Palladium-catalyzed cycloisomerizations of diarylacetylenes: Synthesis, structures, and physical properties of highly substituted naphthalenes and 8,8 a-dihydrocyclopenta[a]indenes

Article abstract of DOI:10.1002/chem.201100437

Depending on the electronic properties of their substituents, the major products generated by palladium-catalyzed cycloisomerizations of diarylalkynes are either highly substituted 8,8a-dihydrocyclopenta[a]indenes 3 or naphthalenes 4. The structures of these compounds were verified by X-ray crystallographic analysis. Many functional groups tolerated the reaction conditions evaluated in this study. The isotope-labeled experiments indicated that added water has a critical role in forming both classes of compounds. The photophysical and electrochemical properties of cycloadducts 3 and their analogues were systematically studied and compared with computational predictions based on density functional theory. Dihydrocyclopenta[a]indenes 3 in either solid or liquid form display strong luminescence, whereas cyclopenta[a]indene 11 j is practically nonfluorescent. The functional groups directly attached to the backbone of compound 3 significantly influenced physical properties. The steric effect arising from the aryl substituents caused different luminescence phenomena, including aggregation-induced and -enhanced emission. Copyright

Full text of DOI:10.1002/chem.201100437

DOI: 10.1002/chem.201100437
Palladium-Catalyzed Cycloisomerizations of Diarylacetylenes: Synthesis,
Structures, and Physical Properties of Highly Substituted Naphthalenes and
8,8a-Dihydrocyclopenta[a]indenes**
Tsun-Cheng Wu,[a] Chia-Cheng Tai,[a] Hsin-Chieh Tiao,[b] Yu-Ting Chang,[a]
Chia-Chun Liu,[b] Ching-Hsiu Li,[b] Cheng-Hao Huang,[b] Ming-Yu Kuo,*[b] and
Yao-Ting Wu*[a]
Abstract: Depending on the electronic
properties of their substituents, the
major products generated by palladi-
um-catalyzed cycloisomerizations of di-
arylalkynes are either highly substitut-
ed 8,8a-dihydrocyclopenta[a]indenes 3
or naphthalenes 4. The structures of
these compounds were verified by X-
ray crystallographic analysis. Many
functional groups tolerated the reac-
tion conditions evaluated in this study.
The isotope-labeled experiments indi-
cated that added water has a critical
role in forming both classes of com-
pounds. The photophysical and electro-
chemical properties of cycloadducts 3
and their analogues were systematically
studied and compared with computa-
tional predictions based on density
functional theory. Dihydrocyclopen-
ta[a]indenes 3 in either solid or liquid
form display strong luminescence,
whereas cyclopenta[a]indene 11j is
practically nonfluorescent. The func-
tional groups directly attached to the
backbone of compound 3 significantly
influenced physical properties. The
steric effect arising from the aryl sub-
stituents caused different luminescence
phenomena, including aggregation-in-
duced and -enhanced emission.
Keywords: aggregation · alkynes ·
cycloisomerization · naphthalene ·
palladium
Introduction
The metal-catalyzed reaction for the carbon–carbon bond
formation is a major field of research in organic synthesis.
This process provides efficient tools for constructing various
interesting and valuable products.[1] In the presence of metal
species, alkynes readily form p-complexes that then undergo
migratory insertion. The reactive intermediates thus formed
participate in further carbon–carbon bond-forming steps.[2]
For example, the reaction of a ruthenium or rhodium cata-
lyst with phenylacetylene generates naphthalene A, and the
vinylidene complex is expected to be the key intermediate
(Scheme 1).[3] The palladium-catalyzed self-dimerization of
2-methylbut-1-en-3-yne produces styrene B by a formal
Scheme 1. Selective examples for metal-catalyzed cycloisomerization and
cycloaddition of alkynes.
