Synthesis of Terminal Vinylindoles via RhIII-Catalyzed Direct C?H Alkenylation with Potassium Vinyltrifluoroborate

Article abstract of DOI:10.1002/chem.201706025

An efficient Rh^III-catalyzed direct C2-alkenylation of indoles using readily available potassium vinyltrifluoroborate under mild conditions has been developed. This protocol features wide substrate scope and excellent functional group compatibi

Full text of DOI:10.1002/chem.201706025

A Journal of
Accepted Article
Title: Synthesis of Terminal Vinylindoles via Rh(III)-Catalyzed Direct C-
H Alkenylation with Potassium Vinyltrifluoroborate
Authors: Liang Wang, Chun Ni Zhou, Hao Hao Xie, Zi Ang Zheng,
Yuan Chao Xiao, Gen Li, Yang Huan Shen, and Wang Ming
Peng
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To be cited as: Chem. Eur. J. 10.1002/chem.201706025
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Synthesis of Terminal Vinylindoles via Rh(III)-Catalyzed Direct C-
H Alkenylation with Potassium Vinyltrifluoroborate
Chun-Ni Zhou, Hao-Hao Xie, Zi-Ang Zheng, Yuan-Chao Xiao, Gen Li, Yang-Huan Shen, Wang-Ming
Peng and Liang Wang*
Dedication ((optional))
Abstract: An efficient Rh(III)-catalyzed direct C2-alkenylation of
indoles using readily available potassium vinyltrifluoroborate under
mild conditions has been developed. This protocol features wide
substrate scope and excellent functional group compatibility,
enabling a facile access to diverse terminal vinylindoles in moderate
to good yields. Furthermore, the C2-alkenylated indole can be easily
transformed into 9H-carbazole through a ring-closing metathesis
/aromatization cascade.
catalyzed direct ortho-olefination of indoles with unactivated
aliphatic olefins.[16] Apart from the C-H alkenylation with
alkenes,[17] hydroarylation of alkynes via C-H bond cleavage
have been demonstrated to be an alternative to install an alkenyl
group into indole molecules.[18] In this regard, Schipper,[19]
Yoshikai,[20] Zeng,[21] Nakao,[22] and others[23] successfully
employed various internal or terminal alkynes as coupling
partners in this transformation. Very recently, Ackermann report
a highly efficient C2-alkenylation of indoles and allenes in the
presence of Cobalt complexes.[24] Yoshino and Matsunaga
documented a powerful methodology for the synthesis of
tetrasubstituted alkenes containing indole moiety via a Co(III)-
catalyzed C-H alkenylation/directing group migration sequence.
Vinylindoles and their derivatives are valuable building blocks
that are often utilized in the construction of a large number of
biologically important compounds such as indole alkaloids,
carbazoles, and carbolines.[1] In particular, terminal vinylindoles
can be not only used in transformations such as [4+2]
cycloadditions, olefin metathesis and Heck-type reactions, but
also served as a handle to access various functional groups.[2]
Consequently, the development of mild and efficient methods for
the synthesis of functionalized vinylindole scaffolds has attracted
considerable attention in modern organic synthesis.[3] In this
context, transition metal-catalyzed direct C-H alkenylation of
indoles with alkenes or alkynes has emerged as one of the most
attractive protocols for preparation of alkenylated indole
derivatives.[4] For examaple, direct C2-alkenylation of indoles
and electron-deficient alkenes catalyzed by various metal
catalysts had been well established by the research groups of
Guant,[5] Ricci,[6] Muira,[7] Carretero,[8] and others.[9] Surprisingly,
however, only one example of Rh-catalyzed oxidative-Heck
coupling of N-acetylindole and styrene had been reported in
2010 by Glorius and co-workers.[10] Shortly after, Wang[11] and
Song[12] independently disclosed the regioselective C2-
alkenylation of indoles using styrene derivatives as alkenyl
sources. In 2014, Xu and Li reported an elegant approach
toward the synthesis of alkenyl substituted indoles via a Rh(I)-
catalyzed decarbonylative direct C-H olefination of indoles with
vinyl carboxylic acids.[13] Recently, selective fluoroalkenylation of
N-pyrimidyl indoles with readily available gem-difluorostyrenes
under mild conditions have been reported by Feng[14] and Li,[15]
respectively. Xu and Zhang disclosed an efficient Rh(III)-
[25]
Although considerable advances had been made, these
existing methods were only limited to constructing vinylindoles
with substituents at the alkene terminus (Scheme 1a). Therefore,
further development of new approaches for the assembly of
terminal vinylindole derivatives is still highly desirable.
