From Indoles to Carbazoles: Tandem Cp?Rh(III)-Catalyzed C-H Activation/Br?nsted Acid-Catalyzed Cyclization Reactions

Article abstract of DOI:10.1021/acscatal.5b01801

A tandem Cp?Rh(III)-catalyzed C-H activation/Br?nsted acid-catalyzed intramolecular cyclization allows a facile synthesis of carbazoles from readily available indoles. The reaction proceeds under rather mild reaction conditions with the generation of wate

Full text of DOI:10.1021/acscatal.5b01801

Letter
pubs.acs.org/acscatalysis
From Indoles to Carbazoles: Tandem Cp*Rh(III)-Catalyzed C−H
Activation/Brønsted Acid-Catalyzed Cyclization Reactions
‡
‡
Jia-Qiang Wu, Zhen Yang, Shang-Shi Zhang, Chun-Yong Jiang, Qingjiang Li, Zhi-Shu Huang,*
and Honggen Wang*
School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, China
*
S Supporting Information
ABSTRACT: A tandem Cp*Rh(III)-catalyzed C−H activation/Brønsted
acid-catalyzed intramolecular cyclization allows a facile synthesis of
carbazoles from readily available indoles. The reaction proceeds under
rather mild reaction conditions with the generation of water and N as the
2
only byproducts. Broad substrate scope, excellent functional group
tolerance, and high yields were observed. The benzannulation of pyroles
for the synthesis of indoles is also feasible using the same protocol.
KEYWORDS: Brønsted acid, carbazoles, C−H bond activation, rhodium(III), tandem catalysis
arbazoles are important skeletons in functional molecules.
C−H bonds, wherein the metal-carbene is proposed to be
C
For instance, they are widely used in photorefractive
formed followed by a concerted C−H insertion event, was
1
materials and organic dyes. In particular, carbazole-containing
small molecules are popular in medicinal chemistry as they
demonstrated to be powerful and practical in organic
12
synthesis. Recently, an alternative reaction mode involving
2
display various bioactivities (Scheme 1a). As a result,
C−H metalation, metal-carbene formation and migration
1
3
insertion was found to be operative in Cp*Rh(III),
1
4
15
Scheme 1. (a) Representative Carbazole-Containing Drugs;
b) Synthetic Strategies toward Carbazoles
Cp*Ir(III), and Cp*Co(III) -catalyzed C−H coupling
(
reactions with diazo compounds (Scheme 2a).
Very recently, the group of Katukojvala disclosed an efficient
synthesis of substituted indoles by using a Rh(II)-catalyzed [4 +
2
] benzannulation of pyrroles with enaldiazo ketones or esters
16
as four-carbon synthons (Scheme 2b). The reaction was
proposed to occur via a Rh(II)-catalyzed carbonoid C−H
insertion/cyclization sequence. Inspired by this elegant work,
17,18
we devised a tandem catalysis
strategy for the synthesis of
carbazoles from indoles. We envisioned that enaldiazo
compounds could serve as intriguing reacting partners in
III
19
Cp*Rh -catalyzed C−H functionalization of indoles to give
an enal or aldehyde intermediate (Scheme 2c). Thereafter, in
the presence of a Brønsted acid, an intramolecular cyclization/
dehydradration reaction could render the synthesis of
carbazoles directly from indoles in a one-pot manner. Herein,
we report our realization of a tandem Cp*Rh(III) and Brønsted
acid catalysis for the benzannulation of indoles, leading to a
variety of substituted carbazoles in high efficiency. The reaction
occurs under rather mild reaction conditions and generates N2
tremendous efforts have been exerted to streamline their
3
synthesis. Traditionally, carbazoles are constructed via the
4
5
6
well-known Fischer−Borsche, Bucherer, Cadogan, and
7
Graebe−Ullmann carbazole syntheses. Recently, with the
advent and development of organo-transition metal chemistry,
methodologies based on transition metal catalysis offer a useful
8
and H O as the only byproducts. We would like to point out
2
and simple alternative. Not surprisingly, the majority of these
that the application of C−H activation reactions in carbazole
methods are targeted on the construction of ring B. In contrast,
syntheses based on the construction of ring C has limited
precedent, with a lot of examples necessitating the
prefunctionalization of starting material indoles at the α-or/
syntheses from different starting materials is an active topic in
20
organic synthesis, and a number of elegant examples exist.
To test our hypothesis, 1-(pyrimidin-2-yl)-1H-indole 1a was
subjected to react with enaldiazo ester 2a under the catalysis of
9
,10
and β-position (Scheme 1b).