[4+2] cycloaddition of enyne with an alkyne moiety.[4] More-
over, alkyne activation by other complexes forms 1-metalla-
cyclopentadiene, which incorporates with another alkyne
molecule or undergoes dimerization to yield benzene deriva-
tives C[5] and cyclooctatetraenes D,[6,7] respectively. Unsub-
stituted C and D (R1 =R2 =H) generated from ethyne were
first observed by Reppe et al.[8] Since then, these reactions
have been studied intensively. Cyclotrimerization is widely
used to prepare numerous (polycyclic) aromatic com-
pounds.[5] Nickel complexes are now known to be most effi-
cient in the catalysis of cyclotetramerization.[6] Under suita-
ble reaction conditions, cycloadducts C or D can be selec-
tively generated as a single regioisomer when asymmetric al-
kynes are used. In contrast to these known reactions, our
preliminary studies indicated that 8,8a-dihydrocyclopen-
ta[a]indenes 3 were formed from diarylacetylenes 1 and that
[a] T.-C. Wu, Dr. C.-C. Tai, Y.-T. Chang, Prof. Y.-T. Wu
Department of Chemistry
National Cheng Kung University
No. 1 Ta-Hsueh Rd., 70101 Tainan (Taiwan)
Fax : (+886)6-2740552
E-mail: ytwuchem@mail.ncku.edu.tw
[b] H.-C. Tiao, C.-C. Liu, C.-H. Li, C.-H. Huang, Prof. M.-Y. Kuo
Department of Applied Chemistry
National Chi Nan University
No. 1 University Rd., 54561 Puli, Nantou (Taiwan)
E-mail: mykuo@ncnu.edu.tw
[**] Metal-Catalyzed Reactions of Alkynes. Part IX. Part VIII: T.-C. Wu,
C.-C. Tai, H.-C. Tiao, M.-Y. Kuo, Y.-T. Wu, Chem. Eur. J. 2011, 17,
1930.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100437.
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Chem. Eur. J. 2011, 17, 7220 – 7227

FULL PAPER
cycloadduct 3a (Ar=Ph) displays aggregation-induced
emissions with blue fluorescence.[9] Here, the full scope and
limitations of the new cyclotrimerization process were stud-
ied. The photophysical and electrochemical properties of
the cycloadducts and their analogues were also analyzed
and compared with computational predictions based on den-
sity functional theory.
guishable by TLC under UV irradiation at 365 nm. In most
cases, compounds 2 are virtually nonfluorescent. Some elec-
tron-deficient alkynes 1 and bis(9,9-di-n-propyl-2-fluoryl)-
acetylene (5), which are less reactive, were not completely
consumed. Alkynes 1k-n and 5 furnished only one regioiso-
mer (Table 1, entries 10–13 and 20).[10] The structures of 3k,
3l, and 3n were determined by 2D-NMR spectroscopic ex-
periments because the through-space interaction between
the hydrogen at position C8 and the methyl or methoxy
group only at position C7 should be observed, and the NOE
effect in 3q and 3r confirms the exception. Compounds 3m
and 3p lack a through-space 13C–19F coupling in their NMR
spectra.[11] The structure of 3m was determined by X-ray
crystallographic analysis.[12] Alkyne 5 could give either 6 or
Results and Discussion
Synthesis: The optimal reaction conditions for synthesizing
cycloadducts 3 have been reported previously (Table 1).
Added water has a critical role in this reaction because, as
indicated by isotope-labeled experiments, most (ca. 75%) of
the hydrogen atoms at position C8 in 3 come from the exter-
nal water. This procedure is compatible with several func-
tional groups, including alkyl, alkoxy, aryl halide, ester, and
ketone. Generally, the small amounts of resulting byprod-
ucts 2 are removable by either careful chromatography or
crystallization. These two classes of compounds are distin-
Table 1. Preparation of dihydrocyclopenta[a]indenes 3 and naphthalenes 4 from alkynes 2.[a]
6’, but the crystals of the cyclo-
adduct were unavailable for the
structural analysis (entry 20).
These two regioisomers are dis-
tinguishable by 2D-NMR spec-
troscopic techniques and com-
putational results. The opti-
mized structure of 6’ indicated
by B3LYP/6-31G** functional
analysis revealed that the dis-
tance between two hydrogen
atoms indicated in the structure
shown above is 2.23 ꢁ, which is
a reasonable value for the NOE
interaction. This through-space
interaction was not observed in
the isolated product. Accord-
ingly, the accurate structure of
the cycloadduct should be 6
rather than 6’. The crowded en-
vironment in 6’ was also unfav-
orable for its formation. Elec-
tron-deficient diarylacetylenes
1, which contain two or three
Entry
Alkyne
R1
R2
R3
Conv. [%]
Prod.
Ratio 2/3[a]
Yield [%][b]
Ref.
