PdCl2-Catalyzed domino reactions of 2-Alkynylbenzaldehydes with Indoles: Synthesis of Fluorescent 5H-Benzo[b]carbazol-6-yl Ketones

Article abstract of DOI:10.1002/chem.201000133

(Chemical Equation Reprentation) Synthetic methods: A new, selective Pd-catalyzed domino reaction of 2- alkynylbenzaldehydes with indoles has been developed for the synthesis of 5H-benzo[b]carbazol-6-yl ketones. The isobenzopyrylium complex is trapped by indole at room temperature to give 3-(1H-isochromen-1-yl)-y1)-indoles, which can be transformed into 5Hbenzo[b]carbazol-6-yl ketones by raising the temperature (see scheme). These ketones exhibit intense fluorescence and interact with metal ions to enhance the fluorescence intensity.

Full text of DOI:10.1002/chem.201000133

COMMUNICATION
DOI: 10.1002/chem.201000133
PdCl2-Catalyzed Domino Reactions of 2-Alkynylbenzaldehydes with Indoles:
Synthesis of Fluorescent 5H-Benzo[b]carbazol-6-yl Ketones
Ri-Yuan Tang[a] and Jin-Heng Li*[a, b]
The selective synthesis of polycyclic aromatic compounds
is of continuing interest in the field of organic chemistry, be-
cause these compounds play important roles in both the
chemical and pharmaceutical industries. For example, some
polycyclic heteroaromatic compounds have been applied as
p-conjugated functional materials, such as organic semicon-
ductors and luminescent materials, due to their special elec-
trochemical and photochemical properties.[1–3] Recently, the
Lewis acid catalyzed [4+2] cycloaddition of 2-ethynylben-
zaldehyde derivatives to unsaturated compounds (alkynes,
alkenes, and enols) has become one of the powerful meth-
tion of indenes 4 from 5 was also proposed, involving the
Nazarov reaction, no direct experimental evidence support-
ed it.[7] Therefore, expanding the scope of the cycloaddition
reaction with respect to indole nucleophiles would represent
an important advance. Here we report the first cycloaddition
of 2-alkynylbenzaldehydes to indoles to form 5H-benzo[b]-
carbazol-6-yl ketones 6, catalyzed by PdCl2 (Scheme 1). We
found that applying indoles 2 as a reaction partner at room
temperature led to products 5,[9] which could then be trans-
formed into the corresponding 5H-benzo[b]carbazol-6-yl ke-
tones 6 by elevating the reaction temperature, under the
same catalyst system.
ods
for
constructing
polycyclic
aromatic
cores
(Scheme 1).[4–8] In 2002, Asao and Yamamoto reported a
novel method for synthesizing naphthyl ketones the by
AuCl3-catalyzed [4+2] benzannulation of 2-alkynylbenzal-
dehydes with alkynes.[4] Dyker and co-workers subsequently
used the method to construct polycyclic heteroaromatic
compounds. In all of these studies, the initial formation of
the isobenzopyrylium complex A was invoked as a key inter-
mediate that led to the observed polycyclic aromatic com-
pounds upon [4+2] cycloaddition with the corresponding
unsaturated partners.[7,9] The isobenzopyrylium complex A
was trapped by Dyker and co-workers by using benzimida-
zole as the nucleophile.[7] In spite of this, no isobenzopyrili-
um products 5 or cycloaddition products 6 have been trap-
ped, but indenes 4 were isolated in the presence of indole or
N-methylindole. Although a possible process for the forma-
We began our investigation by examining the effect of the
conditions on the reaction of 2-(phenylethynyl)benzalde-
hyde (1A) with indole (2a) and the results are summarized
in Table 1. We found that the reaction temperature plays a
crucial role in the selectivity (Table 1, entries 1–6). Whereas
treatment of substrate 1A with indole (2a) in the presence
of PdCl2 at room temperature afforded 3-(3-phenyl-1H-iso-
chromen-1-yl)-1H-indole (5Aa) exclusively, in 85% yield
(Table 1, entry 1), another product, 5H-benzo[b]carbazol-6-
ylACHTNUTRGNEUNG(phenyl)methanone (6Aa), was obtained at increased tem-
peratures (Table 1, entries 2–6). To our delight, only product
6Aa was isolated if the temperature was over 1208C. En-
couraged by these results, the effect of different solvents
was subsequently examined (Table 1, entries 7–12). The re-
sults showed that a mixture of N-methylpyrrolidone (NMP)
and DMSO (v/v=1:4) gave the best results (Table 1,
entry 12). We were pleased to find that the yield of 6Aa was
enhanced to some extent when 15 mol% of PdCl2 was used
(Table 1, entry 13), but the yield of 5Aa was only slightly af-
fected under the same conditions (Table 1, entry 14). Finally,
[a] R.-Y. Tang,+ Prof. Dr. J.-H. Li
College of Chemistry and Materials Science, Wenzhou University
Wenzhou Zhejiang, 325035 (China)
Fax : (+86)577-86689372
a series of other catalysts, including Pd
ACHTUNGTRENN(NUG OAc)2, PtCl2, Cu-
E-mail: jhli@hunnu.edu.cn
AHCTUNGTRENNUNG
[b] Prof. Dr. J.-H. Li
Table 1, entries 15–19). Although these catalysts have activi-
ty in the reaction, they were all inferior to PdCl2.
Key Laboratory Chemical Biology &
Traditional Chinese Medicine Research (Ministry of Education)
With the standard conditions in hand, we turned our at-
tention to the scope and limitations of the transformations
for selectively synthesizing indoles 5 and ketones 6 (Table 2
and Table 3). As shown in Table 2, when the reaction was
Hunan Normal University, Changsha, Hunan, 410081 (China)
[+] Crystallographic investigation
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000133.
Chem. Eur. J. 2010, 16, 4733 – 4738
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4733

