Palladium-catalyzed ring-expansion reaction of indoles with alkynes: from indoles to tetrahydroquinoline derivatives under mild reaction conditions

Article abstract of DOI:10.1002/anie.201001237

(Figure Presented) A cut and shut job: The highly selective title reaction proceeds, using O₂ as the oxidant, to afford tetrahydroquinoline derivatives. This chemistry offers a new approach to polysubstituted 4, 5-dihydrocyclopenta[c]quinolines, and also valuable mechanistic insight into this ring-expansion reaction.

Full text of DOI:10.1002/anie.201001237

Communications
DOI: 10.1002/anie.201001237
Ring-Expansion Reactions
Palladium-Catalyzed Ring-Expansion Reaction of
Indoles with Alkynes: From Indoles to
Tetrahydroquinoline Derivatives Under Mild Reaction
Conditions**
Zhuangzhi Shi, Bo Zhang, Yuxin Cui, and Ning Jiao*
In memory of Xian Huang
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Table 1: Palladium-catalyzed ring-expansion reactions of 1a with 2a.[a]
Polyarylated aromatic and heteroaromatic compounds with
condensed aromatic cores have attracted considerable atten-
tion in the electrochemical, photochemical, and functional
materials fields because of their stability, their enhanced
ability to transport charge, and their fluorescent properties in
the solid state that are brought about by the aryl groups.[1]
Alkynes are important building blocks and synthons for the
construction of these multiply arylated compounds.[2]
Entry Pd(OAc)2 [eqiuv]
Solvent
T [8C] t [h] Yield [%][b]
3aa 4aa
À
Recently, the cyclization reactions of alkynes through C H
activation[3] has presented an attractive and powerful strategy
for the generation of aromatic and heteroaromatic com-
pounds, such as naphthalenes, indoles, isoquinolines, carba-
zoles, benzothiazoles, and pyridines.[4] Herein, we describe an
unprecedented palladium-catalyzed ring-expansion reaction
1
0.1
DMA/PivOH 100
(4:1)
12
0
41
2
3
4
5
0.1
0.1
0.5
0.1
AcOH
AcOH
AcOH
100
RT
RT
RT
12
36
36
36
0
18
31 trace
59 trace
DCE/AcOH
(1:1)
CH3CN/AcOH RT
(1:1)
CH3CN/AcOH RT
(1:1)
CH3CN/AcOH RT
(1:1)
62
78
69
0
0
0
0
0
0
À
of indoles with alkynes through dual C H bond activations,
using O2 as the oxidant, which leads to tetrahydroquinoline
derivatives with highly substituted cyclopentadienyl cores. To
the best of our knowledge, only a few examples of ring-
expansion reactions of indoles have been performed so far.[5]
This chemistry not only offers a new approach to polysubsti-
tuted 4,5-dihydrocyclopenta[c]quinolines, but also valuable
mechanistic insights into this novel ring-expansion reaction.
Recently, we developed the direct dehydrogenative anu-
lation (DDA) reaction of simple anilines or biaryl compounds
with internal alkynes to generate indoles and carbazole
derivatives, respectively,[6] using O2 as the oxidant.[7,8] Our
6
0.1
0.05
none
0.1
36
36
48
48
7
8
9[c]
CH3CN/AcOH RT
(1:1)
59
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.5 mmol), Pd(OAc)2, in
solvent (1.0 mL), under an O2 atmosphere (1 atm). [b] Yield of isolated
product. The diastereoselectivity of this transformation is very high.
[c] The reaction was carried out in air. DMA=N,N-dimethylacetamide,
DCE=1,2-dichloroethane.