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
F
100
100
100
100
97
100
100
70
3a
3b
3c
3d
3e
3 f
3g
3h
3i
5:95
13:87
4:96
11:89
7:93
68
72
59
77
61
40
51
28
[9]
[9]
[9]
[9]
this work
[9]
[9]
[9]
this work
[9]
[9]
this work
[9]
this work
this work
[9]
[9]
[9]
this work
Me
CF3
F
Cl
tBu
OMe
CO2Me
Ac
H
H
H
F
H
H
H
H
OMe
F
5:95
8:92
9:91
11:89
6:94
4:96
10:90
9:91
9
H
92
59
78
10
11
12
13
14
15
16
17
18
19
1k
1l
Me
OMe
CF3
Me
F
CF3
Me
OMe
OMe
F
100
100
100
100
100
100
100
100
100
100
3k
3l
70
1m
1n
1o
1p
1q
1r
3m
3n
3o
3p
3q
3r
78[d]
71
[c]
–
–
17[e]
10[f]
65
[c]
CF3
Me
OMe
OMe
F
8:92
0:100
19:81
85
1s
1t
3s
3t
82
[c]
13[g]
strong
electron-withdrawing
–
groups in an aryl substituent,
this work showed different chemoselectiv-
ity under these conditions, and
[c]
20
5
65
6
–
53
they predominantly formed
[a] The reaction was carried out with alkyne 1 (1.12 mmol), PCy3 (0.56 mmol), Cu
ACHTUNGTREN(NUNG OAc)2·H2O (1.12 mmol),
naphthalenes 4 by undergoing
new cycloisomerization reac-
tions (entries 14, 15 and 19).[13]
For example, the major prod-
ucts yielded by bis(3,5-difluoro-
benzoquinone (BQ, 0.56 mmol), Pd(OAc)2 (55.6 mmol), K2CO3 (1.12 mmol), H2O (1 mL), and acetonitrile
AHCTUNGTRENNUNG
1
(3.5 mL) in a thick-walled Pyrex tube at 1108C for 36 h. The ratios were determined from the H NMR spectra
of the products, which were obtained after purification by simple chromatography. [b] Combined isolated
yields for isomers 2 and 3. [c]Due to the overlap of signals, the ratio cannot be identified. [d] Compound 4m
(6%) was also isolated. [e] Compound 4o (42%) was also isolated. [f] Signals of 4p can be observed in the
MS spectrum of the crude product, but it cannot be obtained in the pure form. [g] Compound 4t (54%) was
also isolated.
phenyl)acetylene
(1o)
and
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Y. -T. Wu, M. -Y. Ku et al.
Table 2. The crossed cycloaddition between dialkylacetylene and alkynes
bis(3,4,5-trifluorophenyl)acetylene (1t) were naphthalenes
4o and 4t, respectively, whereas bis(4-fluorophenyl)acety-
lene (1d) gave only cycloadduct 3d (entry 4). In alkyne 1p,
increased electron deficiency was expected to generate a
high yield of naphthalene 4p, but, in all possible cycload-
ducts, production was unsatisfactory (entry 15). Analysis of
the crude products of this reaction by GCMS revealed sig-
1.[a]
nificant amounts of tetrakisACTHNURTGNEU(GN 3,5-bis(trifluoromethyl)phenyl)-
butadienes. Additionally the position of the electron-with-
drawing group in an aryl ring only slightly affected the che-
moselectivity between compounds 3 and 4. As observed in
bis(4-trifluoromethylphenyl)acetylene (1c), only small
amounts of naphthalene derivative 4m were produced from
bis(3-(trifluoromethyl)phenyl)acetylene (1m). The structure
of 4t was verified by X-ray crystallographic analysis
(Figure 1).[14] Because alkyne 1 contains an enyne skeleton,
Alkyne
R1
R2
Product (isolated yield [%])
Ref.
1a
1a
1t
nPr
nBu
nPr
H
H
F
7a-nPr (40)+9a-nPr (27)
7a-nBu (36)+9a-nBu (18)
7t-nPr (21)+8t-nPr (45)[b]
[9]
[9]
this work
[a] The reaction was carried out with alkyne 1 (0.75 mmol), 4-octyne (or
5-decayne, 1.12 mmol), PCy3 (0.56 mmol), Cu(OAc)2·H2O (1.12 mmol),
benzoquinone (0.56 mmol), Pd(OAc)2 (55.6 mmol), K2CO3 (1.12 mmol),
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
H2O (1 mL), and acetonitrile (3.5 mL) in a thick-walled Pyrex tube at
1108C for 36 h. Cycloadducts 3 and/or 4 generated from 1 were also ob-
tained, but neglected. [b] Pure compounds 7t-nPr and 8t-nPr were not
obtained because they contained some inseparable impurity (ca. 15%).