Potassium vinyltrifluoroborate, which was commercially available
and air-stable, has been widely employed as a class of versatile
vinylating agent in Suzuki-Miyaura cross-coupling reactions in
the past decades, and proven to be powerful C2 building block
toward a variety of desirable styrene derivatives.[26] However, to
the best of our knowledge, there has been no previous report on
the C-H alkenylation utilizing potassium vinyltrifluoroborate.
Following our continuing interest in the indole chemistry,[27] we
herein developed a Rh-catalyzed direct C-H alkenylation of
indoles with potassium vinyltrifluoroborate for the synthesis of
terminal vinylindoles (Scheme 1b).
[a]
[b]
Key Laboratory of Optoelectronic Chemical Materials and Devices,
Ministry of Education, School of Chemical and Environmental
Engineering, Jianghan University, 8 Sanjiaohu Road, Wuhan
430056, China
E-mail: wangliang@mail.jhun.edu.cn
Title(s), Initial(s), Surname(s) of Author(s)
Department
Scheme 1. Transition metal-catalyzed direct C2-alkenylation of indoles.
Reaction optimization was initiated with 3-methyl-N-pyrimidyl
indole (1a) and potassium vinyltrifluoroborate (2) by employing
the commonly used Rhodium catalysts at 1 mol % catalyst
loading (Table 1). To our delight, the reaction did indeed occur
Institution
Address 2
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
o
with the use of AgOAc as oxidant in MeOH at 60 C for 12 h
under an air atmosphere affording the desired terminal
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vinylindole 3aa with 65% yield (entry 1). Further screening of
Ag(I) oxidants revealed that AgCO2CF3, Ag2O and AgBF4 were
inferior to AgOAc (entries 2-4). Subsequently, a brief survey of
the reaction media indicated that this catalytic system was
sensitive to the solvents. In contrast to MeOH, EtOH gave a
slightly lower yield (entry 5). Notably, changing ethanol to
trifluoroethanol (TFE) could obviously accelerate this catalytic
process, whereas only 33% of product was isolated (entry 6).
Gratifyingly, when carried out in a 1:1 mixture of MeOH/TFE, the
the corresponding products in a range of 61-75% yields (3ba,
3ca, 3ea, 3fa, 3ka, and 3oa ). Moreover, the reaction showed
good compatibility with strongly electron-withdrawing groups,
such as cyano (3ia) and nitro (3ja), which are of great value for
the synthesis of amines and acids. Good tolerance to the fluoro
(3la), chloro (3da, 3ga, and 3ma), and bromo (3ha, 3na, and
3pa) is especially noteworthy since they are useful functional
handle for further transformations through standard cross-
coupling reactions. Importantly, polycyclic indole substrate
reaction was completed in 8 h and the yield was improved to 70% participated in the direct C-H alkenylation successfully to furnish
(entry 7). The effect of temperature was further investigated,
which showed that lower reaction temperature could also enable
this transformation. Actually, 40 oC proved to be the most
suitable for the reaction, furnishing the vinylated product in 82%
yield (entry 10). It should be noted that the substituent at the C3-
position of indoles is very important for the success of the
desired reaction, as the reaction with indole without substituent
at the C-3 position resulted in a complicated mixture.
the expected product in 80% yield (3qa). The methyl group in
pyrimidine ring also proved to be no effect in the current catalytic
system and the expected product 3ra was obtained with 81%
yield. In addition, potassium isopropenyltrifluoroborate was also
efficient coupling partners for this transformation to give the
desired product 3sa in 46% yield.
Table 2. Scope of C3-substituted indoles.[a, b]
Table 1. Optimization of reaction conditions.[a]
Me
Me
H
H
H
[RhCp*Cl2]2
N
N
+
KF3B
Oxidant, Solvent
Temp., Air
H
N
N
N
N
1a
2a
3aa
entry
1
oxidant
AgOAc
AgTFA
Ag2O
solvent
MeOH
MeOH
MeOH
MeOH
EtOH
Temp [oC]
t [h]
yield [%][b]
60
60
60
60
60
60
12
12
12
12
12
4
65
37
21
0
2
3
4
AgBF4
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
5
63
33
70
67
39
82
50
6
TFE
7
MeOH/TFE = 1:1
MeOH/TFE = 1:1
MeOH/TFE = 1:1
MeOH/TFE = 1:1
MeOH/TFE = 1:1
60
60
60
40
25
8
8
12
6
9
10
11
8
24
[a] Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), [RhCp*Cl2]2 (0.002
mmol), oxidant (0.3 mmol), solvent (1.0 mL), Air ballon (1 L). [b] Isolated
yield.