À
continued efforts in metal-catalyzed C H bond activation and
cycloaromatization reactions prompted us to explore the
reaction of indoles and alkynes. When N-methylindole (1a)
was treated with diphenylethyne (2a) in the presence of
Pd(OAc)2 in acidic conditions at 1008C under O2 (1 atm),
only 9-methyl-1,2,3,4-tetraphenyl-9H-carbazole (4aa) was
obtained (Table 1, entries 1–2).[9] Interestingly, when the
reaction was carried out at 208C, polysubstituted 4,5-dihy-
droquinoline 3aa was selectively produced in 31% yield
(Table 1, entry 3). Subsequently, various parameters were
screened in the presence of AcOH (Table 1, entries 4–9).
When a mixture of acetonitrile and acetic acid (v/v = 1:1) was
used as solvent, the reaction gave the highest efficiency with
high diastereoselectivity (78% yield; Table 1, entry 6). Use of
a lower loading of Pd(OAc)2 gave 3aa in an acceptable yield
(Table 1, entry 7). Control reactions confirmed that the
reaction did not proceed in the absence of a palladium
catalyst (Table 1, entry 8). Notably, a 59% yield was achieved
even when air was used as an oxidant instead of O2 (Table 1,
entry 9).
Under these optimized reaction conditions, groups such as
benzyl and hydroxyethyl, on the N were tolerated to give the
desired products (3ba and 3ca, 65% and 63%, respectively;
Table 2). 1,5-dimethyl-1H-indole gave the corresponding
polysubstituted tetrahydro-quinoline in 63% yield (3da,
Table 2). Notably, free (NH) indole (1e) was successfully
transformed into the desired products in good yield (3ea,
74%, Table 2). In particular, the halogen-containing motifs
(3 fa and 3ga; Table 2) work well in this transformation.
Moreover, nitrile and nitro groups can also be incorporated
into the benzene ring of the indole units for this reaction. As
expected, indole derivative 2,3-dihydro-1H-pyrrolo[3,2,1-
ij]quinoline 1j can also be converted into the interesting 3ja
in 70% yield (Table 2).
The scope of the internal alkynes was then investigated
with N-methylindole (1a). These results indicated that both
aryl- and alkyl-substituted internal alkynes were tolerant of
this ring-expansion process. 1-Phenyl-1-hexyne (2b) reacted
with 1a to afford 2,3a-dibutyl-5-methyl-4-(1-methyl-1H-
indol-3-yl)-1,3-diphenyl-4,5-dihydro-3aH-cyclopenta[c]qui-
noline (3ab), along with a small amount of an unidentified
isomer[10] in 57% yield and 64:36 regioselectivity (Table 3,
entry 2). Similarly, the reaction of 1e with 2b afforded 3eb
and an isomer[10] (70:30) in a moderate combined yield
(Table 3, entry 3). Even dialkyl-substituted dec-5-yne 2c,
were also converted into the desired product 3ac with a
[*] Z. Shi, B. Zhang, Dr. Y. Cui, Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
Xue Yuan Rd. 38, Beijing 100191 (China)
Fax: (+86)10-8280-5297
E-mail: jiaoning@bjmu.edu.cn
Dr. N. Jiao
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences
Shanghai 200032 (China)
[**] Financial support from Peking University, the National Science
Foundation of China (Nos. 20702002 and 20872003), and the
National Basic Research Program of China (973 Program; Grant
No. 2009CB825300) is greatly appreciated. We thank Riyuan Lin in
this group for reproducing the results of 3ba and 3ga in Table 2.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001237.
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Table 2: Ring-expansion reactions of indoles 1 with alkynes 2a.[a,b]
Table 3: Ring-expansion reactions of indoles 1 with alkynes 2a–2c.[a]
Entry
R1
R, R’
Product
Yield [%][b]
1
2
3
4
Me (1a)
Me (1a)
H (1e)
Ph, Ph (2a)
Ph, nBu (2b)
Ph, nBu (2b)
nBu, nBu (2c)
3aa
3ab
3eb
3ac
78
57 (64:36)[c]
33 (70:30)[c]
35
Me (1a)
[a] Standard reaction conditions: 1 (0.20 mmol), 2 (0.50 mmol), Pd-
(OAc)2 (0.02 mmol), CH3CN/AcOH (1:1; 1.0 mL), RT, O2 (1 atm), 36 h;
[b] Yield of isolated product. [c] The ratio of the regioisomers was
1
determined by H NMR spectroscopy.