Cycloadducts 9t-nPr and 10t-nPr were isolated as an inseparable mixture
in small amounts (about 10%).
tained was 8t-nPr, which contains additional electron-rich
moieties. Notably, compound 11 has a tricyclic skeltone simi-
lar to that of cycloadduct 3a, and the former was synthe-
sized from diketone 12 by using a procedure described in
the literature.[15] Purification and identification of 11a is dif-
ficult because of its low solubility in common organic sol-
vents. Instead of compound 11a, therefore, n-butyl-substitut-
ed derivative 11j was used for comparison with 3a.
Figure 1. The molecular structure of 4t. The fluorine atom is shown in
white.
formal [4+2] cycloaddition was expected to give naphtha-
lene 4 as in B in Scheme 1. However, the isotope-labeled ex-
periments revealed that their formation mechanisms differ.
Whereas B is mainly furnished by intramolecular hydrogen
migration, naphthalene 4 is generated through the protona-
tion of external water. When alkyne 1t was heated with deu-
terium oxide under the reaction conditions described here,
[D1]4t revealed isotope purity exceeding 91% (Table 1). As
in the formation of 3, water was a key cycloisomerization re-
agent.
Dialkylacetylenes are suitable reaction partners in crossed
cycloisomerizations because they do not significantly yield
hexaalkylbenzenes under the reaction conditions described
here. Compounds 7–10 are the likely products generated
(Table 2). Cycloaddition of 4-octyne or 5-decayne with di-
phenylacetylene (1a) gave a mixture of products 7a and 9a.
Other regioisomers of 7a were observed in small quantities.
Cycloadduct 7a has been structurally analyzed by 2D-NMR
spectroscopic techniques, that is, NOE and HMQC. The
photophysical properties also confirm the assignment (see
below). However, electron-deficient alkyne 1t furnished a
complex mixture of cycloadducts, and the major product ob-
Based on the literature and the findings of the above iso-
tope-labeled experiment, a putative mechanism for the for-
mation of compounds 3 and 4 was formulated as shown in
Scheme 2. Reaction of an active PdII species with three
alkyne molecules generates 1-palldacycloheptatriene 17 via
1-palldacyclopentadiene 15 (Route A).[16] PCy3 reduces com-
plex 17 to yield PdII intermediate 18,[17] which rearranges
into a Schrock-type carbene complex 19.[18] Protonation of
the carbene carbon atom with water forms s-complex 20,[19]
which then produces complex 14 through intramolecular
electrophilic palladation[20] followed by deprotonation. The
steric repulsion in 14 or 20 is minimized when the two
phenyl groups adopt a trans configuration. Reductive elimi-
nation of 14 gives the desired product 3a and a Pd0 species.
The active PdII complex can eventually be regenerated by
oxidation with CuACTHNUTRGNEUNG
(OAc)2·H2O and benzoquinone.[21] Appa-
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Palladium-Catalyzed Cycloisomerizations of Diarylacetylenes
FULL PAPER
Scheme 2. Proposed reaction mechanism for the formation of cycloadducts 3 and 4.
rently, the crossed cycloadduct 7 should be generated from
key intermediates 27, which can be formed from 26 by a 1,2-
or 1,5-shift.[22] Complex 27 is more stable relative to 26 be-
cause of the lower steric repulsion between the Pd carbene
moiety and the alkyl group at position C5 in the former.
Use of an asymmetric alkyne for testing migration of the Pd
moiety on the cyclopentadienyl core revealed a higher than
expected number of regioisomers.[23]
Electron-deficient diarylacetylenes 1 predominately un-
dergo cyclodimerization to form naphthalenes 4, which may
support a reaction involving a PdIV intermediate. Instead of
inserting an alkyne (Route A), the PdIV species 15 bearing
numerous groups with high electron-withdrawal is more
easily reduced by PCy3 to yield a PdII species 16 (Route B).
Complex 16 undergoes protonation and ring opening to give
a Schrock-type carbene complex 24 via the intermediate 25.
Complex 24 converts into its conformer 23, which produces
1-palladabenzocycloheptatriene 21 through intramolecular
electrophilic palladation and subsequent deprotonation. Re-
ductive elimination of 21 gives naphthalene 4 and a Pd0 spe-
cies.
