Construction of N-C Axial Chirality through Atroposelective C-H Olefination of N-Arylindoles by Palladium/Amino Acid Cooperative Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b02243

Direct construction of N-C axial chirality via Pd-catalyzed atroposelective C-H olefination of N-arylindoles is reported. The crucial role of chiral amino acid as a cocatalyst in the regio- and stereocontrol has been disclosed. In this reaction, a wide range of arylindoles and functional alkenes could be well tolerated. Moreover, the practicality and synthetic value of this process were demonstrated by the divers and simple transformations of the products.

Full text of DOI:10.1021/acs.orglett.9b02243

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Construction of N−C Axial Chirality through Atroposelective C−H
Olefination of N‑Arylindoles by Palladium/Amino Acid Cooperative
Catalysis
Jitan Zhang,* Qiaoqiao Xu, Jiaping Wu, Jian Fan, and Meihua Xie*
Key Laboratory of Functional Molecular Solids (Ministry of Educatifon), Anhui Key Laboratory of Molecular Based Materials,
College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241002, China
S
* Supporting Information
ABSTRACT: Direct construction of N−C axial chirality via Pd-catalyzed atroposelective C−H olefination of N-arylindoles is
reported. The crucial role of chiral amino acid as a cocatalyst in the regio- and stereocontrol has been disclosed. In this reaction,
a wide range of arylindoles and functional alkenes could be well tolerated. Moreover, the practicality and synthetic value of this
process were demonstrated by the divers and simple transformations of the products.
wing to the ubiquitous occurrence of axially chiral biaryl
scaffolds in the materials, natural products, and
of some more reactive C−H bonds in the heterocycles. Indeed,
following Murai and co-workers’ pioneering exploration on the
dynamic kinetic resolution of biaryl atropoisomers through
Rhodium-catalyzed atroposelective C−H alkylation with
olefins in moderate stereocontrol,11 the groups of Miller,12
You,13 Colobert,14 Waldmann,15 Cramer,16 Shi,17 and others18
have demonstrated their elegant works independently on the
synthesis of axially chiral biaryls via catalytic atroposelective
C−H functionalization. However, to the best of our knowl-
edge, the highly efficient and practical entry to construct N−C
axial chirality by the utilization of an enantioselective C−H
activation strategy was rarely disclosed.19−21 Encouraged by
the aforementioned outstanding findings and as our endeavor
toward the asymmetric C−H activation, we decided to offer an
efficient and general catalytic enantioselective approach to
deliver divers optically pure N−C axially chiral scaffolds with
the tolerance of a broader scope of coupling partners, which
would be of great value with respect to the synthetic efficiency
and potential application. Thus, we, herein, would like to
disclose an efficient means for the Pd-catalyzed atroposelective
C−H olefination of N-aryl heterocycles with the assistance of
catalytic chiral amino acid22 via desymmetrization, dynamic
kinetic resolution, and kinetic resolution procedures, thereby
achieving the direct construction of N−C axial chirality and
providing the first synthesis of N−C axially chiral arylindoles
via C−H activation (Scheme 1c).23 Noteworthy, both the
regiocontrol and stereoselective induction were impacted
significantly by the choice of the amino acid. Various indole
derivatives and functional alkenes, such as bulky internal and
trisubstituted alkenes, could be employed successfully in this
O
pharmaceuticals and the important role in organic synthesis
as chiral catalysts and ligands,1 the construction of these
compounds has attracted considerable attention. As a result, a
variety of practical methodologies for their direct construction
have been established.2 Meanwhile, the N−C axially chiral
backbones have been emerging as promising organic motifs,
which are prevalent in drugs, natural products, asymmetric
synthesis, and molecular devices (Scheme 1a).3 Besides, recent
studies evidenced that the efficient axial-to-point chirality
transfer could be achieved efficiently when the atropisomers
with chiral N−C axis were utilized in the reaction.4 However,
compared with the well-developed chiral biaryl skeletons, the
enantioselective construction of N−C axially chiral molecules
is far unexplored and presents a remarkable challenging task.
Accordingly, over the past decade, several strategies, such as
enantioselective N−C cross-coupling,5 asymmetric desymmet-
rization,6 kinetic resolution,7 and other fairly efficient stereo-
selective transformations,8 have been developed to access N−
C axial chirality. Despite these significant advances, the general
requirement of prefunctionalized substrates makes them
limited. Therefore, the development of novel and straightfor-
ward asymmetric catalytic methodologies for the establishment
of the chiral N−C axis is highly desirable.
