Palladium-catalyzed regioselective C-H fluoroalkylation of indoles at the C4-position

Article abstract of DOI:10.1039/c7cc01274h

An exclusive catalytic C4-selective fluoroalkylation of indoles with highly active (1H, 1H-perfluoroalkyl)mesityliodonium triflate has been described. The key to its high regioselectivity is the appropriate choice of an easily accessible, cheap and removable directing group at the C3 position in the presence of a Pd(OAc)₂ catalyst. Besides indole fluoroalkylation, the application of this strategy in other heteroarenes such as benzo[b]thiophene is also described.

Full text of DOI:10.1039/c7cc01274h

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Palladium-Catalyzed Regioselective C-H Fluoroalkylation of
Indoles at C4-Position
Received 00th January 20xx,
Accepted 00th January 20xx
a
a*
Arun Jyoti Borah and Zhuangzhi Shi
DOI: 10.1039/x0xx00000x
www.rsc.org/
An exclusive catalytic C4-selective fluoroalkylation of indoles with
highly active (1H, 1H-perfluoroalkyl)mesityliodonium triflate has
been described. The key to this high regioselectivity is the
appropriate choice of an easily accessible, cheap and removable
2
directing group at C3 position in the presence of Pd(OAc) catalyst.
Besides indole fluoroalkylation, the application of this strategy to
other heteroarenes such as benzo[b]thiophene is also described.
Fluorine containing organic compounds plays a vital role in the
search for new active pharmaceutical and agrochemical
compounds, as incorporation of fluorine-containing functional
groups into organic molecules can often bring about many
profound changes in their physical, chemical, and biological
1
2
properties. In the past decade, trifluoromethylation process has
been investigated extensively, however, the introduction of other
3
alkyl groups bearing CF group remains largely unexplored. Indole
3
framework as a core structure in many pharmaceuticals, synthesis
of fluoroalkylated indole derivatives have attracted much attention
4
to the synthetic chemists. In 2014, Antonchick and co-workers
reported
a
remarkable method for the synthesis of C2-
indoles with the help of alkene
Figure 1 Synthesis of fluoroalkylated indoles.
trifluoroethylated
heterocycles based on a zinc sulfinate as the active reagent via
4
e
trifluromethylation (Fig. 1a). Undoubtedly, the development of
new methods for the direct and regioselective installation of
fluoroalkyl group at indole core is still in high demand.
10
radical process. Compared to these forcing reaction conditions,
Novák group recently have reported very mild protocols for
11a
11b-c
trifluoroethylation of indoles (Fig. 1b) and other arenes
,2,2-trifuoroethyl(mesityl)-iodonium triflate salt.
As part of our continuous effort on developing C-H bond
using
The installation of trifluoroethyl group at aromatic core is widely
2
based on traditional cross-coupling methods with CF
3
CH X (X = I,
2
5
OTs, NH
2
) reagent. However, direct installation of a trifluoroethyl
12
functionalization at the benzene core of indoles, we decided to
tackle a synthetic route to address C4-fluoroalkylation of indoles.
The challenges of C4-selectivity for this strategy results from the
formation five-membered metalacycle through C-H bond cleavage
at the C2-position which is usually more preferable to the
6
group by C-H bond activation is undoubtedly the most desirable
7
route. The first example in this regard was reported by Ackermann
et al. utilizing 8-aminoquinoline as
a
directing group and
trifluoroethyliodide as the alkyl sources. Subsequently Liu and co-
workers described Catellani-type palladium-catalyzed
8
a
13
corresponding six-membered metallacycle at the C4-position. So
9
alkenylation/trifluoroethylation cascade reaction. Baran’s group
have also described fluoroalkylation of selected nitrogen
selection of a suitable directing group at C3 position of indole is the
key to success. We believed that the simple indole building blocks
could form the starting point for a strategy that would involve
convenient introduction and removal of a directing group to
append fluoroalkyl group at C4 position. Herein, we described a
facile approach for this transformation by installation of a suitable
directing group at C3 position of indoles (Fig. 1c).
a.