3 are not planar and the torsion angle between ring A and
À
C is about 338 (Table 3). The bond length of C8 C8a (d4) is
in the range of 1.582–1.589 ꢁ, which is significantly longer
À
than the usual alkyl C C bond (1.54 ꢁ). Other bond lengths
are not unusual. The computational results generally have a
good fit to the experimental data. Notably, the X-ray crystal
structure of 3p indicates a nonbonded contact distance of
2.996 ꢁ between the C8 and fluorine atom in the trifluoro-
methyl group at position C7.[14] Due to the free rotation of
the CF3 group, the shortest distance should be smaller than
this value. Comparison of the C···F distance with the sum of
the van der Wall radii for carbon and fluorine (3.17 ꢁ)[26] in-
dicated that they should show some interaction. However,
no through-space 13C–19F coupling was observed in the
13C NMR spectrum.
The aryl substituent Ar3 attached to ring A in compound
3 sometimes has an unusual alignment, which can be quali-
fied by determining the dihedral angle between the A ring
and the aryl plane (A/Ar3). Interestingly, the dihedral angle
A/Ar3 is also affected by the electronic properties of Ar3.
Comparison of the 3,5-bis(trifluoromethyl)phenyl group in
3p (6.88) with dimethylphenyl-substituted derivative 3q
(52.28) indicated that the small value of the dihedral angle
A/Ar3 results from an electron-deficient aryl moiety. The
substituents Ar1–Ar4 in both compounds 3 and 7 cause
crowding, and their steric interaction is indicated by the
nonbonded contact distance between two neighboring aryl
Structure analysis and DFT calculations: In an earlier study,
the authors provided X-ray crystallographic analysis of com-
pounds 3a, 3n, and 3q to confirm their structures, but no
further analyses were performed. Therefore, this study ana-
lyzed their structures together with 3p and compared them
with their geometrically optimized results calculated at the
B3LYP/6-31G** level by using the Gaussian 03 Program.[24]
These molecules in solids reveal no p–p stacking, which is
the usual intermolecular interaction in PAHs.[25] Compounds
groups. This explains why diACTHNGUTERNNU(G 2-tolyl)acetylene cannot form
cycloadducts. Unlike compound 3a, the 11j backbone is
nearly planar, and its three aryl substituents do not present
small dihedral angles with respect to ring A.
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Y. -T. Wu, M. -Y. Ku et al.
Table 3. Structure and conformation analysis of compounds 3, 9, and 12.
Bond length [ꢁ]
Compd. Method
Torsion angle [8]
A/C
Ref.
d1
d2
d3
d4
A/Ar1
A/Ar2
A/Ar3
1.355
(1.350)
1.362
1.477
(1.478)
1.485
(1.485)
1.474
1.488
1.370
(1.367)
1.374
(1.374)
1.370
1.375
1.582
(1.586)
1.590
(1.590)
1.584
1.590
34.0
(37.2)
31.9
(31.9)
33.5
30.6
48.7
(49.9)
50.4
(50.4)
50.0
49.7
65.3
(64.1)
56.8
(56.8)
54.8
54.5
23.9
(20.5)
30.1
(30.2)
36.4
36.3
3a[a]
experimental
[9]
B3LYP/6-
31G**
this
work
[9]
this
work
this
work
this
work
[9]
this
work
this
(1.362)
3n
3p
experimental 1.348
B3LYP/6-
31G**
1.358
experimental 1.356
1.483
1.482
1.366
1.373
1.583
1.596
32.5
32.2
46.6
51.4
67.6
60.4
6.8
B3LYP/6-
31G**
1.361
25.5
3q
experimental 1.351
1.475
1.487
1.362
1.369
1.589
1.594
31.3
26.3
57.0
50.1
44.7
51.5
52.2
58.1
B3LYP/6-
31G**
B3LYP/6-
31G**
B3LYP/6-
31G**
1.361
1.363
1.370
7a-
1.493
1.502
1.365
1.378
1.586
1.375
32.4
4.5
47.4
43.6
58.5
72.3
–
nBu[b]
11j
work
this
work
55.0
[a] Two asymmetric molecules of 3a in a unit cell. [b] In compound 7a-nBu, Ar3 =Ar4 =nBu.
To elucidate the role of aryl substituents to the backbone,
the HOMOs and LUMOs of some important compounds
were analyzed (Figure 2). Due to the small dihedral angle
between Ar3 and the A ring in compound 3a, the distribu-
tions of the HOMO and LUMO on Ar3 should be consid-
ered. Therefore, the effective p system in compound 3 is
more extended than expected. The population of the
HOMO and LUMO on the C6 is also larger than those on
C5 and C7.