Scheme 1. Lewis acid catalyzed [4+2] cycloaddition reactions; a) [MLn]; b) AuCl3 or CuCAHTUNGTRENNUNG
paper and may occur by a Diels–Alder, or other, domino reaction process; e) AuCl3, 2;[7] f) AuCl3, 1H-benzimidazole;[7] g) Cu
ACHTUNGTRENNUNG
i) AuCl3;[7] j) the Nazarov process; k) 2; l) PdCl2, reaction discussed in this work.
performed with 1A, a set of indoles 2b–e underwent the cy-
cloaddition at room temperature to afford the correspond-
ing indoles 5Ab–Ae with moderate to good yields, regard-
less of their steric or electronic nature (Table 2, entries 1–4).
Both electron-withdrawing and electron-donating groups on
the aryl moiety of the terminal alkyne were well tolerated,
though the electron-donating groups (methyl and methoxy)
displayed higher activity than the electron-withdrawing
group (acetyl) (entries 5–7). The standard conditions were
also compatible with 2-{2-(pyridin-3-yl)ethynyl}benzalde-
Table 1. Screening for optimal conditions.[a]
Entry Catalyst
Solvent [v/v]
T [oC] t [h]
Yield [%]
5Aa 6Aa
hyde (1E),
a heteroaryl-containing substrate (Table 2,
1
2
3
4
5
6
7
8
9
10
11
12
13[b]
14[b]
15[b]
16[b]
17[b]
18[b]
19[b]
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
CH3CN
DMF
toluene
NMP
NMP/DMSO (1:1) 120
NMP/DMSO (1:4) 120
NMP/DMSO (1:4) 120
RT
50
80
100
120
140
120
120
120
120
24
24
24
24
10
10
16
16
16
10
10
10
10
10
10
10
10
10
10
85
76
15
<5
0
0
0
0
0
0
0
0
0
86
0
0
0
0
0
<5
42
51
65
46
38
19
12
32
55
68
74
0
entry 8). However, it was found that alkyl-substituted
alkyne 1F resulted in a decreased yield (Table 2, entry 9).
Substrates 1G and 1H, containing substituents on the arylal-
dehyde moiety, were also suitable substrates for the reaction
with indole 2a and PdCl2, at room temperature in 63 and
86% yields, respectively (Table 2, entries 10 and 11).
We next evaluated the one-pot reaction between alde-
hydes 1 and indoles 2 at 1208C to form compounds 6
(Table 3). It appears from these results that the yields of this
reaction are dependent on the substitution pattern of both
substrates. For substrate 1A the reactions with various in-
doles 2b–f proceeded smoothly in moderate yields (Table 3,
entries 1–5). Moderate yields were still achieved from sub-
strates 1B and 1C, which have electron-donating aryl
groups on the terminal alkyne (Table 3, entries 6 and 7).
However, an electron-deficient aryl group or a heteroaryl
group somewhat reduced the yield (Table 3, entries 8 and 9).
NMP/DMSO (1:4)
RT
Pd
PtCl2
Cu(OTf)2
N
25
45
56
17
<5
NMP/DMSO (1:4) 120
NMP/DMSO (1:4) 120
NMP/DMSO (1:4) 120
NMP/DMSO (1:4) 120
G
AgOTf
ZnCl2
0
[a] Reaction conditions: 1A (0.22 mmol), 2a (0.2 mmol), catalyst
(10 mol%), and solvent (2.5 mL). [b] Catalyst (15 mol%).
