Rhodium-catalyzed intramolecular cascade sequence for the formation of fused carbazole-annulated medium-sized rings by cleavage of C(sp2)-H/C(sp3)-H bonds

Article abstract of DOI:10.1039/c8cc04428g

The rhodium(iii)-catalyzed intramolecular annulation of alkyne-tethered 3-(indol-3-yl)-3-oxopropanenitriles for the synthesis of fused carbazole scaffolds via C-H activation has been developed. A series of six-, seven-, and eight-membered hydroazepino[3,2

Full text of DOI:10.1039/c8cc04428g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Rhodium-Catalyzed Intramolecular Cascade Sequence for the
Formation of Fused Carbazole-Annulated Medium-Sized Rings by
Cleavage of C(sp2)−H/C(sp3)−H Bonds
Received 00th January 20xx,
Accepted 00th January 20xx
QiuyunLi,aBin Lia and Baiquan Wang*abc
DOI: 10.1039/x0xx00000x
www.rsc.org/
The rhodium(III)-catalyzed intramolecular annulation of alkyne-
tethered 3-(indol-3-yl)-3-oxopropanenitriles for the synthesis of
fused carbazolescaffolds via C−H activation has been developed. A
series of six-, seven-, and eight-membered hydroazepino[3,2,1-
jk]carbazoles were achieved. This reaction proceeded under mild
reaction conditions and with a broad substrates scope. The
reaction involved sequential cleavage of C(sp2)−H/C(sp3)−H bonds
and annulation with the tethered alkyne.
N
O
N
O
NH
NH
O
MeN
N
O
N
N
N
H
Br
NMe2
Ro 32-0432
CRTH2 agonist
5-HT3 receptor antagonist
anti-cancer
Figure 1 Biologically active compounds containing carbazole or 1,2-fused
indoline moieties.
The indole skeletons are found in many biologically active
natural products and are useful moieties in functional
materials and drug design.1 Among them, fused polycyclic
indoles such as carbazoles, which are also the constituents of a
large amount of important skeleton for many bioactive natural
products, photorefractive materials, and organic dyes (Figure
1).2 In addition, corresponding medium-sized-ring analogues
are also the constituents of numerous natural products and
pharmaceutical agents.3 This class of compounds possesses a
functionalized medium-sized ring bridged to the N1- and C3-
positions of the carbazoles. Therefore, it is of great significance
for the construction of these scaffolds. Few methods have
been reported for the synthesis of azaheterocyclo[3,2,1-
jk]carbazoles (Scheme 1).4 So, the development of a general
method for the rapid construction of their analogue library
remains a challenge.
(a) 1,4-Palladium migration for the synthesis of fused carbazoles (ref. 4a)
Pd(OAc)2/dppm
CsO2CCMe3
I
DMF, 100 o
0.5 d
C
N
N
2
(b) Photoinduced cyclization for the synthesis of fused carbazoles (ref. 4c)
O
H
hv
X
H
X
+
acetone
pyridine
N
N
N
n
n
n
Scheme 1 Synthesis of azaheterocyclo[3,2,1-jk]carbazoles.
subsequent cross-coupling with alkynes or alkenes is a rapidly
evolving research field. In 2010, Miura and Satoh,6a Rovis,6b
and Li6c independently reported rhodium(III)-catalyzed
oxidative C−H/N−H funcꢀonalizaꢀon of benzamides in the
coupling with alkynes. In 2012, we reported the annulation of
benzoylacetonitrile with internal alkynes to get substituted
naphtho[1,8-bc]pyrans.6x In 2016, we also reported Rh(III)-
catalyzed carbocyclization reactions of 3-(indolin-1-yl)-3-
oxopropanenitriles with alkynes and alkenes.6q However, the
intermolecular reactions showed low regioselectivity with
unsymmetrical alkynes, especially for the unsymmetrical
dialkylalkynes. Consequently, intramolecular reactions are
highly attractive on account of regioselectivity, high efficiency,
and versatility for fused heterocyclic compounds. The
Recent years have witnessed an explosive growth of
synthetic transformations relying on transition-metal catalyzed
C–H bond activation processes.5 Owning to its high efficiency,
functional-group tolerance, and selectivity, rhodium(III)
catalysis has emerged as a powerful tool in C–H activation in
recent decades. Rh(III)-catalyzed C(sp2)
−H activation with
