Rhodium-Catalyzed C?H Functionalization of Indoles with Diazo Compounds: Synthesis of Structurally Diverse 2,3-Fused Indoles

Article abstract of DOI:10.1002/adsc.201700974

A Rhodium-catalyzed C2-H functionalization of indoles with diazo compounds, followed by intramolecular nucleophilic addition to C=O or C=C bonds, is reported for divergent synthesis of 2,3-fused indoles. Besides acceptor/acceptor diazo compounds, donor/acceptor diazo compounds are broadly tolerated, giving various 2,3-fused indoles with perfect diastereocontrol. Notably, a selective C?H dialkylation reaction at C2 and C7 position of indoles has also been developed by simply changing the reaction conditions. This environmentally benign transformation proceeds under mild conditions and gives dinitrogen as the only by-product. (Figure presented.).

Full text of DOI:10.1002/adsc.201700974

Title: Rhodium-Catalyzed C-H Functionalization of Indoles with Diazo
Compounds: Synthesis of Structurally Diverse 2,3-Fused Indoles
Authors: Mengying Gao, Yaxi Yang, Hua Chen, and Bing Zhou
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700974
Link to VoR: http://dx.doi.org/10.1002/adsc.201700974

10.1002/adsc.201700974
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Rhodium-Catalyzed C-H Functionalization of Indoles with
Diazo Compounds: Synthesis of Structurally Diverse 2,3-Fused
Indoles
Mengying Gao,ab Yaxi Yang, b* Hua Chen a* and Bing Zhou b*
a
Key Laboratory of Chemical Biology of Hebei Province, College of Chemistry and Environmental Science, Hebei
University, Baoding 071002. Email: hua-todd@163.com
Department of Medicinal Chemistry, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu
b
Chong Zhi Road, Shanghai 201203, PR China.
Email: yangyaxi@simm.ac.cn; Email:zhoubing2012@hotmail.com
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
catalyzed C-H functionalizations with diazo
Abstract. A Rhodium-catalyzed C2-H functionalization of
indoles with diazo compounds, followed by intramolecular compounds.
nucleophilic addition to C=O or C=C bonds, is reported for
divergent synthesis of 2,3-fused indoles. Besides
acceptor/acceptor diazo compounds, donor/acceptor diazo
compounds are broadly tolerated, giving various 2,3-fused
indoles with perfect diastereocontrol. Notably, a selective
C-H dialkylation reaction at C2 and C7 position of indoles
has also been developed by simply changing the reaction
conditions. This environmentally benign transformation
proceeds under mild conditions and gives dinitrogen as the
only by-product.
Keywords: Cp*Rh(III) catalyst; C-H activation; Diazo
compounds; 2,3-fused indoles
Figure 1. Representative biologically active 2,3-fused
The indole motif is prevalent in a variety of bioactive
natural products, pharmaceuticals and
indoles.
Very recently, our group reported a Rh(III)-
catalyzed cyclization reaction of nitrones with diazo
compounds, where the C(sp3)-Rh intermediate is not
protonated and undergoes an intramolecular addition
agrochemicals.[1] Among them, 2,3-Fused indoles
have received considerable attention due to their
interesting biological activities (Figure 1).[2] Although
several methods have been developed for the
synthesis of this popular scaffold, [3] the development
of new approaches that allow rapid establishment of
these scaffolds in simple operation from readily
available precursors is still highly desirable.
to the polarized C=N bond, thus producing various N-
[8c]
hydroxyindolines (Scheme 1b).
In this context,
with our continuing interest in the sustainable organic
[9]
synthesis of fused indoles,
we herein report a
Rh(III)-catalyzed C2-H functionalization of indoles
with diazo compounds, where the resulting C(sp3)-Rh
intermediate undergoes an intramolecular addition
reaction to the C=O or C=C bonds, providing various
2,3-fused indoles (Scheme 1c). This reaction
proceeds under mild conditions, giving benign N2 as
the only byproduct.
Over the past several years, the diazo compounds
have been widely used as powerful cross-coupling
partners for transition-metal-catalyzed carbene
transformations. [4] However, intermolecular aromatic
C(sp2)−H insertion into diazo compounds has very
limited precedent. Until recently, a significant
breakthrough has been made by Yu, [5] who reported
the first intermolecular insertion of aromatic
C(sp2)−H bonds into α-diazocarbonyl compounds via
[6]
a Rh(III)-catalyzed C-H activation (Scheme 1a).
