient pathway to the
azulenes 1a–g ethynylated in the five-membered ring, our attentions focused on the alkynylation of
the iodoazulenes 2–4.9 Thus, cross-coupling of 2–4 with trimethylsilyl-acetylene (TMSA) under So-
nogashira–Hagihara conditions10 afforded the trimethylsilyl-protected ethynylazulenes 5–7 in 80–90%
yields as green or blue crystals.11 Subsequent treatment of the (trimethylsilylethynyl)azulenes 5–7 with
potassium hydroxide in methanol followed by chromatography on alumina (BII-III) using n-hexane as
an eluent furnished blue crystals of the mono- and diethynylazulenes 1a, b, d, f and g, respectively, in
90–100% yields (Scheme 1).
Scheme 1. (i) 0.04 mol% PdCl2(PPh3)2, 0.08 mol% CuI, NEt3, 1 or 2 equiv. TMSA, rt; (ii) 1.1 equiv. 1 M KOH in H2O, MeOH,
rt
In analogy, the reaction of 2-(trimethylsilylethynyl)azulene (7) with one or two equivalents of N-
iodosuccinimide leads to the formation of the corresponding mono- and diiodoazulenes 8 and 9,
respectively, which could be coupled with TMSA under Sonogashira conditions to give the 1,2-
bis(trimethylsilylethynyl)azulene (10) and 1,2,3-tris(trimethylsilylethynyl)azulene (11), respectively, as
green crystals in 90–95% yields. Deprotection of the trimethylsilyl groups of 10 and 11 with potassium
hydroxide in methanol and subsequent chromatography on alumina (BII-III) using n-hexane as an eluent
afforded the 1,2-di- and 1,2,3-triethynylazulenes 1c and 1e as greenish blue crystals in 95% and 90%
yields, respectively (Scheme 2).
Because of the difficulty of introducing iodosubstituents on the seven-membered ring of azulenes12
ethynylation of the 5- and 7-position of azulene was achieved by using the Corey–Fuchs6 method for
the conversion of aldehydes into acetylenes. Reaction of the corresponding 5,7-diformylazulene (12)13
with triphenylphosphine and tetrabromomethane in dichloromethane furnished, after chromatography on
alumina (BII-III) using n-hexane as an eluent, greenish blue crystals of the tetrabromodiolefin 13 in 70%
yield. The latter could be converted into the 5,7-diethynylazulene 1h (86%) upon treatment with 6 equiv.
of LDA followed by hydrolysis (Scheme 3).14
The ethynylazulenes so far prepared are slightly stable and can be easily manipulated under ambient
conditions, especially as long as the alkyne groups are protected by trimethylsilyl groups. With the

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Scheme 2. (i) 1 equiv. NIS, CH2Cl2, rt; (ii) 2 equiv. NIS, CH2Cl2, rt; (iii) 0.04 mol% PdCl2(PPh3)2, 0.08 mol% CuI, NEt3, 1 or
2 equiv. TMSA, rt; (iv) 1.1 equiv. 1 M KOH in H2O, MeOH, rt
Scheme 3. (i) PPh3, CBr4, CH2Cl2, rt; (ii) 6 equiv. LDA, THF, −90°C→rt
exception of 5,7-diethynylazulene (1h) the deprotected ethynylazulenes 1a–e are only slightly stable at rt
and form black solids with metallic luster after a few hours which could not be characterized due to their
insolubility. The 6-tert-butyl derivatives 1f and 1g are noticeably more stable than their unsubstituted
derivatives 1a and 1d presumably due to the stabilizing effect of the tert-butyl groups.
