Preparation of 1-arylazulenes through regioselective photoarylation of azulene with aryl iodides

Article abstract of DOI:10.1016/S0040-4039(00)02044-X

Photoirradiation of aryl iodide in the presence of azulene has been shown to be a useful method for regioselective arylation of azulene at the electron rich 1-position.

Full text of DOI:10.1016/S0040-4039(00)02044-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 715–717
Preparation of 1-arylazulenes through regioselective
†
photoarylation of azulene with aryl iodides
‡
Tong-Ing Ho,* Chien-Kuo Ku and Robert S. H. Liu
Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
Received 3 July 2000; accepted 10 November 2000
Abstract—Photoirradiation of aryl iodide in the presence of azulene has been shown to be a useful method for regioselective
arylation of azulene at the electron rich 1-position. © 2001 Elsevier Science Ltd. All rights reserved.
Biaryls are useful compounds for possible applications
in asymmetric catalysis, formation of liquid crystals and
other novel materials. The preferred method of synthe-
sis of biaryls via metal catalyzed coupling reactions is
particularly suitable for the preparation of symmetrical
colored nature of the azulene chromophore (possibly
serving the destructive role of internal filters or a
quencher of excited species), such a photochemical
conversion obviously will have to be performed under
controlled conditions. In this paper, we would like to
report successful execution of photoarylation of azule-
nes resulting in respectable yields of products. Thus, the
reaction has the potential of becoming a viable method
for preparation of some of the unusual arylazulenes.
1
biaryls. The demonstration of a possible use of photol-
ysis of diazonium salts and aryl halides to generate an
2
reactive intermediate for arylation provides a ready
entry to unsymmetrical biaryls. Radical mechanism was
2
initially suggested but carbocation intermediate was
3
We found that photoirradiation of a hexane solution of
azulene and an iodoarene with light between 254 and
subsequently suggested particularly after the recent
finding that the photolysis of chlorobenzene and
fluorobenzene is facilitated by the presence of an elec-
tron donating group at the para-position appears to
3
50 nm led to the decomposition of the iodoarene and
the formation of 1-arylazulene (1) in yields ranging
63–92%. The six aryliodides that we have used so far
varied from phenyl (2), substituted phenyls (3,4), a-
naphthyl (5), 2-thienyl (6) to 1-azulenyl (7) for the aryl
4
favor a reactive carbocation intermediate. In a matrix
isolation spectroscopic study, Winkler and Sander
showed that photolysis of iodobenzene led to phenyl
cation, characterized spectroscopically for the first
7,8
group giving products 1a–f.
In a typical run, we
5
irradiated a 12 ml n-hexane solution containing 17
mmole of azulene and 34 mmole of iodobenzene with a
bank of 16 (24 W) germincidal lamps in a Rayonet
reactor for 24 h. From the irradiated mixture, 10
mmole of unreacted azulene was recovered (41% con-
version) and 6 mmoles of 1-phenylazulene was isolated
time. However, the method of production of the
phenyl cation could not rule out prior involvement of
the phenyl radical. This work is also encouraged by
successful trapping of reactive intermediates produced
from irradiation of aryl iodides by conventional nucle-
6
ophiles such as enolates.
(
85% yield). While the irradiation period was arbitrarily
chosen, we generally detected a significant reduction of
efficiency if the reaction was allowed to proceed fur-
ther. We suspect the trend was due to the increasing
effect of internal filtering contributed by both the azu-
lene and the product. The results of these six cases are
summarized in Table 1.
It occurred to us that such an electron deficient inter-
mediate might be used to effect regioselective arylation
of azulenes attacking preferentially at the electron rich
sites at C-1 and C-3. This could provide a ready entry
to 1-arylazulenes, 1. However, because of the highly
Keywords: regioselective; photoarylation; azulene.
It might appear odd that while proposing a carbocation
intermediate, we preferred to use a non-polar solvent
(hexane) in these photo-arylation reactions. Earlier, not
surprisingly, we found that irradiation of iodobenzene
in methanol led to its complete disappearance within 6
*
Corresponding author.
†
In memoriam of Professor Tetsuo Nozoe
Permanent address: Department of Chemistry, University of
Hawaii, Honolulu, HI 96822, USA.
‡
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02044-X