With the optimal conditions in hand, we next turned our attention
to study the scope of indoles and generality of this process. As
shown in Table 2, a wide range of N-pyrimidyl indoles bearing
substituents with different electronic effects were found to be
suitable for the C-H alkenylation reaction. Substrates bearing
various electron-donating groups (e.g. Me, OMe, and OBn)
coupled smoothly with potassium vinyltrifluoroborate, affording
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), [RhCp*Cl2]2 (0.002
mmol), AgOAc (0.3 mmol), MeOH (0.5 mL), and TFE (0.5 mL), Air balloon
(1 L), 40 oC, 8-12 h. [b] Isolated yield. [c] Performed at 80 oC for 24 h.
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deuterium incorporation was also detected in the recovered 1a
(eq 2). These results suggest that a reversible C-H bond
cleavage process should be involved in this process.
Furthermore, the kinetic isotope effect was measured during
parallel reactions of 1a (or d-1a) with 2 under the standard
conditions. A value of 1.04 was obtained from the ratio of
desired product 3aa (eq 3), which suggested that the C-H bond
cleavage was not involved in the rate-determining step.
To further expand the substrate scope of this protocol, a broad
range of C3-substituted indoles under the optimal reaction
conditions were screened (Table 3). Indoles containing a variety
of alkyl groups at C3-position, regardless of their electronic and
steric properties, proceeded smoothly to deliver the terminal
vinylindoles in moderate to good yields (3ab-3ah). It should be
noted that allyl, ester and ether group on the alkyl chain of
indoles were well tolerated under the standard conditions (3af-
3ah). The direct C-H alkenylation was not restricted to the alkyl
substituted substrate, but also tolerated a 3-phenyl group (3ai).
We were pleased to find that both acetoxyl (3aj) and bromo (3ak)
group remained intact during the course of the reaction, giving
the corresponding products in 78% and 83% yields, respectively.
Unfortunately, the reactions of substituted indoles 1la and 1ma
with strongly electron-withdrawing groups, such as ester or
cyano at the C3-position did not proceed under the standard
conditions.
Table 3. Scope of C3-substituted indoles.[a, b]
Scheme 2. Isotopic experiments
On the basis of these experimental results,
a plausible
mechanism is proposed as described in Scheme 3. This process
is likely to be initiated by the coordination of the nitrogen atom of
pyrimidyl group to cationic rhodium complex A, which was
formed through the adsorption of chlorine by AgOAc. With the
assistance of the pyrimidyl group, the direct cyclometalation via
C−H bond cleavage affords a five-membered rhodacycle C.
Then,
the
following
transmetalation
with
potassium
vinyltrifluoroborate generates a new RhIII intermediate D, which
undergoes a reductive elimination to produce the desired
vinylindoles together with RhI species. Finally, the catalytically
active species A was regenerated by oxidation of RhI species in
the presence of AgOAc and oxygen.
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), [RhCp*Cl2]2 (0.002
mmol), AgOAc (0.3 mmol), MeOH (0.5 mL), and TFE (0.5 mL), Air balloon
(1 L), 40 oC, 8-12 h. [b] Isolated yield.
In order to gain some insight into the mechanism, the H/D
exchange experiment was conducted in CD3OD for 3 h, and 49%
deuterium incorporation at C2-position was observed (Scheme 2,
eq 1). When the reaction was performed in the presence of 2a,
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Scheme 3. Proposed mechanism
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As shown in Scheme 4, the synthesis of 9H-carbazole was
conducted to demonstrate the utility of this synthetic
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Scheme 4. Carbazole synthesis
In summary, we have developed an efficient method for
synthesis of valuable terminal vinylindoles via RhIII-caalyzed
direct C-H alkenylation of indoles with potassium
vinyltrifluoroborate under mild reaction conditions. This reaction
shows broad substrate scope and high functional group
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Keywords: Terminal vinylindole •Rhodium • C-H Alkenylation •
Potassium Vinyltrifluoroborate
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Chun-Ni Zhou, Hao-Hao Xie, Zi-Ang
Zheng, Yuan-Chao Xiao, Gen Li, Yang-
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Wang*
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Synthesis of Terminal Vinylindoles
via Rh(III)-Catalyzed Direct C-H
An efficient Rh(III)-catalyzed direct C2-alkenylation of indoles using readily
available potassium vinyltrifluoroborate under mild conditions has been developed.
This protocol features wide substrate scope and excellent functional group
compatibility, enabling a facile access to diverse terminal vinylindoles in moderate
to good yields. Furthermore, the C2-alkenylated indole can be easily converted into
9H-carbazole through a ring-closing metathesis /aromatization cascade.
Alkenylation
with
Potassium
Vinyltrifluoroborate
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