State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, 210093, China
†Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, characterization of starting materials and products and
crystallographic data. See DOI: 10.1039/x0xx00000x
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a
a
Table 1 Optimization studies
Table 2 Scope of various indole
O
R
F3C
O
R
[
Pd], Additive
DCM, rt
F3C
N
OTf
N
Me
I
Me
1
Mes
3
2a
X-ray structure of 3aa
R = Me)
(
[
Pd]
Additive
(equiv)
yield
(%)
entry
R
b
(
mol%)
1
2
3
4
5
6
7
8
9
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
(10)
(10)
(10)
(10)
(10)
(10)
TsOH∙H O (1.0)
2
16
0
-
AcOH (1.0)
PivOH (1.0)
TFA (1.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
0
0
67
83
54
0
Pd(OAc)
2
PdCl
2
(10)
-
H (1a-1)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
(10)
(10)
(10)
(10)
35
0
1
1
1
0
1
2
OH (1a-2)
t
O Bu(1a-3)
0
i
NPr
2
(1a-4)
0
a
Reaction conditions: 1 (0.1 mmol), 2a (1.1 equiv), Pd-catalyst (10 mol%)
b
solvent (1.5 mL), 24 h, Ar atmosphere; Isolated yield.
We began our investigations by examining the reactivity of
commercially available 1-(1-methyl-1H-indol-3-yl)ethanoneindole
1a with 2,2,2-trifuoroethyl(mesityl)-iodonium triflate salt (2a, 1.1
equiv) in presence of palladium catalyst (Table 1). Delightedly we
observed the formation of C4-trifluoroetylated product 3aa in 16%
yield in presence of catalytic Pd(OAc)
2
(10 mol%) and 1.0 equiv
TsOH∙H O as an additive in DCM at room temperature (entry 1) and
2
the product was confirmed by X-ray analysis. The use of acid
additive is crucial in the process and less acidic acids were found
ineffective for this transformation (entries 3-4). We found that TFA
14
was the best additive and 83% yield of 3aa was generated in the
presence of 3.0 equiv TFA (entry 6). Other palladium sources such
2
as Pd(TFA) were also proven successful in this reaction, albeit in
low reactivity (entry 7). Control experiments revealed that the
reaction does not occur in absence of the catalyst (entry 8). Indole
1
5
1
a-1 bearing the formyl group at C3 position afforded low yield of
the expected C4 trifluoroethylation product (entry 9). However, the
experiments were found unsuccessful with other directing groups
like carboxylic acid 1a-2, ester 1a-3 and amide 1a-4 (entries 10-12).
These results confirmed the importance of the acetyl moiety for this
high regioselectivity.
a
Reaction conditions: 10 mol% Pd(OAc)
2
, 1a (0.1 mmol), 2a (0.11 mmol),
CH
2
Cl
5 mol% Pd(OAc)
highlighting the potential of this process in combination with
2
(1.5 mL), TFA (3equiv), 24 h, Ar atmosphere, Isolated yield;
b
c
1
2
; 1 equiv TFA; NR=no reaction
With the best conditions in hand, we have examined the scope of further conventional cross-coupling reactions. It has been observed
indole motif for this direct C4-trifluoroethylation process (Table 2). that functional groups possessing coordination ability to the Pd
Cross-coupling reactions of 2,2,2-trifuoroethyl(mesityl)-iodonium require comparatively higher loading of the catalyst (15 mol%, 1d
triflate (2a) with a broad range of indoles (1b-1v) were first and 1h) to achieve satisfactory yield. The compatibility of boron
examined. Various functional groups could be tolerated under the ester functionality (1j) was not appreciable and provided only 42%
present reaction condition. Electron-neutral and indoles bearing of the desired product. In case of C6-substitution, the electron
electron donating substituents particularly at C2 and C7 position donating groups exhibited comparatively superior results (1n-1o,
(1a-1i and 1k-1m) underwent facile trifluoroethylation affording the 60-70%) over halo substituents (1p-1r, 48-55%). However methoxy
desired products in 67-85% yield. The halogen-containing motifs (F, group at this position exerted lower efficiency (1s). Highly electron
Cl and Br) are well tolerated under the reaction condition (1e-1g),
withdrawing cyandide substitution (1t) did not undergo the
2
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transformation. The C5-substituted indoles greatly suffer from
steric congestion and furnished very low yield of the desired
trifluoroethylated product (1u-1v, 30-38%). The protocol is highly
regioselective and in all cases only C4-isomer formation was
observed. Gratifyingly, the present strategy can be suitably utilized
in C4-trfluroethylation of 3-benzo[b]thiophene derivative 1w
1
6
affording the desired product 3wa in 80% yield (eq 1).