Recently, transition-metal-catalyzed enantioselective C−H
bond functionalization has emerged as a new and powerful
avenue for developing asymmetric catalysis due to its atom-
and step-economic nature.9 Although the great progress has
been witnessed for the past several years, the enantioselective
synthesis of optically pure atropisomers,10 especially N−C
axially chiral skeletons via C−H bond activation, remains an
appealing challenge, which might arise from the low
atropostability of the N−C bonds and the competitive reaction
Received: June 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02243
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Important Compounds with a Chiral N−C Axis
and the Construction of N−C Axial Chirality via Catalytic
Enantioselective C−H Activation
Table 1. Optimization of the Reaction Conditions
b
c
entry
L
base
yield (%)
ee (%)
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L1
L1
L1
NaOAc
LiOAc
KOAc
NaHCO3
NaOPiv
NaTFA
KHCO3
59
53
58
77
67
34
65
29
0
32
50
45
40
62
60
69
92
91
90
94
91
92
92
91
d
9
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
−
10
11
12
13
14
15
L2
L3
L4
L5
L1
L1
L1
40
90
90
27
92
90
79
e
f
g
16
a
Reaction conditions: 1a (0.1 mmol), 2a (0.3 mmol), Pd(OAc)2
(0.01 mmol), L (0.02 mmol), BQ (0.01 mmol), base (0.2 mmol),
HFIP/HOAc (1:1, v/v, 1.0 mL), O2, 35 °C, 60 h. BQ =
benzoquinone. HFIP = hexafluoroisopropyl alcohol. TFE =
b
trifluoroethyl alcohol. Isolated yield by flash column chromatog-
c
d
raphy. The ee value was determined by HPLC. The C-3 alkenylated
asymmetric transformation. Furthermore, the attempt to
modify the product was carried out by enabling the formation
of several kinds of interesting chiral molecular structures.
We initiated our studies on the desymmetrization of 1-
phenyl-1H-indole-2-carbaldehyde 1a with butyl acrylate 2a
through asymmetric C−H olefination using chiral amino acids
as chiral ligands (Table 1, more details in the Supporting
Information). Gratifyingly, in the presence of 10 mol %
Pd(OAc)2, 20 mol % L-valine, 10 mol % BQ, and 2.0 equiv of
NaOAc in HFIP/HOAc (1:1, v/v) at 35 °C using O2 as the
terminal oxidant, the atroposelective C−H olefination/
desymmetrization afforded the desired product in 59% yield
and with 92% enantiomeric excess (entry 1). The sequential
screening of other bases demonstrated that NaHCO3 showed a
significant effect on the reaction to give the product in 77%
yield and 94% ee (entries 2−7), while a low yield was obtained
without the addition of bases (Table 1, entry 8). It is
noteworthy that the reaction did not give the desired product
but the C3-alkenylated indoles instead in the absence of L-
valine (entry 9). Given the crucial role of the ligand in this
transformation, we aimed to improve the efficiency of the
reaction by employment of different chiral amino acids, but it
failed (entries 10−13). Besides, the use of TFE/HOAc (1:1, v/
v) as the solvent was accompanied by a decrease in the yield
(entry 14). Last, either the absence of BQ or the increase of
the reaction temperature all resulted in the diminished yield
and enantioselectivity (entries 15−16).
e
f
indole was formed. TFE/HOAc (1:1, v/v, 1 mL) was used. Without
BQ. 45 °C.
g
metrization of N-aryl indoles under optimal reaction
conditions. By utilization of butyl acrylate 2a as the coupling
partner, an array of N-aryl indoles was well tolerated, and the
corresponding alkenylated products (3aa−3ja) could be
afforded in moderate to good yields with excellent
enantiocontrol (up to 96% ee). To our delight, its analogues
with 3,5-disubstituted phenyl rings could also participate in the
reaction, generating the corresponding products in excellent
enantioselectivity (3ka−3ma, up to 96% ee), albeit in
moderate yields. Meanwhile, the reactivity of a variety of
functional alkenes was also examined. The reaction could
proceed well with styrene and vinyl sulfone providing 3ab and
3ac in moderate yields with good to excellent enantioselectiv-
ity. Notably, sterically challenging alkenes, such as 1-phenyl-
1H-pyrrole-2,5-dione and 3-methylenedihydrofuran-2(3H)-
one, conducted this reaction smoothly to deliver the axially
chiral N-aryl indoles (3ad and 3ae) in reasonable yields with
excellent ee values (up to 98% ee). On the other hand, the
catalytic C−H olefination/dynamic kinetic transformation was
principally axially chiral but configurationally unstable since
sterically more or less unhindered N-aryl indoles were
achieved. The generality of this transformation was evidenced
by the tolerance of a wide range of substrates, providing the
desired products (3na−3sa) in moderate yields and satisfying
atroposelectivity (91% to 95% ee). Furthermore, the C−N
biaryl substrates could be varied to a series of N-aryl pyrroles.
