Domino thermal radical cycloaromatization of non-conjugated aromatic hexa- and heptaynes: Synthesis of fluoranthene and benzo[a]rubicene skeletons

Article abstract of DOI:10.1016/S0040-4039(99)02312-6

Non-conjugated aromatic hexa- and heptaynes underwent domino thermal radical cycloaromatization at 25°C to yield indenol ring-fused fluoranthene and indenol ring-fused benzo[a]rubicene skeletons with helicity. This multicyclization reaction proceeded regi

Full text of DOI:10.1016/S0040-4039(99)02312-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1447–1451
Domino thermal radical cycloaromatization of non-conjugated
aromatic hexa- and heptaynes: synthesis of fluoranthene and
benzo[a]rubicene skeletons
Kazuhiro Miyawaki, Tomikazu Kawano and Ikuo Ueda ∗
The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Received 11 November 1999; accepted 3 December 1999
Abstract
Non-conjugated aromatic hexa- and heptaynes underwent domino thermal radical cycloaromatization at 25°C to
yield indenol ring-fused fluoranthene and indenol ring-fused benzo[a]rubicene skeletons with helicity. This multi-
cyclization reaction proceeded regioselectively to yield [6]helicene derivatives via annulation at the final step. The
structure and thermal racemization of [6]helicene derivatives (6 and 7) is also described. © 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: domino thermal radical cycloaromatization; aromatic non-conjugated polyyne; benzo[a]rubicene; [6]helicene.
Recently, we reported that the aromatic acyclic tetraynes underwent thermal radical cycloaromatization
under mild conditions to yield polycyclic ring systems such as fluorenol and fluoreno[3,2-c]isochromenol
skeletons.1,2 In the hope of developing this reaction and discovering a new type of polyphenylenes, we
planned to study the thermal radical cycloaromatization of aromatic acyclic polyynes. In this paper, we
describe domino thermal radical cycloaromatization of hexaynes (3a–c and 4a–c) and heptaynes (3d and
4d) which are converted into indenol ring-fused fluoranthene and indenol ring-fused benzo[a]rubicene
skeletons with helicity.
∗
Corresponding author. Tel: +00 81 6 6879 8470; fax: +00 81 6 6879 8474; e-mail: ueda@sanken.osaka-u.ac.jp (I. Ueda)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02312-6
tetl 16240

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Polyynes (3 and 4) were synthesized from 1 and 2 with 4-trimethylsilylbuta-1,3-diyn-1-yl lithium3
according to the method described in the literature (Scheme 1).1,2
Scheme 1.
Thermolysis of 3 and 4 in benzene (5 mM) was carried out at 25°C for 72 h in the dark under argon
atmosphere (Scheme 2).
Scheme 2.
Thermolysis of 3a afforded 5 in 60% yield, which was brought about by an intermolecular Diels–Alder
reaction of 1,2-benzyne intermediate (D) illustrated in Scheme 3 with benzene used as the solvent.
Reaction of 3b and 4b under the same conditions afforded 6 and 7 in 20% and 23% yields, respectively,
yielding an isochromene skeleton. Treatment of 3c and 4c under the same conditions afforded 8 and 9
in 21% and 22% yields, respectively, these results were brought about by an intramolecular Diels–Alder
reaction of the 1,2-benzyne intermediate (D) with a benzene ring in the parent molecules. On the other
hand, 3d and 4d underwent the thermal radical cyclization to yield 10 and 11 in 25% and 10% yields,
respectively, which had a benzo[a]rubicene skeleton in their molecule. The structure of 5 was determined
on the basis of spectral data and elementary analysis and finally confirmed by a single crystal X-ray
analysis.2 The ORTEP drawing of 5 is shown in Fig. 1. The structures of 6–11 were also assigned on the
basis of their spectral data and elementary analyses by comparing with the data of 5.4
Compounds (6–11) were shown to be a mixture of diastereoisomers because of the helical structures.
Among them, compounds (6, 7, 9 and 11) were separated as two diastereoisomers by silica gel column

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Scheme 3.
Fig. 1. The ORTEP drawing of 52
chromatography with benzene as an eluent. Thus, the mixture could be distinguished by their 1H NMR
spectral data and Rf-values on TLC (silica gel, eluted with benzene). The isomers with the lower Rf-
values, of which two proton signals were observed between δ 5.84 and δ 5.77 ppm for the Ha-hydrogen
and between δ 3.21 and δ 2.95 ppm for the Hb-hydrogen of the hydroxy group, were assigned to be
in (R*–M*)-configurations, while the other isomers with higher Rf-values, of which two proton signals
were observed between δ 6.26 and δ 6.14 ppm for the Ha-hydrogen and between δ 3.85 and δ 3.72 ppm
for the Hb-hydrogen of the hydroxy group, were assigned to be in (R*–P*)-configuration. Fig. 2 shows
schematic models of 7 as a typical example. The 1H NMR spectral data of 7 indicate that the four rings,
A, B, C and D are arranged in a coplane. The hydrogen atom located on the 5-position of (R*–P*)-7 is