It was determined that product 3aa was not generated
from a rearrangement of 1a and 4aa. When 1a and 4aa were
heated in our catalytic system, both starting materials were
recovered [Eq. (1)]. Moreover, when a control experiment
[a] Standard reaction conditions: 1 (0.20 mmol), 2 (0.50 mmol), Pd-
(OAc)2 (0.02 mmol), CH3CN/AcOH (1:1; 1.0 mL), RT, O2 (1 atm), 36 h;
[b] Yield of isolated product. The diastereoselectivity of this trans-
formation is very high.
was carried out with [D4]AcOH/CH3CN, 3aa was obtained in
70% yield, but no deuterium-labeled product was observed at
the C2 position [Eq. (2)]. When 1,3-dideuteroindole [D]-1e
was employed in the reaction under the standard conditions,
slight lower yield (Table 3, entry 4). Furthermore, the struc-
ture of 3aa and 3eb were confirmed by single-crystal X-ray
analysis (see the Supporting Information, Figure S1 and S2).
The possibility of cross-reactions between two different
indoles was also investigated. The reaction of 1a, 1k, and 2a
in a 1:1:5 molar ratio afforded three products (3aa:5:3ka =
4:5:1) in 78% overall yield with the indole product 5 as the
major product (Scheme 1). However, the reaction of 1a and
2a in the presence of other electron-rich (hetero)arenes, such
as 1,2-dimethyl-1H-indole, furan, 1-methyl-1H-pyrrole, ben-
zo[b]thiophene, and para-xylene in our catalytic system,
produced 3aa as the sole product.
Scheme 1. The reaction of 1a and 1k with 2a.
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3ea was exclusively obtained in 40% yield, but again, no
deuterated product was observed [Eq. (3)]. Further study
showed that the reaction of 2-deutero-1-methylindole (94%
sequent insertion of two alkyne molecules forms a dienylpal-
ladium intermediate C.[13] Then, the intramolecular 5-exo-dig
insertion[14] of an indole leads to spiro palladium intermediate
D, with subsequent trapping by another indole ring to
generate intermediate E (route A). Next, intermediate E
undergoes reductive elimination to give intermediate F and
the Pd0 that is formed is reoxidized by O2 to PdII to complete
the catalytic cycle. Finally, the spiro intermediate F may
highly selectively rearrange into the more stable ring-fused
product 3aa, having a cis relationship between the phenyl
group and D owing to the steric hindrance. Alternatively,
intermediate D may highly selectively rearrange into the
more stable ring-fused intermediate G (route B). Then a
successive second intermolecular electrophilic aromatic pal-
ladation of another indole molecule would generate inter-
mediate H, which undergoes reductive elimination to give the
product 3aa.
D) and 2a in our catalytic system gave [D2]-3aa in 83% yield
with 94% isotopic purity at the C2 position [Eq. (4)]. These
results indicate that all of the hydrogen atoms at the C2
position in 3aa come from the C2 position of 1a.[11]
Some of the synthesized polysubstituted 4,5-dihydrocy-
clopenta[c]quinolines showed solid state fluorescence in the
range of 500–560 nm (see the Supporting Information).
Remarkably, 3aa shows a strong emission compared with a
typical emitter, such as tris(8-hydroxyquinolino)aluminum
(Alq3), which is a well-known green fluorescence emitter in
the solid state (Figure 1, A versus B). In contrast, the free
(NH) substrate 3ea was found to exhibit weaker lumines-
cence compared with that of Alq3 (Figure 1, A versus C). In
particular, compound 3aa exhibits an intense green lumines-
cence peak at around 520 nm, and the intensity is at least
three times stronger than that of Alq3. Furthermore, the
bandwidth of 3aa is much narrower than that of Alq3, and is
observed as a high-purity green emission. This strongly
suggests that the remarkably intense luminescence of 3aa
may be applied to a green emitter for electroluminescence
(EL) devices.
