Synthesis, structure, and photophysical properties of highly substituted 8,8a-dihydrocyclopenta[a]indenes

Article abstract of DOI:10.1002/anie.200802560

(Chemical Equation Presented) A triple round: A palladium-catalyzed cyclotrimerization of 1,2-diarylacetylenes has been developed to synthesize highly substituted 8,8a-dihydrocyclopenta[a]indenes. One such cycloadduct (1) displays unusual aggregation-induced emission with a strong blue fluorescence (see picture). The structures of the products have been confirmed by X-ray crystal analysis.

Full text of DOI:10.1002/anie.200802560

Angewandte
Chemie
DOI: 10.1002/anie.200802560
Polycycles
Synthesis, Structure, and Photophysical Properties of Highly
Substituted 8,8a-Dihydrocyclopenta[a]indenes**
Yao-Ting Wu,* Ming-Yu Kuo, Yu-Ting Chang, Chien-Chueh Shin, Tsun-Cheng Wu,
Chia-Cheng Tai, Tzu-Heng Cheng, and Wei-Szu Liu
Dedicated to Professor Armin de Meijere on the occasion of his 70th birthday
Table 1: Optimization of reaction conditions for the preparation of 3a.[a]
Alkynes are important building blocks and synthons for the
construction of a variety of interesting and useful molecules.[1]
Since the discovery of the formation of benzene from
acetylene by Berthelot[2] and Reppe et al.,[3] this formal
[2+2+2] cyclotrimerization has become an elegant method
for the generation of benzene derivatives and polycyclic
aromatic compounds, and several metal catalysts have been
developed for this reaction.[4] We recently synthesized a new
cycloadduct 3a, which is formed from diphenylacetylene (2a)
in the presence of a palladium catalyst (Scheme 1). To the
Entry Catalyst
Additives (equiv) Base
Yield [%]
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
[Pd(PPh3)4]
Pd/C
PPh3 (0.5)
PPh3 (0.5)
PPh3 (0.5)
–
PCy3 (0.25)
PCy3 (0.5)
PCy3 (0.5)
PCy3 (0.75)
PCy3 (0.5)
PCy3 (0.5)
PCy3 (0.5)
PCy3 (0.5)
PCy3 (0.5)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
32[b]
4[c,d]
0[c,e]
3[c]
32
68
13[c,f]
67
9
60
67
10
11
12
13
14
Na2CO3
NaOAc or NEt3 5[c]
K2CO3
K2CO3
K2CO3
67
23
56
[PdCl2(PCy3)2] PCy3 (0.5)
[a] Reaction was carried out in a thick-walled sealable tube. The amounts
of catalysts, oxidants, additives, and base are relative to alkyne 2a. Pd
catalyst (5 mol%), Cu(OAc)2·H2O (1.0 equiv), BQ (0.5 equiv), base
(1.0 equiv), water, and CH3CN were used in this reaction. [b] 1,2,3,4-
Tetraphenylnaphthalene (16%) was also obtained. [c] Most of the alkyne
2a remained. [d] In the absence of BQ. [e] In the absence of Cu-
(OAc)2·H2O. [f] In the absence of water.
Scheme 1. Metal-catalyzed cyclotrimerizations of alkyne 2a.
best of our knowledge, this is the first example of a tricyclic
[5.5.6]-fused skeleton generated by the trimerization of
diarylalkynes.[5] Herein, we describe the systematic optimiza-
tion of this reaction and the applicability of this method to
several alkynes.
roles in this reaction (Table 1). In the absence of any one of
them, 3a was only obtained in very low yield.