aState Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai
University, Tianjin 300071, P. R. China. E-mail: bqwang@nankai.edu.cn; Fax: +86
(22) 23504781; Tel: +86 (22) 23504781
bCollaborative Innovation Center of Chemical Science and Engineering, Tianjin
300071, P. R. China
c State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
† Electronic supplementary information (ESI) available: Fullexperimental details,
characterization and NMRspectra of the target products. CCDC 1834647. For ESI
and crystallographic data in CIF or other electronic format see
DOI:10.1039/b000000x/
rhodium-catalyzed intramolecular C−H activation reactions
also have been reported recent years,7 but most of them are
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J. Name., 2017, 00, 1-4 | 1
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limited to Rh(III)-catalyzed C(sp2)
−
H
activation with mass spectra, and single-crystal X-ray diffraction analysis
Rh(III)-catalyzed (Figure 2). Then, the electronic effects of aryl groups attached
hydroarylation
C(sp2) H/C(sp3)
with N-tethered alkene have never been reported. Based on intramolecular
the pioneering works (Scheme 2), we became interested in the corresponding products in good yields (Table 2, 2o
of
alkene
tethers,
−
−H activation of the intramolecular reactions to the alkyne were examined, which were all available for this
annulation
reaction,
affording
the
). 2-
−
r
Rh(III)-catalyzed intramolecular reaction of alkyne-tethered Naphthalene group could be accommodated in the reaction,
indol-3-yl-propanenitrile. Herein we report the rhodium- giving product 2s in 67% yield. Compound 2t with an
catalyzed intramolecular annulation of alkynes via a C−H bond oxazepino-7-membered ring was also produced in 53% yield by
activation pathway to efficiently construct fused carbazoles this method. Satisfyingly, the intramolecular reaction could be
(Scheme 2d).
extended to generate fused carbazoles 2u and 2v with 6- and
8-membered rings in 89% and 44% yields, respectively.
(a) Our previous work (ref. 6x):
R1
Table 1 Optimization of reaction conditionsa
R2
R1
O
O
CN
CN
CN
Rh(III)
O
HO
+
CN
R1
Ph
R2
[Cp*RhCl2]2 (5 mol %)
R2
CsOAc (2 equiv)
N
N
additive, solvent, temp
(b) Bergman's & Ellman's work (ref. 7a):
4
Ph
2a
1a
Bn
N
Bn
Yield (%)b
N
Entry
1
Solvent
CH3CN
DMF
DCE
Oxidant
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Ag2CO3
T/°C
120
120
120
120
120
120
120
100
100
80
Rh(I)
R1
R2
50
X
X
R1
n
n
2
55
R2
3
trace
47
(c) Glorius's work (ref. 7e):
O
O
O
4
t-AmOH
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
NH
R1
R2
NHOR
NH
R1
5
15
Rh(III)
or
H
n
R2
N
6
AgOAc
18
X
R1
X
n
n
R2
7
DDQ
trace
trace
92
(d) This work:
O
8
DTBP
CN
CN
9
TEMPO
TEMPO
HO
H
H
R
10
11c
12
13d
14e
15f
16g
85
Rh(III)
n = 3, 4, 5
N
N
TEMPO
100
100
100
100
100
100
n.r.
21
n-2
n
R
-
Scheme 2 Rhodium-catalyzed activation and annulation.
TEMPO
45
We initiated our studies by investigating the reaction of
indole-derived propanenitrile (1a) as model substrate. When
the reaction was performed in the presence of [Cp*RhCl2]2 (5
mol %), CsOAc (0.4 mmol, 2 equiv), and Cu(OAc)2·H2O (0.4
mmol, 2 equiv) in MeCN at 120 °C for 12 h under Ar
atomosphere (entry 1, Table 1), to our delight, the desired
TEMPO
43
TEMPO
trace
TEMPO
51
aReaction conditions: 1a (0.2 mmol), [Cp*RhCl2]2 (5.0 mol %), CsOAc
product 2a was obtained in 50% isolated yield. Changing the (0.4 mmol, 2.0 equiv), additive (0.4 mmol), solvent (1 mL), 100 °C,
b
c
d
solvent, oxidant, and temperature (entries 2−10), the yield was
12 h. Isolated yield. Without [Cp*RhCl2]2, [Cp*IrCl2]2 (5.0 mol %),
eCp*Rh(CH3CN)3(SbF6)2 (5.0 mol %), Cp*Co(CO)I2 (5.0 mol %), g[(p-
f
improved to 92% when DMF as the solvent and TEMPO as the
oxidant in 100 °C (entry 9). Without catalyst no reaction was
occurred (entry 11). In the absence of oxidant 2a was obtained
in 21% yield (entry 12). Different catalysts were also tested
cymene)RuCl2]2 (5.0 mol%).
(entries 13−16), [Cp*RhCl2]2 got the highest yield.
