Copper(I)-Catalyzed Intramolecular Asymmetric Double C-Arylation for the Formation of Chiral Spirocyclic Bis-oxindoles

Article abstract of DOI:10.1021/acs.orglett.9b01373

A copper-catalyzed intramolecular asymmetric double C-arylation reaction was developed. The method provides a facile approach to chiral spiro bis-oxindoles in high yields and with good to excellent enantioselectivities. It also shows a broad substrate sco

Full text of DOI:10.1021/acs.orglett.9b01373

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper(I)-Catalyzed Intramolecular Asymmetric Double C‑Arylation
for the Formation of Chiral Spirocyclic Bis-oxindoles
Ting Liu,†,∇ Jiajie Feng,‡,∇ Chen Chen,† Zhuoji Deng,† Rajendraprasad Kotagiri,† Guangxiong Zhou,*,†
Xinhao Zhang,*,‡,§ and Qian Cai*,†
†International Cooperative Laboratory of Traditional Chinese Medicine Modernization and Innovative Drug Development of
Chinese Ministry of Education, College of Pharmacy, Jinan University, No. 601 Huangpu Avenue West, Guangzhou, 510632, China
‡Lab of Computational Chemistry and Drug Design, State Key Laboratory of Chemical Oncogenomics, Peking University Shenzhen
Graduate School, Shenzhen 518055, China
§Shenzhen Bay Laboratory, Shenzhen, 518055, China
S
* Supporting Information
ABSTRACT: A copper-catalyzed intramolecular asymmetric double C-arylation reaction was developed. The method provides
a facile approach to chiral spiro bis-oxindoles in high yields and with good to excellent enantioselectivities. It also shows a broad
substrate scope and good functional group tolerance. Density functional theory (DFT) calculations were conducted and
revealed that the enantioselectivity is determined at the oxidative addition of Cu(I) into the second C−I bond.
opper-catalyzed coupling reaction of aryl halides with
nucleophiles has become one of the most important
oxindoles from N1,N3-diarylmalonamides through a PIFA[PhI-
(TFA)2]-promoted intramolecular oxidative coupling (Scheme
1a).12,13 In 2014, Gong and co-workers developed such
reactions further, achieving an elegant catalytic asymmetric
version with chiral iodoarenes and oxidants (Scheme 1b).14
C
methods for the construction of aryl C−C and C−heteroatom
bonds.1 However, the research on copper-catalyzed asym-
metric aryl C−C coupling reaction is rare. That was until 2006,
when the pioneering study for copper-catalyzed asymmetric C-
arylation was reported by Ma and co-workers.2 In this work,
the enantioselective coupling of 2-methylacetoacetates with 2-
halotrifluoroacetanilides afforded the C-arylation product with
a quaternary chiral carbon center under the catalysis of CuI
and trans-4-hydroxyl-L-proline. Moderate to high yields and
enantioselectivities were obtained. No further research on
copper-catalyzed asymmetric coupling of aryl halides with C-
nucleophiles was reported since then,3−7 and it has remained a
great challenge in coupling chemistry.
Scheme 1. Copper-Catalyzed Intramolecular Asymmetric
Double C-Arylation for the Formation of Spiro Bis-
oxindoles
The spirooxindole moiety has been found as a core structure
in many bioactive natural products and synthetic pharmaceut-
ical compounds.8,9 The unique structures and broad biological
activities of such compounds have attracted great attention in
the synthetic community and many elegant methods have been
developed for their syntheses.10,11 Among various spiroox-
indoles, spirocyclic bis-oxindoles, in which two oxindoles are
connected through a single carbon at the 3-position, have a
very rigid and crowded spirocarbon stereogenic center. The
presence of two bulky aryl rings in such compounds makes
their synthesis challenging, but also provides a good
opportunity for intramolecular double C-arylation of N1,N3-
diarylmalonamides to access the spirocyclic structures. In 2012,
Du and Zhao reported a synthesis of racemic spirocyclic bis-
Received: April 18, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01373
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a
Table 1. Reaction Condition Screening
b
c
entry
L*
base
solvent
temperature (°C)
yield (%)
enantiomeric excess, ee (%)
1
2
3
4
5
6
7
8
9
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L4
L4
L4
L4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K3PO4
toluene
DMF
THF
80
80
80
80
80
60
60
60
60
60
60
60
60
60
60
60
82
80
95
98
34
76
91
72
60
80
86
72
91
88
−
33
20
20
23
53
62
56
52
41
<10
91
55
90
88
−
MeCN
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
10
11
12
d
13
e
14
f
15
g
16
67
85
a
Reagents and conditions: entries 1−14, 1a (0.2 mmol), CuI (0.04 mmol, 20 mol %), L* (0.08, 40 mol %), base (0.75 mmol, 3 equiv), solvent 2
b
c
d
e
f
mL, 24 h. Isolated yield. Determined by chiral HPLC. CuI (10 mol %), L* (20 mol %), 48 h. CuI (5 mol %), L* (10 mol %), 48 h. 1a′ was
g
used. 1a′′ was used.
However, because of the incompatibility of some functional
groups with oxidative conditions and the complexity of
reactions mediated by iodine reagents, serious limitations
and challenges to these synthetic efforts still persisted,
including limited substrate scope, moderate reaction efficiency,
and poor regioselectivity.15 Further efforts may also be needed
for further improvement of the enantioselectivity of such
reactions. Consequently, a highly efficient and enantioselective
method for the asymmetric synthesis of spiro bis-oxindoles will