Photophysical properties: The photophysical properties of
cycloadducts 3 and 6 and their analogues 7 and 11 were
studied (Table 4). At a concentration of 10 mm, most of these
Table 4. Photophysical properties of the cycloadducts.[a]
lmax,em
F (quantum yield [%])
Compd.
labs [nm]
[nm]
W0
W30
W60
W75
W90
3a
3b
3c
3d
3g
3h
3k
3n
3p
3q
3r
259, 288, 367
259, 289, 375
290, 375
368
470
470
440
463
475
490
475
461
446
475
478
495
501
400
–
0
0
0
1.3
4.9
5.9
1.8
2.7
5.9
2.3
10.5
7.0
4.8
76.3
39.6
8.5
2.0
0.0
30.2
9.6
1.2
3.9
0
1.3
4.4
0
2.3
4.8
0.1
2.6
5.6
0.0
5.0
5.6
2.9
56.6
38.1
8.5
0.0
0.0
29.9
25.0
39.3
28.3
8.7
43.2
41.0
24.6
17.2
24.3
8.7
295, 379
305, 390
258, 368
277, 370
267, 354
262, 367
279, 378
303, 390
281, 311, 409
340
0
0
3.6
0.0
3.3
1.9
1.1
26.9
19.9
49.2
0.0
0.0
4.8
0.0
4.2
3.8
1.8
28.0
25.1
50.5
0.0
0.0
3s
6
7a-nBu
11j
34.5
0.0
295, 430
[a] All samples (ca. 10À5 m) were measured in THF. Coumarin was used
as the standard for the determination of quantum yields. Wn=the water
contents (V%) in the solution.
compounds are nonluminescent or only weakly luminescent
in THF. The exceptions are compounds 3r, 3s, and 6. Sub-
stituents in the aryl side arms are less important relative to
those directly attached to the backbone since the X-ray
structures demonstrate that the periphery aryl rings are
twisted from the central aromatic core. In some cases, how-
ever, no significant twisting was noted in Ar3.[27] Although
the absorption and emission spectra of compounds 3 are
very similar, the electron-donating and -withdrawing groups
attached at position C6 in 3 are significantly red- and blue-
shifted, respectively, in both their absorption and emission
Figure 2. The optimized structures (Opt) and isosurface plots of frontier
orbitals of 3a, 3b, and 3h calculated at the B3LYP/6-311+G**//B3LYP/
6-31G** level.
7224
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Palladium-Catalyzed Cycloisomerizations of Diarylacetylenes
FULL PAPER
bands. Figure 2 shows that the population of HOMO on po-
sition C6 is larger than that of LUMO on the same atom.
The electron-donating group at position C6 causes a larger
potential increase in the HOMO than in the LUMO, and
leads to a redshift in both the absorption and emission
bands. Unlike other electron-withdrawing groups, such as
fluoro and trifluoromethyl, the ester group causes a batho-
chromic shift in 3h and displays a tendency similar to that
observed in the methoxy-substituted product 3s. The exten-
sion of the p system generated by the ester group in the
LUMO, as shown in Figure 2, decreases its potential and
redshift in both the absorption and emission bands. Dialkyl-
substituted cycloadduct 7a-nBu, which lacks the aryl group
at position C1, causing shorter wavelengths in both absorp-
tion and emission bands relative to 3a. As noted above, the
dihedral angle A/Ar3 was small in compound 3p. Therefore,
Ar3 would significantly contribute to the p-system of the
backbone that causes the redshift in both the absorption
and emission bands. However, this tendency was not ob-
served when the photophysical properties of 3a, 3p, and 3q
are compared. Perhaps the effect of electron-withdrawing
substituents located at positions C5 and C7 was larger than
that of the coplanarity of A/Ar3. The absorption spectra of
compound 11j are very similar to 3a, but a solution of the
former shifts bathochromically. Like dibenzopentalenes
13,[28] compound 11j contains 4n p electrons, and it is practi-
cally nonfluorescent in either liquid or solid form. Accord-
ing to the computational results based on time-dependent
DFT (TD-DFT), its HOMO-to-LUMO transition is symme-
try forbidden (see the Supporting Information).
The aggregation-induced emission (AIE) of 3a was ob-
served in our previous study.[9,29] The solution containing 3a
in dichloromethane or THF is practically nonfluorescent,
but it becomes highly luminescent in crystal (or solid) form.