It was found that both N-(o-tolyl)pyrrole and N-naphthale-
With the optimal reaction conditions in hand, we started to
explore the generality of this catalytic atroposelective C−H
transformation. As shown in Scheme 2, we first investigated the
substrate scope of atroposelective C−H olefination/desym-
B
DOI: 10.1021/acs.orglett.9b02243
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Letter
With the slight modification of the reaction conditions by
using AgTFA as the oxidant, the kinetic resolution (KR) of N-
arylindoles bearing sterically more demanding substituents was
realized via palladium-catalyzed C−H olefination (Scheme 3).
Scheme 2. Atroposelective C−H Olefination of N-aryl
a b
,
Indoles
Scheme 3. Atroposelective C−H Olefination/KR of N-Aryl
a b c
, ,
Indoles
a
Reaction conditions: rac-4 (0.1 mmol), 2 (0.3 mmol), Pd(OAc)2
(0.01 mmol), L-tert-leucine (0.02 mmol), AgTFA (0.2 mmol),
NaHCO3 (0.2 mmol), TFE/HOAc (1:1, v/v, 1.0 mL), N2, 60 °C,
a
Reaction conditions: 1 (0.1 mmol), 2 (0.3 mmol), Pd(OAc)2 (0.01
b
c
48 h. Isolated yield by flash column chromatography. s = ln[(1 −
mmol), L-valine (0.02 mmol), BQ (0.01 mmol), NaHCO3 (0.2
mmol), HFIP/HOAc (1:1, v/v, 1.0 mL), O2, 35 °C, 60 h. Isolated
yield by flash column chromatography. 1 (0.1 mmol), 2 (0.3 mmol),
b
C)(1 − ee4)]/[(1 − C)/(1 + ee4)], C = ee4/(ee4 + ee5).
c
Pd(OAc)2 (0.01 mmol), L-tert-leucine (0.02 mmol), AgTFA (0.2
In general, the C−H olefination/KR of rac-4 could tolerate all
the acrylate derivatives with different substitution patterns,
generating the alkenylated products (5a−5g) in excellent
yields with excellent enantioselectivities, and the enantioen-
riched starting materials were recovered in 45−76% ee. The
reaction could also be conducted smoothly using other
polarized olefins like phosphonate ester and vinyl sulfone to
give the chiral N-aryl indoles (5h−5i) in 48% yield, 97% ee
and 47% yield, 98% ee, respectively, and more than good
enantiomeric excess of the remaining 4a was obtained.
mmol), NaHCO3 (0.2 mmol), TFE/HOAc (1:1, v/v, 1.0 mL), N2, 60
d
°C, 72 h. 48 h.
nylpyrrole worked well to afford the atropoisomer (3ta and
3ua) with excellent enantiocontrol. Besides, other functional
olefins could also be tolerated in the oxidative coupling
reaction of N-aryl pyrroles, giving the corresponding optically
pure products (3uc and 3ud), albeit with a depressing yield for
3ud, largely due to the incomplete reaction of pyrrole.
C
DOI: 10.1021/acs.orglett.9b02243
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Letter
Moreover, the absolute configuration of the biaryl axis present
in 5i was confirmed through single crystal X-ray analysis and
found to be R (CCDC 1902839). Meanwhile, the conforma-
tional stability of 5i was also investigated by heating it at 130
°C for 48 h, yet no erosion of the ee value was observed (see
details in the Supporting Information). The functional group
tolerance of the reaction was further demonstrated through
screening a range of N-aryl indoles. The substrates bearing
substituents in both indole core and N-arene furnished the
corresponding products (5j−5m) in good to excellent yields
(40% to 48%) with little impact on enantioselectivity (97% to
99% ee). Excellent selectivity factors between 155 and 459
were reached in this transformation.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02243.
■
S
Experimental details, spectral and analytical data, copies
of 1H NMR and 13C NMR spectra, and X-ray
crystallographic structures for the new compounds
(PDF)
Accession Codes
CCDC 1902839 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The additional key advantage of the herein presented
strategy relies on the readily accessible late-stage derivatization
of the chiral N-arylindoles, which paves the way toward the
diverse application of our transformation (Scheme 4).
Scheme 4. Derivatization of Product 5i
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zhangjt@ahnu.edu.cn.
*E-mail: xiemh@mail.ahnu.edu.cn.
ORCID
Jitan Zhang: 0000-0002-8249-8329
Meihua Xie: 0000-0002-1921-790X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Natural Science
Foundation of China (Nos. 21702237 and 21272004) and
the Start-up Research Fund of Anhui Normal University for
financial support.
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D
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DOI: 10.1021/acs.orglett.9b02243
Org. Lett. XXXX, XXX, XXX−XXX