Since then, other groups[7] and our group[8] have
displayed the successful exploration of Rh(III)-
1
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10.1002/adsc.201700974
Advanced Synthesis & Catalysis
Encouraged by this result, various additives were
next screened (entries 4-6). The addition of acid
decreased the yield of product 3a (entry 4).
Pleasingly, the situation changed significantly when
adding some acetate additives (entries 5-6), with
CsOAc being optimal to afford 3a as the sole product
in 92% yield, as shown in entry 6. Interestingly, when
employing [Cp*RhCl2]2 (2.5 mol %) and AgSbF6 (10
mol %) as catalyst, 4a could be obtained as the main
product in 42% yield (entry 7). To our delight,
introduction of Cu(OAc)2 as the additive further
improved the yield of 4a to 82% (entry 8).
With the optimized reaction conditions in hand, we
embarked on the investigation of the scope of this
annulation reaction. As shown in scheme 2, indoles
containing both electron-donating (3b, 3c) and -
withdrawing (3d, 3e) groups were readily coupled
diazomalonate (2a) to provide the corresponding
products in good to excellent yields. Some important
functional groups such as halogen and ester, were
remarkably compatible, offering the opportunity for
further synthetic transformations. The scope of diazo
coupling partners has also been established and
various α-diazomalonates underwent this coupling
reaction smoothly to provide products 3f-3h in
satisfactory yields. In addition to α-diazomalonates,
other acceptor-acceptor diazo compounds with one
ester group replaced by another electron-withdrawing
group such as amide (3i), acetyl (3j-3l), sulfonyl
(3m), were also found to be favorable in current
transformation, giving the corresponding products in
excellent yields. It is noteworthy that aryl substituted
α-diazoacetylacetates (3k, 3l) and sulfonylacetate
(3m) exhibited good diastereoselectivity (dr up
to >20:1).
Scheme 1. Rh(III)-catalyzed C-H functionalization with
diazo compounds.
Table 1. Optimization of the reaction conditions.[a]
yield of 3a
yield of 4a
entry
R
additive
(%)
(%)
1
2
(CH3)2NCO
Ac
-
0
0
0
0
-
-
3
2-pyrimidyl
2-pyrimidyl
2-pyrimidyl
2-pyrimidyl
2-pyrimidyl
2-pyrimidyl
43
12
16
< 10
<2
42
82
4
PivOH
AgOAc
CsOAc
-
< 10
75
5
6
92
Scheme 2. Scope of indoles and acceptor/acceptor diazo
compounds.
7[b]
8[c]
[a]
< 5
<2
Cu(OAc)2
Reaction conditions: 0.2 mmol of 1a, 2a (1.5 equiv.),
catalyst (2.5 mol %), additive (100 mol %) and DCE (2
mL) at 60°C for 8 h in a sealed tube. Yield isolated by
column chromatography. [b] Using [Cp*RhCl2]2 (2.5 mol %)
and AgSbF6 (10 mol %) as catalyst [c] 0.2 mmol of 1a, 2a
(4 equiv.), [RhCp*Cl2]2 (2.5 mol %), AgSbF6 (10 mol %),
Cu(OAc)2 (20 mol %) and DCE (2 mL) at 80°C for 16 h in
a sealed tube.
The initial experiments were performed with a
variety of N-substituted indoles (1a) and
diazomalonate (2a) in the presence of 2.5 mol%
[Cp*Rh(MeCN)3](SbF6)2
in
1,2-dichloroethane
(Table 1, entry 1). The choice of indole protecting
group was found to be crucial (Table 1, entries 1-3),
with 2-pyrimidyl being optimal (Table 1, entry 3). To
our delight, the coupling reaction occurred smoothly
to provide the desired product 3a and di-alkylation
product 4a with a ratio of ca. 3:1 (entry 3).
Reaction conditions: 0.2 mmol of 1, 2 (1.5 equiv),
[RhCp*(CH3CN)3](SbF6)2 (2.5 mol %), CsOAc (100
2
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10.1002/adsc.201700974
Advanced Synthesis & Catalysis
mol %) and DCE (2 mL) at 60°C for 8 h. Yield isolated by
column chromatography.