To understand this abnormal AIE, the quantum yield (FF)
of compounds 3, 6, and 7a-nBu (ca. 10À5 m) was examined in
a mixture of THF/water (Table 4). In compounds 3a–d, 3g,
3h, 3k, 3n, 3p, and 3q, fluorescence is weak to absent when
the water content is below 60% (W60). However, they
become fluorescent at W75 or higher because aggregations
of these molecules can efficiently inhibit the free vibration
and rotation of aryl groups, thus increasing quantum yield.
The photoluminescent behaviors of these compounds clearly
reveal their AIE properties. Dialkyl-substituted 7a-nBu has
a less crowded environment. Its AIE properties must be en-
hanced under more aggregated conditions. In contrast to the
aforementioned compounds, the THF solutions of 3r and 3s
are photoluminescent, and the bulky substituents of these
two molecules partly block the nonradiative pathway, even
under nonaggregated conditions. Their emission behaviors
are classified as aggregation-enhanced emission (AEE).[29]
However, as water is added to the solutions (exceeding
W75), the FF values dramatically decrease, possibly due to
the precipitation of molecules. Notably, the THF solution of
compound 6 is a bright fluorescent green (FF =49.2%),
even under ambient lighting because such bulky side arms
completely block the nonradiative pathway. Therefore, com-
pound 6 resembles a normal luminescent molecule, and its
FF value is inversely related to water content. Moreover,
the aggregation properties of compounds 3g and 3r in the
solution were analyzed by dynamic light scattering (DLS).
As expected, no significant particles are observed in their
W0 solutions, whereas 3g (W90) and 3r (W75) aggregate in
the aqueous solutions with the particle size in the 170–420
and 120–150 nm range, respectively.
Electrochemistry: Table 5 shows the redox properties of cy-
cloadducts and their analogues as characterized by cyclic
voltammetry. The first oxidation potential depends on sub-
stituent type and position and also on the extension of the
p-system in the backbone. An electron-donating substituent,
that is, the methyl and methoxy groups at the C6-position in
compounds 3 reduces the oxidation potential, whereas an
electron-withdrawing group, such as fluoro, trifluoromethyl,
and methyl ester, increases the value. However, an electron-
donating substituent at C5- and C7-positions would only
slightly reduce the oxidation potential as revealed by com-
paring 3b (0.49 V) vs. 3k (0.58 V) and 3g (0.27 V) vs. 3r
(0.69 V). These results are consistent with the computational
Table 5. Cyclic voltammetric analyses for cycloadducts.[a]
Compd.
Eox (V vs. Fe/Fe+)
Ered (V vs Fe/Fe+)
3a
3b
3c
3d
3g
3h
3k
3n
3r
3s
6
0.70
0.49, 0.87 (Ep), 0.95 (Ep)
1.12 (Ep)
0.79 (Ep), 1.04 (Ep), 1.19 (Ep)
0.27, 0.57 (Ep)
1.02 (Ep)
À2.05 (Ep)
À2.48 (Ep)
À2.00 (Ep)
0.58
0.69, 1.03 (Ep), 1.17 (Ep)
0.69 (Ep)
0.54 (Ep), 0.99 (Ep)
0.41
À0.75
À1.64
11j
0.46, 0.98 (ir)
[a]Experimental conditions: compounds (concentration ca. 10À3 m) in a
background electrolyte solution of 0.1m TBAPF6 (tetrabutylammonium
hexafluorophophate) in CH2Cl2. Working electrode: glass carbon (GC)
electrode, reference electrode: Ag/AgNO3 (0.1m AgNO3 and 0.1m N-
AHCTNUTGREGN(NNU nBu)4ClO4 in acetonitrile), counter electrode: platinum wire. Scan rate:
50 mVsÀ1. Internal standard: ferrocene.
Figure 3. The redox potentials determined by CV (open) and HOMO
and LUMO levels generated from the DFT computation (solid).
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Y. -T. Wu, M. -Y. Ku et al.
predictions that, compared to a substituent at positions C5
and C7, a substituent at position C6 would have a larger
impact on redox potential. Compound 11j with a fully p-
conjugated core has lower oxidation and reduction poten-
tials relative to 3a. Qualitatively, the computational predica-
tions of the HOMO and/or LUMO of compounds 3 have a
good fit to the results obtained by CV (Figure 3).