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5H-Benzo[b]carbazol-6-yl Ketones
COMMUNICATION
Table 2. Palladium-catalyzed synthesis of 3-(1H-isochromen-1-yl)-1H-in-
doles 5.[a]
Table 3. Palladium-catalyzed synthesis of 5H-benzo[b]carbazol-6-yl ke-
tones (6).[a]
Entry
Substrate 1
Indole 2
Yield [%]
Entry Substrate 1
Indole 2
T
[h]
Yield
[%]
1
2
3
4
1A
1A
1A
1A
2b
2c
2d
2e
73 6Ab
53 6Ac
42 6Ad
53 6Ae
1
24
24
36
24
87 5Ab
81 5Ac
43 5Ad
88 5Ae
5
1A
63 6Af
2
3
4
1A
1A
1A
6
7
8
1B
1C
1D
1E
1F
2a
2a
2a
2a
2a
2a
2a
60 6Ba
64 6Ca
44 6Da
39 6Ea
56 6Fa
56 6Ga
67 6Ha
9
10
11
12
1G
1H
13
14
2a
57 6Ia
1I
2e
53 6Ie
5
6
30
24
85 5Ba
72 5Ca
[a] Reaction conditions: 1 (0.22 mmol), 2 (0.2 mmol), PdCl2 (15 mol%),
and NMP/DMSO (v/v=1:4; 2.5 mL) at 1208C for 10 h.
2a
2a
2a
We also investigated the effect of substituents on the aryl-
AHCTUNGTREGaNNNU ldehyde moiety and these substituents were tolerated by
the reaction (Table 3, entries 11–14). Electron-rich arylalde-
hyde 1I, for instance, was treated with indoles 2a and 2e
successfully in the presence of PdCl2 at 1208C, affording the
target products 6Ia and 6Ie in 57 and 53% yields, respec-
tively (Table 3, entries 13 and 14).
The structures of the products 5 and 6 were unambiguous-
ly confirmed by the X-ray single-crystal diffraction analysis
of 5Ae[10] and 6Aa.[11]
7
8
30
36
59 5Da
51 5Ea
Having trapped product 5, we performed two further con-
trolled experiments as summarized in Scheme 2. The trap-
ped product 5Aa, generated from the reaction of 2-(phenyl-
ethynyl)benzaldehyde (1A) with indole (2a) at room tem-
perature, then smoothly underwent the domino rearrange-
ment reaction in the presence of PdCl2 at 1208C and was
transformed into the cycloaddition product 6Aa in 78%
yield. However, reaction of compound 5Aa under Dykerꢁs
conditions produced no product 4, but did form the cycload-
dition product 6Aa, the ring-opened product 7Aa, and two
unidentified products (8Aa and 9Aa); these results do not
support Dykerꢁs proposed process for the formation of in-
denes 4.[7]
9
10
11
2a
2a
2a
36
36
36
50 5Fa
63 5Ga
86 5Ha
[a] Reaction conditions: 1 (0.22 mmol), 2 (0.2 mmol), PdCl2 (10 mol%),
and NMP/DMSO (v/v=1:4; 2.5 mL) at room temperature.
Possible mechanisms for the present reactions were pro-
posed as outlined in Scheme 3, which were based on the re-
ported mechanism[4–8] and our results. Complexation of the
triple bond with PdCl2 takes place readily to afford inter-
mediate D, followed by nucleophilic attack of the carbonyl
We were delighted to find that the standard conditions were
reliable with alkyl-substituted alkyne 1F (Table 3, entry 10).
Chem. Eur. J. 2010, 16, 4733 – 4738
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4735