Over the past decade,
transition-metal-catalyzed direct C−H functionalization has
experienced a fruitful development, allowing for the assembly of
diverse C−C and C−heteroatom bonds in an atom- and step-
Received: August 15, 2015
Revised: September 27, 2015
1
1
economic manner. In this regard, the carbenoid insertion into
©
XXXX American Chemical Society
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DOI: 10.1021/acscatal.5b01801
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Letter
Scheme 2. (a) Cp*Rh(III), Cp*Ir(III), and Co*Co(III)-
Catalyzed C−H Coupling with Diazo Compounds; (b)
Indole Syntheses via Rh(II)-Catalyzed Carbenoid C−H
Insertion/Cyclization Sequence; (c) This Work: Tandem
Cp*Rh(III) and Bronsted Acid Catalysis for the
a comparable yield (35%, entry 4); however, its stronger acidic
analogue, p-NO PhCOOH, showed superior reactivity (entry
2
5). Moreover, the use of p-TsOH and (PhO) POOH further
2
improved the yield to 78% and 91%, respectively (entries 6 and
7). Gratifyingly, the loading of the acid could be decreased to
10 mol %, wherein a similar yield was maintained (entry 8).
Control experiments showed that Cp*Rh(III) was essential for
this reaction as its omission led to no formation of the desired
product (entry 9). A satisfactory yield of 63% was also obtained
when 1 mol % of Cp*Rh(III) was employed (entries 10 and
Benzannulation of Indoles toward Carbazole Synthesis
1
1). Based on a survey of different solvents, CHCl gave the
3
best result (entries 12−16). Interestingly, the use of CHCl /
3
DMF (9:1) as cosolvent delivered 3aa in an excellent yield of
9
6%. It is noteworthy that only a slight excess (1.2 equiv) of
enaldiazo ester 2a was used, and no slow addition of 2a was
needed in this reaction (entry 17).
With the optimized reaction conditions (Table 1, entry 17),
the generality of this transformation was then explored
(
Scheme 3). It was found that 2b and 2c, bearing a tert-butyl
Scheme 3. Benzannulation Reaction of Indoles with
Enaldiazo Esters
[
Cp*Rh(CH CN) ](SbF ) (5 mol %) in DCE at 35 °C for 20
3 3 6 2
h (Table 1). Delightedly, the desired benzannulation product
a
Table 1. Optimization of the Reaction Conditions
Cp*Rh(III)
yield
[%]
entry
[mol %]
solvent
DCE
additive [equiv]
1
2
3
4
5
6
7
8
9
5
5
5
5
5
5
5
5
0
2.5
1
5
5
5
5
5
5
-
10
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
MeOH
DMF
THF
PhMe
CHCl3
CsOAc (1.0)
PivOH (1.0)
PhCOOH (1.0)
trace
33
35
p-NO PhCOOH (1.0)
65
2
p-TsOH (1.0)
78
(PhO) POOH (1.0)
91
2
(PhO) POOH (0.1)
90
2
(PhO) POOH (0.1)
0
2
10
11
12
13
14
15
16
17
(PhO) POOH (0.1)
81
2
(PhO) POOH (0.1)
63
2
(PhO) POOH (0.1)
trace
trace
64
and benzyl ester, respectively, were also suitable coupling
partners for this reaction (3ab and 3ac). To our delight, a wide
range of substituted indoles were converted smoothly to the
corresponding carbazoles in generally good to excellent yields.
Functionalities, regardless of the substitution positions and
electronic nature, including ether (3cb, 3fb, 3nb), cyano (3ib),
ester (3db, 3jb), nitro (3kb, 3ob), fluoro (3lb), chloro (3gb,
3mb, 3qb), bromo (3hb), and even formyl (3eb) group, were
well tolerated. No corresponding product was obtained when
biheterocyclic 1H-pyrrolo[2,3-c]pyridine 1r was used, probably
arising from the strong coordination ability of the pyridine
moiety, which hampers the C−H activation event (3rb). It is
worth noting that enaldiazo ester 2d, bearing a methyl
substituent, was also tolerated, giving carbazole 3ad in 55%
yield.
2
(PhO) POOH (0.1)
2
(PhO) POOH (0.1)
2
(PhO) POOH (0.1)
76
2
(PhO) POOH (0.1)
92
2
CHCl /DMF
(PhO) POOH (0.1)
96
3
2
(9:1)
a
1
a (0.1 mmol), 2 (0.12 mmol), [Cp*Rh(CH CN) ](SbF ) (5 mol
3 3 6 2
%
), additive: solvent (1.0 mL), 20 h, 35 °C, isolated yield.
3
aa was obtained, albeit in low yield (10%, entry 1). On the
basis of our working hypothesis, we reasoned that the use of
acid might facilitate the reaction. Indeed, while the use of
CsOAc as additive killed the reactivity (entry 2), the use of
PivOH (1.0 equiv) was beneficial for the reaction, giving an
improved 33% yield (entry 3). Encouraged by this result, a
variety of Brønsted acids were thus screened. Benzoic acid gave
It was found that not only enaldiazo esters but also enaldiazo
ketones were effective for this benzannulation reaction
6
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Scheme 4). In these cases, 1.6 equiv of enaldiazo ketones were
Letter
(
directing group went smoothly upon the treatment of NaOMe
subjected to maintain a higher yield. Again, the reaction was
in DMSO, giving the free carbazole 6 in 78% yield (eq 2).