Exhaustive studies of the reaction conditions for the
synthesis of cycloadduct 3a from diphenylacetylene (2a)
showed that the palladium catalyst, the organophosphine,
water, benzoquinone (BQ), and Cu(OAc)2·H2O all play key
Pd(OAc)2 and [Pd(PPh3)4] were found to be the most
efficient catalysts in this reaction. PCy3 was determined to be
the best reagent for the reaction; PPh3 led not only to the
desired product, but also to 1,2,3,4-tetraphenylnaphthalene.[6]
It was necessary to use half an equivalent of PCy3 to get the
optimal yield (Table 1, entries 1, 5, 6, and 8). The use of a
carbonate base slightly improved the yield of the reaction,
whereas triethylamine or NaOAc inhibited the formation of
3a (Table 1, entry 11). Under the optimized conditions, the
desired product 3a was obtained in 68% yield, along with
trace amounts of by-product 1a (Table 1, entry 6). Crystals of
3a[7] were grown from CH2Cl2/MeOH, and the structure was
determined by X-ray diffraction analysis. The phenyl groups
at positions C8 and C8a in 3a adopt a trans configuration and
the torsional angle between the cyclopentadienyl and benzo
planes is approximately 348. The overcrowded and nonplanar
structure causes this compound to exhibit unusual photo-
physical properties (see below).
[*] Prof. Y.-T. Wu, Y.-T. Chang, C.-C. Shin, T.-C. Wu, C.-C. Tai, T.-H. Cheng
Department of Chemistry, National Cheng Kung University
No. 1 Ta-Hsueh Rd., 70101 Tainan (Taiwan)
Fax: (+886)6274-0552
E-mail: ytwuchem@mail.ncku.edu.tw
Prof. M.-Y. Kuo, W.-S. Liu
Department of Applied Chemistry, National Chi Nan University
No. 1 University Rd., 54561 Puli, Nantou (Taiwan)
[**] Metal-Catalyzed Reactions of Alkynes. Part III. This work was
supported by the Landmark Project of National Cheng Kung
University and the National Science Council of Taiwan (NSC 96-
2113M-006-008-MY2). We also thank Prof. S.-L. Wang and C.-Y.
Chen (National Tsing Hua University, Taiwan) for the X-ray structure
analyses. Part II: Y.-T. Wu, W.-C. Lin, C.-J. Liu, C.-Y. Wu, Adv. Synth.
Catal. 2008, 350, 1841.
The reactivity of several alkynes was examined under the
optimized conditions (Table 2). Alkynes 2b–2m underwent
the cyclotrimerization to generate 3 in moderate to excellent
yields (28–85%). The lowest yield was obtained for 3g, which
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802560.
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Communications
Table 2: Preparation of tricyclic adducts 3 from alkynes 2.
It was determined that cycloadduct 3a is not generated
from a rearrangement of hexaphenylbenzene (1a). When 1a
was heated in the presence of our catalytic system, it
remained unchanged. Moreover, when a control experiment
was carried out with deuterium oxide, instead of water, [D1]-
3a was obtained in 62% yield with 70% isotopic purity.[13,14]
Further study of the reaction with [D10]diphenylacetylene
([D10]-2a) in water gave a mixture of [D29]-3a and [D30]-3a in
a ratio of approximately 75:25.[15] These results indicate that
the majority of the hydrogen atoms at position C8 in 3a come
from the external water, but some of them (ca. 25%) are
provided by the phenyl ring in 2a (see Scheme 3, route B).[16]
Based on literature precedent and the above control
Entry Alkyne R1
R2
R3
Product 1/3[a] Yield [%][b]
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2 f
2g
2h
2i
H
H
H
H
H
H
H
Me
OMe
Me
Me
OMe
H
H
H
H
H
H
H
H
H
H
H
Me
3a[c]
3b[c]
3c[c]
3d[c]
3e[c]
3 f[c]
3g[d]
3h[c]
3i[c]
5:95
13:87 72
4:96 59
11:89 77
68
Me
CF3
F
tBu
OMe
CO2Me
H
H
F
H
H
5:95
8:92
9:91
6:94
4:96
9:91
8:92
40
51
28[e]
78
70
71
65
9
10
11
12
13
2j
3j[c]
2k
2 l
2m
3k[c]
OMe 3l
0:100 85
19:81 82
experiments,
a
proposed mechanism was formulated
OMe OMe
OMe 3m[d]
(Scheme 3). The reaction of an active PdII species with three
molecules of alkyne 2a generates 1-palladacycloheptatriene
8,[17,18] which can either undergo a reductive elimination to
form hexaphenylbenzene (1a)[19] or can be reduced to PdII
complex 9 by PCy3.[20] Intermediate 9 subsequently rearranges
[a] The ratios were determined from the 1H NMR spectra of the products,
which were obtained after purification by simple chromatography. [b] The
combined yields for isomers 1 and 3. [c] Separation from 1 by
crystallization from CH2Cl2/MeOH. [d] Separation from 1 by chromatog-
raphy. [e] 30% 2g was recovered.