Encouraged by these results, we further examined
donor/acceptor diazo compounds which were often
less
reactive
(Scheme
3).[10]
Pleasingly,
donor/acceptor diazo compounds were broadly
tolerated with reasonable scope (3n-3w), irrespective
of the electronic nature of the substituents on the
phenyl ring (3o-3u). Cyclic diazo substrates also
delivered the desired spiro compounds in high yields
under the reaction conditions (3v, 3w). Notably, all
the products showed perfect diastereocontrol (dr >
20:1), except 3s, 3v and 3w (dr = 1:1). The
configuration was verified via NOESY spectra (see
supporting information for detail). The good
diastereocontrol may be understood by a favored six-
membered cyclic transition state AA when the O-Rh
intermediate undergoes the intramolecular aldol
reaction.[11]
[a]
Reaction conditions: 0.2 mmol of 9 or 10, 2a (1.5 equiv),
[RhCp*(CH3CN)3](SbF6)2 (2.5 mol %), CsOAc (100
mol %) and DCE (2 mL) at 60°C for 8 h.
Finally, the scope of the double C-H
activation/alkylation was also examined (Scheme 5).
Various substrates containing both electron-donating
(4b, 4c) and electron-withdrawing groups (4d-4f) at
C5 position of the indole ring were successfully
coupled with dimethyl diazomelonate, providing C2,
C7-dialkylation products 4b-f in good to excellent
yields. Besides dimethyl diazomelonate, ethyl,
isopropyl and benzyl diazomelonates also worked
well to provide the corresponding dialkylation
products 4g-i in good yields.
Scheme 3. Scope of donor/acceptor diazo-compounds.[a]
Scheme 5. Rh-catalyzed C2, C7-dialkylation of indole C-H
bonds.[a]
[a]
Reaction conditions: 0.2 mmol of 1, 0.8 mmol of 2,
[a]
Reaction conditions: 0.2 mmol of 1, 0.3 mmol of 2,
[RhCp*Cl2]2 (2.5 mol %), AgSbF6 (10 mol %), Cu(OAc)2
(20 mol %) and DCE (2 mL) at 80°C for 16 h.
[RhCp*(CH3CN)3](SbF6)2 (2.5 mol %), CsOAc (100
mol %) and DCE (2 mL) at 60°C for 8 h. Yield isolated by
column chromatography.
To illustrate the synthetic utility of this reaction,
treatment of 3f with DIPEA resulted in
decarbonylation to afford 5f in 72% yield (Scheme
6a). In addition, 3f could undergo
decarbonylation/elimination of hydroxyl
group/oxidation to give carbazole derivative 6f in 67
yield when treating 3f with LiOH in EtOH and H2O
at 60 C (Scheme 6b). Finally, the pyrimidyl group
could be successfully removed by the treatment of
NaOEt in DMSO at 100 oC for 3 h to give 7f in 75%
yield.
To further expand this coupling/annulation reaction
with diazo compounds, we wondered whether a
intramolecular Michael addition of the C(sp3)–Rh
intermediate to acrylate might be possible (Scheme 4).
To our delight, the indole with acrylate skeleton (9)
delivered the 2,3-fused indole 3x in 77% yield under
the standard reaction conditions. More significantly,
the C(sp3)-Rh intermediate could undergo an
intramolecular Grignard-type addition reaction to
ester, providing the seven-membered 2,3-fused indole
3y in good yield.
a
o
Scheme 6. Transformation of product 3f
.
Scheme 4. Synthesis of 2,3-fused indoles.[a]
3
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10.1002/adsc.201700974
Advanced Synthesis & Catalysis
Based on the literature precedents,[5-8] we
tentatively propose the following reaction
mechanism (Scheme 8). First, a five-membered
cyclometalated Rh(III) complex A is formed via a
Rh(III)-catalyzed C(sp2)−H bond cleavage.
Coordination of the diazo compound with A
gives the intermediate B which undergoes an
intramolecular 1,2-migratory insertion of the aryl
group to afford the intermediate C. Subsequent
intramolecular nucleophilic addition to aldehyde
afford the rhodium alkoxide D which undergoes a
protonation to yield the product 3a and regenerate
the rhodium catalyst.
Several experiments were carried out to obtain
a more mechanistic insight (scheme 7). First, a
H/D scrambling was observed at the C2-position
of indole when 1a was reacted with the
[RhCp*(CH3CN3)](SbF6)2 catalyst in DCE and
D2O in the absence of 2a, indicating the C–H
bond cleavage is reversible (Scheme 7a).
However, in the presence of 2a, no H/D
Scheme 8. Proposed mechanism.
scrambling was observed in the recovered 1a
,
indicating that C–H functionalization is much
faster than the deuteration (Scheme 7b). To
explain why Cu(OAc)2 facilitate the second C7-H
functionalization to form the dialkylation product
4a, several control experiments were carried out.