With the optimal conditions in hand, we surveyed various
substrates to determine the scope of the reaction. When n = 4,
the reaction proceeded smoothly to give a series of fused
carbazoles in moderate to excellent yields. Ignoring the
substituted groups at the 4-, 5-, 6-, or 7-position, both the
electron-donating and electron-withdrawing groups could
proceed smoothly to afford the corresponding carbazoles in
synthetically useful yields (Table 2, 2a−n). The molecular
structure of 2a was confirmed by its H and 13C NMR spectra,
1
Figure 2 Molecular structure of 2a
2 | J. Name., 2017, 00, 1-4
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Synthetic applications of this protocol have been demonstrated
(Scheme 4). Fused carbazole 2a was synthesized in 89% yield on a
gram scale even under reduced catalyst loading (Scheme 4, eqn (a)).
Furthermore, the hydroxyl group of 2a can act as an efficient
directing group for further rhodium(III)-catalyzed C−H
functionalization at the 9-position of the fused carbazole derivatives.
Reactions of 2a with ethyl acrylate and diphenylacetylene gave the
corresponding olefination product 8 and fused carbazoles 10 in 38%
and 87% yields, respectively (Scheme 4, eqn (b) and (c)).
Table
2
Rh(III)-catalyzed intramolecular cyclization to fused
carbazolesa
CN
CN
O
HO
[Cp*RhCl2]2 (5 mol%)
CsOAc (2 equiv)
H
R2
H
TEMPO (2 equiv)
N
N
R1
R1
DMF, 100 o
C
n-2
R2
n
1
2
CN
CN
CN
HO
HO
N
HO
R3
N
N
R1
O
R3 = F, 2o (88%)
Cl, 2p (70%)
2t (53%)
CN
R1 = H, 2a (92%)
4-Me, 2b (86%)
4-Cl, 2c (81%)
5-Me, 2d (88%)
HO
Me, 2q (71%)
OMe, 2r (89%)
5-OMe, 2e (81%)
5-F, 2f (85%)
N
CN
HO
5-Cl, 2g (77%)
5-Br, 2h (84%)
2u (89%)
CN
HO
5-CO2Me, 2i (71%)
5-NO2, 2j (88%)
6-Me, 2k (92%)
6-Br, 2l (84%)
N
2s (67%)
N
7-Me, 2m (83%)
7-Cl, 2n (80%)
2v (44%)b
aReaction conditions: 1a (0.2 mmol), [Cp*RhCl2]2 (5.0 mol %), CsOAc
(0.4 mmol, 2.0 equiv), TEMPO (0.4 mmol, 2.0 equiv), DMF, 100 °C,
12 h, isolated yield.bCp*Rh(CH3CN)3(SbF6)2 (5 mol%).
Scheme 4 Gram-scale synthesis and derivatization reactions of carbazole
Based on the literature work,6f a possible mechanism for this
rhodium-catalyzed cascade reaction is proposed as shown in
Scheme 3. The first step is likely to be the acidic C(sp3)−H bond
activation process affording intermediate 3, then C(sp2)−H bond
activation through the CMD mechanism gives a five-membered
rhodacycle 4. The coordination and insertion of the tethered alkyne
leads to the six, seven or eight-membered rhodacycle intermediate
5. After ketone enolization intermediate 6 is formed and undergoes
reductive elimination to afford product 2a and Cp*Rh(I). Cp*Rh(I) is
oxidized by TEMPO to afford Cp*Rh(OAc)2 for the next catalytic
cycle.
In summary, we have developed a mild and efficient method for
the synthesis of fused carbazoles, based on a rhodium(III)-catalyzed
C−H bond activation and subsequent intramolecular oxidative
annulation of the tethered alkynes by cleavage of
C(sp2)−H/C(sp3)−H bonds. This reaction has high selectivity and a
broad substrate scope, contributing to the formation of heterocyclic
scaffolds of utmost importance in agrochemistry or medicinal
chemistry. Further applications of this method in the synthesis of
other targets are in progress.
The authors wish to thank the National Natural Science
Foundation of China (No. 21672108, and 21421062) and the Natural
Science Foundation of Tianjin (16JCZDJC31700) for financial support.
Notes and references
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DOI: 10.1039/C8CC04428G
Qiuyun Li, Bin Li, and Baiquan
Wang*
Rhodium-Catalyzed
Cascade Sequence for the Formation
of Fused Carbazole-annulated
Intramolecular
The rhodium(III)-catalyzed intramolecular annulation of alkyne-tethered 3-(indol-3-yl)-
3-oxopropanenitriles for the synthesis of fused carbazole scaffolds via C−H activation
has been developed. A series of six-, seven-, and eight-membered hydroazepino[3,2,1-
jk]carbazoles were achieved.
Medium-Sized Rings by Cleavage of
C(sp2)−H/C(sp3)−H Bonds