was isolated together with 30% of the starting material
(Table 2, entry 7). In general, by-products 1 were also
produced in small amounts under these reaction conditions,
but they can be removed by either careful chromatography or
crystallization. The three 3,3’-disubstituted diarylethynes 2h–
2j furnished only one regioisomer (Table 2, entries 8–10).[8]
The structures of 3h and 3i can be assigned on the basis of the
crystal structures of 3j and 3k[7,9] and their 2D NMR spectra.
It was also determined that sterically congested di(2-tolyl)-
ethyne and heteroaryl acetylenes, such as di(2-thiophenyl)-
ethyne and di(3-pyridinyl)ethyne, did not form the desired
product under these conditions.[10]
The possibility of crossed cycloadditions between differ-
ent alkynes was also investigated. The reaction of 2a with
dialkylalkynes 4 in our catalytic system produced cyclo-
adducts 5 and 6 (Scheme 2).[11] Other tricyclic regioisomers of
6 were not observed in appreciable amounts. Unfortunately, a
mixture of 1,8-bis(phenylethynyl)naphthalene and alkyne 2a
did not furnish the desired product, but 7,8,9,10-tetraphenyl-
fluoranthene (7) was obtained in 65% yield.[12]
Scheme 2. Crossed cycloadditions between alkynes 2a and 4.
Scheme 3. Proposed mechanism for the formation of cycloadduct 3a.
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Angewandte
Chemie
to a Schrock-type carbene complex 10,[21] in which the
carbene metal atom is electrophilic and the carbene carbon
atom is nucleophilic.[22] Protonation of the carbene carbon
atom with water leads to the formation of s-complex 11,[23]
which produces complex 13 by intramolecular electrophilic
palladation[24] and subsequent deprotonation (route A).
cence. Further studies of the photophysical and electrolumi-
nescence properties of functionalized derivatives of 3 are
currently underway, as these molecules may potentially find
use as organic light-emitting diodes (OLEDs).[28]
Alternatively, the carbene complex 10 could cyclize to yield Experimental Section
Preparation of 3a: A mixture of alkyne 2a (200 mg, 1.12 mmol), PCy3
the zwitterionic intermediate 12 (route B). An intramolecular
proton shift or reaction with water would then furnish 13. The
steric repulsion in 12 or 13 can be minimized when the two
phenyl groups adopt a trans configuration. Reductive elim-
ination of 13 gives the desired product 3a and a Pd0 species.
The active PdII complex can eventually be regenerated by
oxidation with Cu(OAc)2·H2O and BQ.[25] However, the
formation of 6 should occur via key intermediates 15, which
could be generated from 14 by a 1,2- or 1,5-shift, as is
observed in other cyclopentadiene derivatives.[26] This re-
arrangement is likely promoted by the steric repulsion
between the palladium–carbene moiety and the phenyl
group at C5 in 14.
Compound 3a is virtually nonfluorescent in dichloro-
methane or THF, but is highly luminescent in the solid state
(or crystal). To quantify this aggregation-induced emission,[27]
a solution of 3a (ca. 10À5 m) in a mixture of THF and water
was excited at 372 nm.[11] A dramatic change in the fluores-
cence intensity and relative quantum yield (F) could be
observed as the amount of water in the solution was increased
(Figure 1). The F value of a solution of 3a in THF/water (10/
90, W90) is about 70 times higher than in pure THF (W0).
Moreover, the size of the particles plays a key role in their
luminescent properties. In a mixture of THF and water
(ca. 1:2), 3a aggregates to form spherical nanoparticles with a
mean diameter of approximately 10–30 nm.[11] The aggrega-
tion of these molecules can efficiently inhibit the nonradiative
vibration and rotation of phenyl groups, and thus enhance the
luminescence.[27b] As with other compounds that exhibit
aggregation-induced emission,[27k] 3a could potentially be
used as a sensor for detecting organic solvents.