a
Scheme 1 Synthetic Applications. (a) 10 mol% Pd(OAc)
1.1 equiv), CH Cl , TFA (3 equiv), 24 h, Ar atmosphere; (b) p-TsOH.H
equiv), Etylene glycol, dry benzene, reflux, 6h; (c) 2,6-di-tert-butylpyridine (2
equiv), 2a (1.3 equiv), CH Cl , RT.
2
, 1a (1 equiv), 2a
(
2
2
2
O (1.1
Then we have extended our observations with different
analogues of trifluroalkylating coupling partners. Using Umemoto
2
2
1
7
A major benefit of this mild fluoroalkylation process is its
amenability to gram-scale applications (Scheme 1). Under the
standard conditions, gram-scale synthesis of 2a in high yield (82%
brsm) proved the practicality of the process. With the free position
at C3 of the indoles framework, they can be subjected to further
transition metal catalyzed cross coupling processes. We successfully
protocol , we have synthesized two more (1H, 1H-
Perfluoroalkyl)mesityliodonium triflates (2b and 2c). To the best of
our knowledge, the synthetic utility of these electrophilic
fluoroalkytion reagents have never been reported. We have
observed that both 2b and 2c are very effective in the present
investigation and their reaction with 1-(1-methyl-1H-indol-3-
yl)ethanoneindole 1a under the standard reaction conditions
provided the desired C4-products 3ab and 3ac in good yields (eqs 2-
1
1a
exemplified trifluroetylation of 4aa using Novák’s protocol.
Gratifyingly, the desired bi(trifluoroethylated) product 5aa was
isolated in 67% yield.
3). However, C4-trifluoromethylation of our indole substrate 1a
In order to illustrate the selectivity of our reaction, we proposed
a catalytic cycle as shown in Figure 2. Palladium species coordinates
the acetyl of the substrate, followed by (1) C-H activation at indole
C4 position; (2) oxidative addition by 2,2,2-trifuoroethyl(mesityl)-
iodonium triflate salt; and (3) reductive elimination to C4-arylation
using Togni’s reagent under the standard reaction protocol was
completely unsuccessful.
II
products, and regeneration of the active Pd species. The selection
of acetyl directing group is crucial to the relative stabilities of B and
B', which affect the selectivity of reaction.
We found the removal of acetyl group readily accomplished by a
reverse Friedel-Crafts reaction in the presence of TsOH and glycol
and the corresponding products were obtained in good to excellent
yields (Table 3). Notably, in situ removal of the directing group was
18
also found very effective. The easy removal of the directing group
from the products under mild reaction conditions led us to realize a
full synthetic utility of this method.
a
Table 3 Selected examples for removal of directing group
Figure 2 Proposed catalytic cycle
In conclusion, we have developed a mild synthetic protocol for
the direct installation of fluoroalkyl groups at C4-position of indoles
by palladium-catalyzed C-H activation with the aid of a removable
directing group. The method exhibits moderate to high yields of the
desired product with good functional group tolerance. We see this
as a highly complementary and competitive strategy to traditional
approaches, and current studies are focused on further
development of new cross-coupling reactions at benzene core of
the indole.
Acknowledgement:
We thank the “1000-Youth Talents Plan”, the “Jiangsu Specially-
Appointed Professor Plan”, and NSF of China (Grant 2167020084)
for financial support.
a
2
Reaction conditions: 3 (0.1 mmol), p-TsOH.H O (1.1 equiv), etylene glycol
0.1 mL), dry benzene, reflux, 6h
(
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4
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