Given the result of the above control experiments, two
plausible pathways were proposed for this kind of trans-
formation (Scheme 2). In the initial step, regioselective direct
electrophilic aromatic palladation at the 3-position of the
indole forms an indol-3-ylpalladium species A.[12] The sub-
In summary, we have demonstrated the first
palladium-catalyzed highly selective ring-expan-
sion reaction of readily available indoles with
alkynes, leading to polysubstituted tetrahydro-
quinoline products. Compound 3aa displays an
intense luminescence in the solid state. This kind of
À
transformation involves dual C H bond activa-
À
À
tions, one C C bond cleavage, five new C C bond
formations, and uniquely, ring-expansion of the
indole through a rearrangement pathway, under
mild reaction conditions. Moreover, O2 (1 atm) is
used as the oxidant in this catalytic cycle. This work
may broaden the research into the areas of
palladium catalysis and the ring-expansion of
indoles. Studies to clarify the reaction mechanism
and additional synthetic applications of this reac-
tion are ongoing in our laboratory.
Experimental Section
5-Methyl-4-(1-methyl-1H-indol-3-yl)-1,2,3,3a-tetra-
phenyl-4,5-dihydro-3aH-cyclopenta[c]quinoline (3aa): 1a
(25.0 mL, 0.20 mmol), 2a (89.0 mg, 0.50 mmol), and Pd-
(OAc)2 (4.50 mg, 0.02 mmol) were added to a 20 mL
Schlenk tube and the tube was purged three times with O2
Scheme 2. Proposed mechanism for the palladium-catalyzed ring-expansion reac-
tion of indoles.
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À
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Figure 1. Fluorescence spectra of Alq3 (A), 3aa (B), and 3ea (C) in the
solid state upon excitation at 380 nm.
(1 atm), followed by addition of 1:1 CH3CN/AcOH (v/v; 1.0 mL). The
mixture was stirred at room temperature under O2 (1 atm) for
36 hours and monitored by TLC. The solution was then quenched
with H2O (10 mL) and extracted with ethyl acetate (3 ꢀ 10 mL). The
combined organic phases were washed with saturated sodium
bicarbonate solution, dried over MgSO4, filtered, and evaporated
under vaccum. The crude product was purified by column chroma-
tography on silica gel (eluent: petroleum ether/ethyl acetate = 30:1)
to afford 48.0 mg (78%) of 3aa: light-yellow/green solid; IR: (KBr)
nmax = 1596, 1484, 1296, 1199, 739, 694 cmÀ1 1H NMR: (400 MHz,
;
CDCl3) d = 7.56 (d, J = 7.6 Hz, 2H), 7.29–7.13 (m, 9H), 7.10–6.85 (m,
8H), 6.80–6.67 (m, 4H), 6.60–6.48 (m, 3H), 6.31 (t, J = 7.4 Hz, 1H),
6.14 (d, J = 7.6 Hz, 2H), 5.49 (s, 1H), 3.61 (s, 3H), 2.91 ppm (s, 3H);
13C NMR: (100 MHz, CDCl3) d = 146.8, 145.9, 144.6, 144.4, 139.4,
136.2, 135.9, 135.83, 135.78, 129.6, 129.5, 129.1, 128.7, 128.6, 128.1,
127.9, 127.6, 127.1, 126.82, 126.78, 126.4, 126.3, 125.8, 120.7, 119.1,
118.5, 117.0, 115.0, 111.8, 110.7, 108.6, 62.4, 60.5, 37.6, 32.6 ppm; MS
(70 eV): m/z (%): 616.6 (100) [M]+; HRMS m/z (ESI) calcd for
C46H36N2 [M]+ 616.2873, found 616.2869.
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Received: March 1, 2010
Published online: April 21, 2010
À
Keywords: alkynes · C H activation · indoles · oxidation ·
palladium
.
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