7
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T.-I. Ho et al. / Tetrahedron Letters 42 (2001) 715–717
h. This rapid loss of halide is largely unaffected by the
addition of azulene while only minute amounts of the
arylation product was detected. Clearly, methanol not
only assisted the heterolysis of iodobenzene but also
efficiently trapped the resulting carbocation. This inter-
pretation is consistent with the electrochemical work
reported earlier. Thus, such a polar solvent, while
facilitating the photolysis, is not suitable for our
arylations.
>450 nm (absorbed selectively by the weak S -band of
1
azulene) for the same period of time failed to produce
any detectable of the coupled product. This negative
result clearly showed that homolysis of iodoazulene can
1
0
only take place from the S state (or the T state) but
2
2
not the lower S (or the T ) state. Thus, entry 6 is
1
1
9
another rare case of chemical reaction originating from
an upper state, a chemical example violating the
Kasha’s Rule.
In an effort to extend similar reactions to other aryl
halides, we found that irradiation of a mixture of
bromobenzene or chlorobenzene with azulene failed to
give any arylated azulenes. This is in contrast with
reports that bromobenzene and chlorobenzene undergo
similar photolysis of the CꢀX bond as in iodobenzene.
The difference is perhaps not surprising when one
considers that azulene with a low lying excited state is
We have further noticed that the reactive intermediate
thus generated is sufficiently reactive toward other aryl
systems. Thus, when 1-iodoazulene was irradiated in
the presence of benzene as solvent, the same 1-phenyl-
azulene (cf entry 1, Table 1) was obtained in improved
94% yield (50% conversion). And, in a preliminary
experiment, we have prepared the highly crowded 1,1%-
bi-guaiazulenyl (8) by a similar photochemical proce-
dure. Thus, it is our intention to apply similar
photochemical procedure for the preparation of other
similarly crowded azulenic compounds, which are azu-
likely to be an efficient quencher whether of the S or
1
the T state of the aryl halides. Only in the case of the
1
weak C-I, its homolysis from the reactive excited state
apparently proceeded at a rate competitive with
bimolecular quenching by azulene.
11
lene anaogs of 1,1%-binaphthyl.
In summary, in spite of the potential problems of
internal filtering and efficient quenching, we have
shown that photolysis of aryl iodides can be used as a
viable entry to 1-arylazulenes.
The six reactions shown in Table 1 are not very sensi-
tive to variation of excitation wavelength from 254 nm
to 350 nm. The observation of minor difference in
yields (or extent of conversion within the same period
of irradiation) is more likely due to changing internal
filtering effect rather than the difference in excitation
energy. The case of 1-iodoazulene with azulene (entry 6
in Table 1) is a particularly interesting one. While
excitation within the 254–350 nm window (absorbed
Acknowledgements
This work was supported by a grant from the National
Science Council of the Republic of China (Taiwan).
RSHL acknowledges with thanks a fellowship from the
Foundation for the Advancement of Outstanding
primarily by the strong S -band of azulene) brought
2
about similar conversion and yield of 1,1%-bi-azulenyl
(
1f), however we observed that irradiation with light
a
Table 1. Photolysis of aryl iodides 2–7 in the presence of azulene
b
Entry
Aryl iodide, cpd
Product
Conversion/%
Yield /%
1
2
3
4
5
6
Phenyl, 2
p-Tolyl, 3
m-Tolyl, 4
a-Naphthyl, 5
2-Thienyl, 6
1-Azulenyl, 7
1-Phenylazulene
41
37
34
18
50
28
85
75
72
92
63
67
1-p-Tolylazulene
1-m-Tolylazulene
1-a-Naphthylazulene
1-(2%-Thienyl)azulene
1,1%-Bi-azulenyl
a
Irradiated for approx. 24 h at 254, 300 or 350 nm. Usually the best yield was achieved with 350 nm light.
Based on consumption of azulene.
b

T.-I. Ho et al. / Tetrahedron Letters 42 (2001) 715–717
717
Scholarship (Taiwan). We are grateful to Professor P.
W. Yang for the sample of azulene.
7. Compounds 1a, 1b, 1d and 1f are known compounds.
Spectral characterization data can be found in: Nefedov,
V. A.; German, N. A.; Lutsenko, A. I. J. Org. Soc. Chem.
USSR (Engl), 1987, 23, 154-162 (for 1a, b, d) and Morita,
T.; Yakase, K. Bull. Chem. Soc. Jpn. 1982, 55, 1144-1152
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2
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.