J.-H. Li and R.-Y. Tang
As is known, polycyclic aro-
matic compounds have been
widely used in the field of or-
ganic materials chemistry due
to their interesting electro-
chemical and fluorescent prop-
erties.[1–3] Moreover, metal-com-
plexed conjugated systems are
attracting fervent interest due
to the possibility of interaction
with DNA and their role in flu-
orescent labeling.[13] In connec-
tion with the highly conjugated
molecules obtained by our
domino reaction, it was as-
sumed that these products, with
two heteroatoms, could be che-
lated with a metal ion, leading
to new physical properties.
Thus, preliminary experiments
using compound 6Aa as the or-
ganic molecular probe have
been conducted. As illustrated
in Figure 1, we introduced
2.5 equivalents of M+ (includ-
Scheme 2. Controlled experiments.
ing Co2+
,
Zn2+
,
Ni2+
,
Cr3+
,
,
Cu2+, Fe3+, Pb2+, Mn2+, Cd2+
and Ag+) to a solution of 6Aa
in CH3CN/H2O to test the
change in the fluorescence
emission intensity.[14] These ex-
periments demonstrated that
compound 6Aa could combine
with Cr3+, Fe3+, or Cu2+ to en-
hance their fluorescence emis-
sion and shift the emission
wavelength considerably at
room temperature. The en-
hancement of fluorescence may
be attributed to ligand chela-
tion to the metal center, which
involves energy transfer from
Scheme 3. Possible mechanisms.
the conjugated ligand to the
metal center. Particularly, the
oxygen atom on the alkyne (intermediate D) to form the Pd
complex E.[4–8] At room temperature, complex E then under-
goes a Friedel–Crafts reaction to give product 5.[7,9] A ring-
opening reaction of product 5 yields intermediate B in the
presence of PdCl2 at 1208C. Intermediate B may then under-
go two pathways (1 or 2) to provide the cycloaddition prod-
uct 6 and regenerate the active Pd species: pathway 1) a
[4+2] Diels–Alder reaction or pathway 2) a thermal elec-
trocyclization.[12] Under Dykerꢁs conditions,[7] there is com-
petition between these cyclization reaction pathways and a
second Friedel–Crafts reaction, which is caused by the stron-
ger Lewis acidity of AuCl3. Studies into the detailed mecha-
nism are in progress.
combination of 6Aa with Cr3+ resulted in an almost 500%
increase in the fluorescent intensity and the peak emission
wavelength was shifted from 500 to 575 nm. Moreover, the
Fe3+ complex may be promising in future DNA research
due to its fluorescent properties and benign effect on human
health.
In summary, we have demonstrated that, by application of
a palladium-catalyzed domino strategy, a variety of function-
al 5H-benzo[b]carbazol-6-yl ketones can be readily con-
structed from a reaction between 2-alkynylbenzaldehydes
and indoles. This work is the first to disclose that the isoben-
zopyrylium complex A can, in the presence of PdCl2, be
trapped by indole nucleophiles at room temperature to
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5H-Benzo[b]carbazol-6-yl Ketones
COMMUNICATION
Acknowledgements
We thank the NSFC (No. 20872112), NCET (No. NCET-06-0711), Zhe-
jiang Provincial Natural Science Foundation of China (Nos. Y407116 and
Y4080169) and Wenzhou University (No. 2007L004) for financial sup-
port.
Keywords: alkynes · domino reactions · indoles · ketones ·
palladium
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,
À
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preliminary fluorescence emission experiments on product
6Aa showed that this type of compound might exhibit in-
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Cr3+, Fe3+, and Cu2+, to enhance the fluorescence emission
intensity, which provides a new route for the design and dis-
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measured reflections, 231 parameters, m=0.083 mmÀ1, R1=0.0417
for 2097 observed reflections [I>2s(I)], wR2=0.1071 for all reflec-
Experimental Section
Typical experimental procedure for the synthesis of 3-(1H-isochromen-1-
yl)-1H-indoles 5: A mixture of 2-alkynylbenzaldehyde 1 (0.22 mmol),
indole 2 (0.2 mmol), and PdCl2 (10 mol%) in NMP/DMSO (v/v=1:4;
2.5 mL) was stirred at room temperature, for the indicated time, or until
complete consumption of the starting material had occurred, as moni-
tored by TLC and GC-MS analysis. After the reaction was finished, the
mixture was filtered through a crude column with ethyl acetate as the
eluent, followed by evaporation of the solvent in a vacuum. The residue
was purified by flash column chromatography (hexane/ethyl acetate 4:1)
to afford the desired product 5.
Typical experimental procedure for the synthesis of 5H-benzo[b]carba-
zol-6-yl ketones 6: A mixture of 2-alkynylbenzaldehyde 1 (0.22 mmol),
indole 2 (0.2 mmol), and PdCl2 (15 mol%) in NMP/DMSO (v/v=1:4;
2.5 mL) was stirred at 1208C, for the indicated time, or until complete
consumption of the starting material had occurred, as monitored by TLC
and GC-MS analysis. After the reaction was finished, the mixture was fil-
tered through a crude column with ethyl acetate as eluent, followed by
evaporation of the solvent in a vacuum. The residue was purified by flash
column chromatography (hexane/ethyl acetate 5:1) to afford the desired
product 6.
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reflections (SHELXL-97)[14] for all non-hydrogen atoms with aniso-
tropic thermal parameters.
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4737

J.-H. Li and R.-Y. Tang
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Received: January 19, 2010
Published online: March 26, 2010
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Chem. Eur. J. 2010, 16, 4733 – 4738