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in the axial position and that of (R*–M*)-7 is in the equatorial position. In comparison with the chemical
shift of the proton of the hydroxy group, the hydroxy group of (R*–P*)-7 will be on the coplane with the
ethynyl group on the C-ring, while that of (R*–M*)-7 will be far from the coplane with the ethynyl group
on the C-ring.
Fig. 2. Schematic models of 7
1
The thermal racemization of 7 in bromobenzene-d5 using the H NMR method was studied. The
inversion rate constant of [(R*–P*)-6 and -7] and [(R*-M*)-6 and -7] was deduced from the first-order
plot which was obtained from the decrease of the peak height of the hydrogen at the 5-position. The
rate constant of 6 at 45°C was k1=3.7×10−5s−1 for (R*–P*)-6 and k−1=6.8×10−5s−1 for (R*–M*)-
6, respectively. The half-life time calculated was 110 minutes for (R*–P*)-6. The rate constant of 7 at
125°C was k1=2.0×10−3s−1 for (R*–P*)-7 and k−1=2.2×10−3s−1 for (R*–M*)-7, respectively. The half-
life time calculated was 164 minutes for (R*–P*)-7. The inversion rate constants indicate that [(R*–P*)-
6 and -7] are thermodynamically more stable than [(R*–M*)-6 and -7]. The difference in the thermal
stability between the (R*–P*)- and (R*-M*)-derivatives may be explained by estimating stereoelectronic
interactions between the hydroxy group at the 5-position and the aromatic core. This inversion was not
observed by radiation under a 500 W mercury lamp.
A plausible mechanism of the overall transformation for 3 (4) is outlined in Scheme 3. At the first
step, 3 (4) gives an outer-ring diradical (A) generated by intramolecular annulation reaction between
both buta-1,3-diyne functions of 3 (4). The diradical A is transformed into a fluorenol-1,2-benzyne
intermediate (B), one radical of which attacks the terminal carbon of a buta-1,3-diyne moiety, giving
an exomethylene radical yielding σ,σ-diradical (C). The diradical (C) is then converted into another
1,2-benzyne intermediate (D) which is the final active intermediate in this transformation. The inter- and
intramolecular Diels–Alder reaction of D yield 5, 8 and 9, respectively. The 1,2-benzyne intermediate
(D) bearing methoxymethyl group as R2 group yields 6 and 7 via a radical intermediate (E), forming
active species such as methyl cation, methyl radical and carbene.5 The diradical (F), which is formed
from D bearing phenylethynyl group as R2 group, is converted into benzo[a]rubicene derivatives (10 and
11) with helicity.
In conclusion, we have demonstrated a novel method for the preparation of indenol ring-fused
fluoranthene and indenol ring-fused benzo[a]rubicene skeletons with helicity by domino thermal radical
cycloaromatization of non-conjugated aromatic hexa- and heptaynes. This multi-cyclization reaction
proceeded regioselectively to yield [6]helicene derivatives through annulation at the final step. The
inversion rate constants and half-lives for the racemization of 6 and 7 were also determined.

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Acknowledgements
The authors are indebted to the Material Analysis Center of ISIR-Sanken for the elementary analyses.
References
1. Miyawaki, K.; Suzuki, R.; Kawano, T.; Ueda, I. Tetrahedron Lett. 1997, 38, 3943–3946.
2. Miyawaki, K.; Kawano, T.; Ueda, I. Tetrahedron Lett. 1998, 39, 6923–6926.
3. Holmes, A. B.; Jennings-White, C. L. D.; Schulthess, A. H. J. Chem. Soc., Chem. Commun. 1979, 840–842
4. All new compounds gave satisfactory analytical and spectral data. Selected physical data are as follows: (R*–P*)-7: Stage
1
orange needles (benzene–hexane), mp 253.3–253.8°C (dec.), Rf=0.43 (benzene), H NMR (CDCl3, 400 MHz): 8.37–8.35
(d-like, 1H), 7.83–7.81 (d-like, 1H), 7.63 (s, 1H), 7.53 (d, 1H, J=7.1 Hz), 7.41 (td, 1H, J=7.3, 1.2 Hz), 7.37 (td, 1H, J=7.3, 1.2
Hz), 7.25 (d-like, 1H), 7.22 (td, 2H, J=7.6, 1.5 Hz), 6.97 (td, 1H, J=7.8, 1.2 Hz), 6.92 (d, 1H, J=7.3 Hz), 6.84 (d, 1H, J=7.1
Hz), 6.18 (d, 1H, J=2.9 Hz), 5.35 (d, 1H, J=12.0 Hz), 5.08 (d, 1H, J=12.0 Hz), 3.81 (d, 1H, J=2.9 Hz), 1.74 (s, 3H), 0.45
(s, 9H) ppm. Anal. calcd for C36H28O2Si: C, 83.04; H, 5.42%. Found: C, 82.93; H, 5.17%. (R*–M*)-7: Stage yellow needles
(benzene–hexane), mp 252.3–252.8°C (dec.), Rf=0.38 (benzene), 1H NMR (CDCl3, 400 MHz): 8.37 (d, J=7.3 Hz, 1H), 7.82
(d, J=7.1 Hz, 1H), 7.63 (s, 1H), 7.58 (d, 1H, J=7.1 Hz), 7.42–7.35 (m, 2H), 7.26–7.18 (m, 3H), 7.00–6.95 (m, 2H), 6.84 (d,
1H, J=7.6 Hz), 5.83 (d, 1H, J=3.3 Hz), 5.35 (d, 1H, J=12.1 Hz), 5.06 (d, 1H, J=12.1 Hz), 3.21 (d, 1H, J=3.3 Hz), 1.70 (s,
3H), 0.45 (s, 9H) ppm. Anal. calcd for C36H28O2Si: C, 83.04; H, 5.42%. Found: C, 82.76; H, 5.32%.
5. Ueda, I.; Sakurai, Y.; Kawano, T.; Wada, Y.; Futai, M. Tetrahedron Lett. 1999, 40, 3919–322.