(157 mg, 0.56 mmol), Cu(OAc)2·H2O (224 mg, 1.12 mmol), benzo-
quinone (61.0 mg, 0.56 mmol), Pd(OAc)2 (12.5 mg, 55.6 mmol),
K2CO3 (155 mg, 1.12 mmol), H2O (1 mL), and acetonitrile (3.5 mL)
in a thick-walled pyrex tube was purged with nitrogen for 5 min. The
sealed tube was kept in an oil bath at 1108C for 36 h. After cooling the
mixture to room temperature and filtration over celite, the solvent of
the filtrate was removed under reduced pressure. The residue was
subjected to chromatography on silica gel with hexane/CH2Cl2 (12:1)
as the eluent to afford a mixture of 1a and 3a as a yellow solid
(136 mg, 68%, 1a/3a = ca. 5:95). By-product 1a could be removed by
either crystallization from CH2Cl2/MeOH or precipitation from
diethyl ether. A suitable crystal of 3a (m.p. 194–1958C) for X-ray
diffraction analysis was grown from CH2Cl2/MeOH. 1H NMR
(300 MHz, CDCl3): d = 5.15 (s, 1H), 6.66 (d, 3J = 6.6 Hz, 2H), 6.81
(d, 3J = 5.8 Hz, 2H), 6.86–6.90 (m, 3H), 7.05–7.15 (m, 12H), 7.21–7.26
(m, 7H), 7.41 (d, 3J = 7.0 Hz, 2H), 7.50 ppm (d, 3J = 5.8 Hz, 1H);
13C NMR (75.5 MHz, CDCl3, plus DEPT): d = 57.0, 78.3, 122.2, 125.6,
126.2, 126.4, 126.5 ꢀ 2, 126.82, 126.83, 127.1 ꢀ 2, 127.3, 127.4, 127.6 ꢀ 2,
127.7, 128.0 ꢀ 4, 128.1 ꢀ 2, 128.2 ꢀ 2, 129.6 ꢀ 2, 129.7 ꢀ 2, 129.9 ꢀ 2,
135.2, 135.4, 135.7, 136.3, 140.3, 141.6, 142.4, 147.8, 148.7, 153.7,
157.5 ppm (one CH carbon atom cannot be observed because of
signal overlap); EIMS (70 eV), m/z (%): 534 (100) [M+]; HRMS (EI)
calcd for C42H30: 534.2347; found: 534.2343. Elemental analysis calcd
(%) for C42H30 (534.7): C 94.34, H 5.66; found: C 94.19, H 5.66.
Received: June 2, 2008
Revised: September 22, 2008
Published online: November 14, 2008
Keywords: aggregation · alkynes · cyclotrimerizations ·
.
fluorescence · palladium
[1] Acetylene Chemistry: Chemistry, Biology, and Material Science
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In summary, we have developed a new and versatile
synthetic method for the preparation of highly substituted
8,8a-dihydrocyclopenta[a]indenes 3 in one pot by the cyclo-
trimerization of diarylalkynes. Compound 3a displays an
aggregation-induced emission with a strong blue fluores-
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[5] Selected examples of the synthesis of cyclopenta[a]indenyl
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Figure 1. Aggregation-induced emission of 3a (ca. 10À5 m) in a THF/
water mixture under illumination with UV light (365 nm). Quantum
yields of 3a were investigated at an excitation wavelength of 372 nm,
and quinine sulfate was used as the standard (F=0.546).[11]
Angew. Chem. Int. Ed. 2008, 47, 9891 –9894
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 9893

Communications
Kakiuchi, J. Am. Chem. Soc. 2002, 124, 3806; d) molybdenum-
PdIV/PdII reduction process should be easier than the PdII/Pd0
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[21] The reaction of Pd(OAc)2 with diazomethane in situ generates
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(Eds.: E. Negishi, A. de Meijere), Wiley, New York, 2002,
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[6] The reaction of PPh3 with diphenylethyne in the presence of Pd