First, treatment of 3a with diazomalonate in the
o
presence of [Cp*Rh(MeCN)3](SbF6)2 at 60 C for
30 min gave only a trace amout of 4a (<5% yield)
and addition of Cu(OAc)2 as an additive gave 4a
in 85% yield (Scheme 7c). However, 6f without a
hydroxyl group can undergo a Rh(III)-catalyzed
C7-H alkylation with diazomalonate smoothly to
give 6g in good yield in the absence of Cu(OAc)2
(Scheme 7d). Based on the above data, we
speculated that the hydroxyl group of 3a may
react with Rh(III) catalyst to form stable O-Rh
species
D (See Scheme 8 for structure), thus
inhibiting the second C7-H functionalization to
form 4a and additon of Cu(OAc)2 may lead to a
transmetalation of the intermidiate
D with the
In summary, an efficient Rh(III)-catalyzed C2-
alkylation of indoles with diazo compounds, followed
by intramolecular addition reaction, has been
developed for the synthesis of various 2,3-fused
indoles. Besides acceptor/acceptor diazo compounds,
donor/acceptor diazo compounds are broadly
tolerated, providing various 2,3-fused indoles with
perfect diastereocontrol. Notably, a C-H dialkylation
reaction at C2 and C7 position of indoles has also
been developed by simply changing the reaction
conditions. This transformation proceeds under mild
conditions, obviates the need for oxidants, and
releases N2 as the single byproduct, thus providing an
environmentally benign method of 2,3-fused indole
synthesis.
copper salt, thus facilitating to release the active
Rh(III) species to catalyze the C7-H alkylation.[12]
Scheme 7. Control experiments.
Experimental Section
General procedure for the synthesis of compound 3: To
a dram screw-cap vial mixture of 0.2 mmol of 1,
[RhCp*(MeCN)3](SbF6)2 (2.5 mol%), CsOAc (100 mol %),
DCE (2 mL) and 0.3 mmol of diazomalonate (2a) at 60 oC
under Ar for 8 h. Then the reaction is cooled to room
temperature, the solvent was removed and the residue was
4
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10.1002/adsc.201700974
Advanced Synthesis & Catalysis
purified by flash chromatography on silica gel (petroleum
ether/EtOAc = 3:1) to give the desired product 3a in yield
92%.
4 a) J. Barluenga, C. Valde´s, Angew. Chem., Int. Ed.
2011, 50, 7486; b) Z. H. Shao, H. B. Zhang, Chem.
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General procedure for the synthesis of compound 4: To
a dram screw-cap vial mixture of 0.2 mmol of 1,
[RhCp*Cl2]2 (2.5 mol %), AgSbF6 (10 mol %), Cu(OAc)2
(20 mol %), DCE (2 mL) and 0.8 mmol of diazomalonate
(2a) at 80 oC under Ar for 16 h. Then the reaction is cooled
to room temperature, the solvent was removed and the
residue was purified by flash chromatography on silica gel
(petroleum ether/EtOAc = 1:1) to give the desired product
4a in yield 82%.
5 W. W. Chan, S. F. Lo, Z. Y. Zhou, W. Y. Yu, J. Am.
Chem. Soc. 2012, 134, 13565.
6 For reviews on Cp*Rh(III) catalyzed C-H activation,
see: a) D. A. Colby, R. G. Bergman, J. A. Ellman,
Chem. Rev. 2010, 110, 624; b) T. Satoh, M. Miura,
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Tsai, R. G. Bergman, J. A. Ellman, Acc. Chem. Res.
2012, 45, 814; d) G. Song, F. Wang, X. W. Li,
Chem. Soc. Rev. 2012, 41, 3651; e) F. W. Patureau,
J. Wencel-Delord, F. Glorius, Aldrichimica Acta.
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Adv. Synth. Catal. 2014, 356, 1443; g) G. Song, X.
W. Li, Acc. Chem. Res. 2015, 48, 1007. h) Kim, J.
Acknowledgements
Support of this work by “1000-Youth Talents Plan”, “the CAS
Pioneer Hundred Talents Program”, Shanghai Rising-Star
Program (Grant No. 17QA1405000), and Youth innovation
promotion association (2017333) is gratefully acknowledged.
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Advanced Synthesis & Catalysis
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6
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10.1002/adsc.201700974
Advanced Synthesis & Catalysis
COMMUNICATION
Rhodium-Catalyzed C-H Functionalization of
Indoles with Diazo Compounds: Synthesis of
Structurally Diverse 2,3-Fused Indoles
Adv. Synth. Catal. Year, Volume, Page – Page
Mengying Gao,ab Yaxi Yang, *b Hua Chen*a and
Bing Zhou*b
7
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