Contrary to alkyl groups, the ethynyl substituents effect in all positions of the bicyclic system a
bathochromic shift of the light absorption of azulene due to both inductive and mesomeric effects,
respectively.15
Physical data of compounds 1a, d, and e.16,17 1a: blue crystals, m.p. 36–37°C (dec.); HRMS:
1
3
m/z=152.0631; H NMR (CDCl3): δH=3.51 (s, 1H, -C≡C-H), 7.25 (t, J=10.0 Hz, 1H, 5-H), 7.27 (t,
3J=9.6 Hz, 1H, 7-H), 7.30 (d, 3J=4.0 Hz, 1H, 3-H), 7.67 (t, 3J=9.8 Hz, 1H, 6-H), 8.00 (s, 1H, 2-H), 8.32
(d, J=9.8 Hz, 1H, 4-H); 8.61 (d, J=9.5 Hz, 1H, 8-H); 13C NMR (CDCl3): δC=80.22, 81.39, 109.14,
117.30, 124.10, 124.69, 136.13, 137.26, 138.56, 139.77, 141.27, 142.09; UV–vis (n-hexane): λmax=259
nm, 263, 271 sh, 275 sh, 280, 285, 290, 296, 302, 315, 341, 346, 349, 358, 362 sh, 368, 371, 377, 388,
500 sh, 525 sh, 540 sh, 559, 579, 602, 631, 660, 699, 734; 1d: blue crystals, m.p. 69–71°C (dec.); HRMS:
m/z=176.0649; 1H NMR (CDCl3): δH=3.45 (s, 2H, -C≡C-H), 7.26 (t, 3J=10.0 Hz, 2H, 5/7-H), 7.62 (t,
3J=9.9 Hz, 1H, 6-H), 8.05 (s, 1H, 2-H), 8.49 (d, 3J=10.0 Hz, 2H, 4/8-H); 13C NMR (CDCl3): δC=79.09,
81.06, 108.86, 126.00, 137.5, 139.92, 142.20, 142.24; UV–vis (n-hexane): λmax=243 nm sh, 270, 283,
288, 294, 300, 306, 313, 320, 340, 345, 359, 363, 369, 373, 378, 383, 389, 400, 513 sh, 535 sh, 548
sh, 575, 597, 615 sh, 620, 628 sh, 655, 679 sh, 685, 705 sh, 729, 766; 1e: greenish blue crystals, m.p.
77–78°C; MS (FD): m/z (%)=200 (100) [M+]; 1H NMR (CDCl3): δH=3.63 (s, 2H, 1/3-C≡C-H), 3.90 (s,
3
3

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1H, 2-C≡C-H), 7.35 (dd, 3J1=9.5 Hz, 3J2=9.5 Hz, 2H, 5/7-H), 7.68 (tt, 3J1=9.9 Hz, 1H, 5J2=1 Hz, 1H,
6-H), 8.50 (d, 3J=9.3 H, 2H, 4/8-H); 13C NMR (CDCl3): δC=77.65, 78.82, 84.17, 89.18, 112.13, 127.12,
134.21, 137,63, 140.61, 141.72; UV–vis (n-hexane): λmax=231 nm, 253, 271, 276, 284, 287, 318 sh, 323,
339, 359, 371, 377, 388, 393, 578, 580, 615, 672, 692, 708 sh, 748, 774.
Acknowledgements
This work was generously supported by the Fonds der Chemischen Industrie and the Dr. Otto Röhm
Gedächtnisstiftung, Darmstadt. Dr. A. H. M. Elwahy thanks the Alexander von Humboldt Foundation for
a research fellowship.
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NIS, respectively, in dichloromethane. The solvent was then removed in vacuo below 10°C followed by chromatography
on alumina (BII-III) using n-hexane as an eluent. The greenish blue residue was immediately resolved in triethylamine for
further cross-coupling reactions. For the synthesis of 1-iodo-and 1-bromoazulene as well as 1,3-dibromoazulene see also:
Hafner, K.; Patzelt, H.; Kaiser, H. Liebigs Ann. Chem. 1962, 656, 24–33. For the synthesis of 2-iodoazulene (4) see: Nozoe,
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11. The trimethylsilylethynylazulenes 5a, 6a could also be obtained in 25–35% yields from the Pd-catalyzed cross-coupling
reaction of the corresponding bromoazulenes9 with TMSA in refluxing triethylamine.
12. McDonald, R. N.; Richmond, J. M.; Curtis, J. R.; Petty, H. E.; Hoskins, T. L. J. Org. Chem. 1976, 41, 1811–1921; Morita,
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14. Attempts to synthesize 1a, b, d, f and g by the Corey–Fuchs method, starting from the corresponding formylazulenes were
unsuccessful presumably due to the deactivation of the carbonyl function at the five-membered ring.
15. Depending on the number and position of the ethynyl groups in the azulene-system the bathochromic shift ranged from
3–48 nm.
16. The described new compounds gave correct elemental analyses.
17. NMR spectra were recorded with a Bruker NMR spectrometer WM 300 in CDCl3 with tetramethylsilane as internal
standard. UV–vis spectra were recorded with a Beckman UV-5240 spectrometer. Mass spectra (MS) were obtained with
a Varian 311A instrument.