catalysts produces 1,2,3,4-tetraphenylnaphthalene: Y.-T. Wu, K.-
H. Huang, C.-C. Shin, T.-C. Wu, Chem. Eur. J. 2008, 14, 6697.
[7] 3a: C84H62, monoclinic, space group P21/c, a = 15.7618(17), b =
16.6604(19), c = 22.911(3) ꢂ, a = g = 90, b = 99.747(2)8, V=
5929.5(11) ꢂ3. 3j·CH3OH: C49H40F6O, monoclinic, space group
P21/n, a = 18.2763(5), b = 11.1214(4), c = 21.3674(7) ꢂ, a = g =
90, b = 113.2580(10)8, V= 3990.2(2) ꢂ3. 3k: C54H54, monoclinic,
space group C2/c, a = 22.8764(15), b = 17.7788(11), c =
22.9868(14) ꢂ, a = g = 90, b = 112.624(4)8, V= 8629.7(9) ꢂ3.
CCDC 685514 (3a), 688340 (3j), and 685513 (3k) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[8] A regioisomer of 3h was also obtained in trace amounts. The
ratio between 3h and this regioisomer is approximately 95:5.
[9] According to the X-ray structure of 3k, the distance between the
hydrogen atom at position C8 and the hydrogen atom of the
methyl group at position C7 is about 2.5–2.7 ꢂ. NOE correla-
tions between these two were observed.
[24] Examples of intermolecular electrophilic palladation of arenes:
a) M. Tani, S. Sakaguchi, Y. Ishii, J. Org. Chem. 2004, 69, 1221;
b) C. Jia, W. Lu, T. Kitamura, Y. Fujiwara, Org. Lett. 1999, 1,
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[10] Di(2-naphthalyl)acetylene and bis(9,9-di-n-propyl-2-fluoryl)-
acetylene can also generate corresponding cycloadducts 3.
Elucidation of their structures is in progress.
[11] See the Supporting Information.
[12] 7,8,9,10-Tetraphenylfluoranthene (7) can be efficiently synthe-
sized from the same starting materials in the presence of the
Wilkinson catalyst: Y.-T. Wu, T. Hayama, K. K. Baldrige, A.
Linden, J. S. Siegel, J. Am. Chem. Soc. 2006, 128, 6870.
[25] A Pd0 complex can be oxidized to a PdII species by benzoqui-
none: H. Grennberg, A. Gogoll, J.-E. Bꢅckvall, Organometallics
1993, 12, 1790.
[13] A kinetic isotope effect was not observed in this reaction. The
reaction of 2a in a mixture of water and deuterium oxide (40:60)
gave [D1]-3a with an isotope purity of about 40%.
[14] Compound [D1]-3a is not generated from 3a by H/D exchange:
Compound 3a remained unchanged when heated with deute-
rium oxide in the presence of our catalytic system.
[15] In independent experiments, 3a (10.055 mg; 18.81 mmol) and a
mixture of [D29]-3a and [D30]-3a (15.093 mg; ca. 26.77 mmol)
were dissolved in CDCl3 (1.0 mL; tetramethylsilane was used as
the internal standard) for 1H NMR studies. The ratio of [D29]-3a
and [D30]-3a can be obtained from the 1H NMR spectra.
[16] It also explains that in the absence of water, 3a was obtained in
only 13% yield (Table 1, entry 7).
[26] For a detailed discussion of thermal sigmatropic rearrangements,
see: a) C. W. Spangler, Chem. Rev. 1976, 76, 187; for the 1,2-shift
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Schirmer, F. Funke, S. Mꢆller, T. Labahn, M. Nꢁtzel, A.
de Meijere, Eur. J. Org. Chem. 2004, 724; for the 1,2-shift of an
acyl group, see: c) M. Zeya, R. F. Childs, J. Am. Chem. Soc. 1972,
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[17] Several PdII catalysts have been found to produce hexasubsti-
tuted benzenes 1 by [2+2+2] cyclotrimerzation of internal
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[18] To the best of our knowledge, the chemistry of 1-palladacyclo-
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van Belzen, H. Hoffmann, C. J. Elsevier, Angew. Chem. 1997,
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[19] 3-Hexyne is relatively unreactive in the presence of this catalytic
system.
[28] More recently, fluorenonearylamine derivatives with aggrega-
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