Scheme 4. Benzannulation Reaction of Indoles with
Enaldiazo Ketones
Scheme 6. Gram-Scale Synthesis, Removal of Directing
Group, and Mechanistic Studies
a
5
0 °C was used. An additional 3 mol % of [Cp*Rh(CH CN) ]-
3 3
6 2
(
SbF ) was added after 24 h, and the reaction was stirred for another
8
h.
exceptionally general while reacting with diversely substituted
enaldiazo ketones. Numerous commonly encountered function-
alities as well as heteroaryl groups including furan and
thiophene were well tolerated.
Having established the benzannulation of indoles for
carbazoles synthesis, we set to investigate the feasibility of
benzannulation of pyrroles using the same protocol (Scheme
To probe the reaction mechanism, indole was subjected to
the reaction instead of 1a, and no formation of carbazole 7 or 8
was detected (eq 3), suggesting the importance of pyrimidyl
group for this transformation. This result might also indicate a
5). The reaction of 2-(1H-pyrrol-1-yl)pyrimidine 4a with 2b
16
reaction mechanism distinct from Katukojvala’s work. In
Scheme 5. Benzannulation Reaction of Pyrroles with
Enaldiazo Esters
addition, when rhodacycle A, prepared according to a literature
21
procedure, was used as a the catalyst, a high yield of 84% was
obtained (eq 4). Furthermore, the stoichiometric reaction of
complex 9 with 2b without 1a gave a yield of 35% (eq 5).
These results support a C−H activation/carbene formation
sequence is operative in this reaction. On the basis of the
above-mentioned experimental results, a plausible mechanism
was outlined in Scheme 7. Initially, Cp*Rh(III)-catalyzed C−H
activation takes place to give a rhodacycle A, which is followed
by the metal-carbene formation with enaldiazo compounds to
give intermediate B. A subsequent α-insertion gives alkyl-
Rh(III) species C, which upon protonation delivers the
regioisomeric alkenes D. Several intermediates, which were
converted slowly to the final product, were found by TLC
a
2
b (0.32 mmol, 1.6 equiv) was used.
Scheme 7. Mechanistic Rationale
delivered bis-benzannulation product carbazole 5ab′ in 26%
yield, along with indole 5ab in 11% yield. With substituted
pyrroles, the corresponding indole products were successfully
constructed in satisfactory to good yields (5bb-5eb). Notably,
the electron-withdrawing acetyl and ester groups at the α-
position were tolerated, which is complementary to Katukojva-
la’s work wherein only electron-withdrawing groups located at
the β-position of pyrroles were presented.
A gram-scale reaction was conducted to evaluate the reaction
efficacy on preparative scale. In total, 1.49 g of carbazole 3ab
was obtained in 86% yield with 3 mol % of Cp*Rh(III) catalyst
when the reaction was run for 32 h, demonstrating that the
reaction is practical (eq 1, Scheme 6). The removal of the
6
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Letter
analysis but were unstable upon isolation. We suspect these
intermediates might be a mixture of D. Thereafter, the double
bond could be isomerized in the presence of Brønsted acid,
thereby setting the stage for the intramolecular cyclization. A
Friedel−Crafts cyclization and a subsequent dehydration
reaction provides the final product 3. These two steps might
also be promoted by the Brønsted acid.
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1
system, which allows the facile construction of carbazoles
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conditions with the generation of H O and N as the only
byproducts. Broad substrate scope, excellent functional group
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ASSOCIATED CONTENT
■
(
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01801.
Org. Lett. 2012, 14, 6198−6201. (b) Kong, W.; Fu, C.; Ma, S. Chem.
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Experimental details of the synthesis and characterization
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of starting materials and final compounds; copies of H
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(
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AUTHOR INFORMATION
■
*
*
(
Corresponding Authors
E-mail: wanghg3@mail.sysu.edu.cn.
E-mail: ceshzs@mail.sysu.edu.cn.
6
24−655. (c) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110,
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Author Contributions
These authors contributed equally (J.-Q.W. and Z.Y.).
L.; O’Hara, F.; Gaunt, M. J. Chem. Soc. Rev. 2011, 40, 1885−1898.
g) Yeung, C. S.; Dong, V. M. Chem. Rev. 2011, 111, 1215−1292.
(h) Cho, S. H.; Kim, J. Y.; Kwak, J.; Chang, S. Chem. Soc. Rev. 2011,
0, 5068−5083. (i) Wencel-Delord, J.; Droge, T.; Liu, F.; Glorius, F.
‡
(
Notes
4
̈
The authors declare no competing financial interest.
Chem. Soc. Rev. 2011, 40, 4740−4761. (j) Kuhl, N.; Hopkinson, M. N.;
Wencel-Delord, J.; Glorius, F. Angew. Chem., Int. Ed. 2012, 51, 10236−
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ACKNOWLEDGMENTS
■
We thank Prof. Dr. Frank Glorius for the helpful discussions
and reading of the manuscript. This work was supported by the
1000-Youth Talents Plan”, a Start-up Grant from Sun Yat-sen
University, and the National Natural Science Foundation of
China (81402794, 21472250, 81330077, 81273433).
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DOI: 10.1021/acscatal.5b01801
ACS Catal. 2015, 5, 6453−6457