À
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Conclusion
The palladium-catalyzed cycloisomerization of alkynes gen-
erated highly substituted 8,8a-dihydrocyclopenta[a]indenes 3
or naphthalenes 4 as the major products. The former class of
compounds displayed interesting photophysical and redox
properties. Further studies of the applications of these mole-
cules in organic materials are currently underway.
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Experimental Section
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Instruments: 1H and 13C NMR spectra: Bruker 300 (300 and 75.5 MHz)
and 400 (400 and 100 MHz). MS: Bruker Daltonics Apex II30. X-ray
crystal structure determination: The data were collected on a Stoe–Sie-
mens-AED diffractometer. Melting points were determined with a Bꢂchi
melting point apparatus B545 and are uncorrected. UV spectra were re-
corded by HP 8453. PL experiments were accomplished by Jasco FP-6300
and Cary Eclipse fluorescence spectrophotometer. Coumarin (F=
0.73),[30] was used as the standard for the determination of quantum
yields. The electrochemical experiments were accomplished by Princeton
Applied Research Corporation (PAR) model 263A potentiostat con-
trolled by PAR model 270 software. A three-electrode cell was used for
the electrochemical experiments. DLS properties were measured by Ze-
tasizer nano ZS (Malvern Instruments). Computation: The geometry op-
timizations and frequencies of selected compounds were calculated at the
B3LYP functional[31] and a 6-31G (d,p) basis set with Gaussian 03 Pro-
gram.[24] The HOMO and LUMO levels and orbital surfaces were ob-
tained by using B3LYP/6-311+G
ACHTUNGTRENNUNG
tures optimized at B3LYP/6-31GAHCNUTGTRENNUNG
LUMO orbital surfaces (isodensity value=0.025) were generated by
using the GaussView 4.1.2 program.[32]
General procedure for palladium-catalyzed cycloisomerization of diary-
lalkynes: A mixture of alkyne 1 (200 mg, 1.12 mmol), PCy3 (157 mg,
0.56 mmol), Cu
ACHTUNGTRENNUNG
(61.0 mg, 0.56 mmol), PdAHCTUNTGRENNUNG
[7] In the presence of Cu reagents under different conditions, reaction
of zirconacyclopentadienes with alkynes selectively gives cycload-
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um and Zirconium in Organic Synthesis (Ed.: I Marek) Wiley-VCH,
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1.12 mmol), H2O (1 mL), and acetonitrile (3.5 mL) in a thick-walled
Pyrex tube (1ꢃ10 cm) was purged with nitrogen for 5 min. The sealed
tube was kept in an oil bath at 1108C for 36 h. After cooling to room
temperature and filtration over Celite, the solvent of the filtrate was re-
moved under reduced pressure. The residue was subjected to chromatog-
raphy on silica gel to give cycloadducts 3 and/or 4. Crystals for the X-ray
diffraction analysis were grown from CH2Cl2/MeOH.
[8] a) W. Reppe, O. Schlichting, K. Klager, T. Toepel, Justus Liebigs
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[10] Compound 3k contains very small amounts of another regioisomer
(<5%), but the byproduct cannot be observed in the 13C NMR
spectrum.
Acknowledgements
This work was supported by the National Science Council of Taiwan
(NSC 98-2113-M-006-002-MY3 and NSC-96-2113-M-260-006-MY2). We
also thank Prof. S.-L. Wang and P.-L. Chen (National Tsing-Hua Univer-
sity, Taiwan) for the X-ray structure analyses, and the National Center
for High-performance Computing for providing computational resource.
Prof. D.-H. Chen and B.-H. Lai (National Cheng Kung University) are
also acknowledged for the DLS analysis.
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Palladium-Catalyzed Cycloisomerizations of Diarylacetylenes
FULL PAPER
13
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[14] Data for the X-ray structure analysis: 3p: crystals from CH2Cl2/
MeOH, C54H18F36; monoclinic crystals of space group P21/n1; unit
cell dimensions: a=13.5723(5), b=25.0822(8), c=15.7767(4) ꢁ; a=
g=90, b=104.9710(10)8; V=5188.5(3) ꢁ3; Z=4. 4t: crystals from
CH2Cl2/MeOH, C28H8F12; monoclinic crystals of space group P21/c;
unit cell dimensions: a=11.3562(2), b=11.7305(3), c=18.1443(4) ꢁ;
a=g=90, b=107.1070(10)8; V=2310.13(9) ꢁ3; Z=4. CCDC806046
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Received: February 9, 2011
Published online: May 12, 2011
Chem. Eur. J. 2011, 17, 7220 – 7227
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7227