be very useful.
To overcome the regioselective and enantioselective
problems, as well as substrate limitations in the asymmetric
synthesis of spiro bis-oxindoles, we envisioned that copper-
catalyzed double C-arylation may be a possible solution.
During our research on asymmetric coupling reactions, we
have developed several CuI/chiral ligand catalytic systems for
asymmetric aryl C−N/O bond coupling.16,17 The high
efficiency of these catalytic systems encouraged us for further
exploration in asymmetric C-arylation. In this work, an
enantioselective double aryl C−C coupling was developed
for the formation of chiral spirocyclic bis-oxindoles (Scheme
1c). The details of this research are reported in this paper.
The reaction of N1,N3-dimethyl-N1,N3-bis(2-iodophenyl)-
malonamide (1a) was taken as a model case. As shown in
Table 1, the CuI-catalyzed double C-arylation of 1a was
performed in toluene at 80 °C, with (1R,2R)-N1,N2-
dimethylcyclohexane-1,2-diamine (L1) as a chiral ligand and
Cs2CO3 as the base. The reaction proceeded smoothly,
furnishing the spirocyclic product (2a) in 82% yield and
33% enantiomeric excess (ee) (see Table 1, entry 1). Further
screening of solvents revealed that better enantioselectivity
(53% ee) but a reduced yield (34% yield) was obtained in 1,4-
dioxane (Table 1, entries 2−5). The poor yield in 1,4-dioxane
is mainly due to the competitive hydrolysis of amide bonds at
80 °C. Thus, we reduced the reaction temperature to 60 °C
and a better yield (76%) and enantioselectivity (62% ee) were
obtained (Table 1, entry 6). Other bases were explored, and
better reaction efficiency, but slightly reduced enantioselectiv-
ity, was achieved by changing the base to K3PO4 (see Table 1,
entry 7). Further screening of different chiral diamine ligands
(L2−L5) revealed that good yields and excellent enantiose-
lectivity were obtained with ligand L4 at 60 °C (Table 1, entry
11, 86% yield, 91% ee). Reducing the catalyst loading to 10
mol % or 5 mol % of CuI (20 mol % or 10 mol % of L4), the
reaction still worked well and the spiro bis-oxindole product 2a
was obtained in 91% yield and 90% ee or 88% yield and 88%
ee, respectively, albeit a prolonged reaction time of 48 h was
needed for complete conversion (see Table 1, entries 13 and
14). Although the formation of spiro bis-oxindole 2a was
supposed to proceed through a double C-arylation process, it is
noteworthy that no monocyclized byproduct was observed
during the reaction. This means that the monocyclized
intermediates are very reactive and can easily undergo a
second C-arylation step. To compare with the reaction of
diiodo substrate 1a, the reactions of dibromo substrate 1a′ and
N1-(2-bromophenyl)-N3-(2-iodophenyl)-N1,N3-dimethylmalo-
namide 1a′′ were explored under the same reaction conditions.
Spirocyclic product 2a was not detected when the less-reactive
dibromo substrate 1a′ was used (Table 1, entry 15).
Meanwhile, the reaction of 1a′′ proceeded smoothly to deliver
B
DOI: 10.1021/acs.orglett.9b01373
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the desired spirocyclic product 2a in moderate yield and good
enantioselectivity (Table 1, entry 16, 67% yield, 85% ee).
With these optimized conditions (Table 1, entry 11), we
explored the reaction scope with a variety of symmetric
diamide substrates, obtaining the results shown in Scheme 2.
A variety of unsymmetric diamide substrates were then
explored, and the results are shown in Scheme 3. The reactions
Scheme 3. Substrate Scope of Symmetric Diamides
Scheme 2. Substrate Scope of Symmetric Diamides
of unsymmetric diamides were expected to afford a mixture of
two monocyclic intermediates at the first C-arylation step, but
lead to the same chiral spirocyclic products through the second
C-arylation. Several examples of unsymmetrical diamides with
different substituents on the N atom or aryl rings were
explored. All these reactions were observed to proceed
smoothly under our optimized conditions, delivering the
corresponding spiro bis-oxindoles in high yields and with good
to excellent enantioselectivities. It is noteworthy that the
substrates bearing electron-withdrawing groups such as F,
ester, or carbonyl groups were found to deliver high
enantioselectivities at 40 °C. The absolute configuration of
product 4b was confirmed as S by X-ray crystallography.18 The
absolute configurations of other products were assigned by
analogy.
The formation of spirocyclic bis-oxindoles was supposed to
proceed through two C-arylation steps (see Scheme 4). The
nucleophilic carbon in monocyclic intermediate INT1,
activated by two amide bonds and a phenyl group, can be
easily deprotonated and undergo racemization under basic
conditions.2,6 Thus, the enantioselectivity is mainly determined
in the second C-arylation step. To understand the origin of the
high enantioselectivity, density functional theory (DFT)
calculations were conducted.18 The potential energy surface
of the second CuI/CuIII catalytic cycle is shown in Figure S1 in
the Supporting Information (SI).19 A systematic conforma-
tional search was conducted to obtain the lowest pathway
leading to the products (see Figures S1 and S2 in the SI). The
transition state for the oxidative addition (TS4) is generally
comparable or higher in free energy than the transition state
associated with reductive elimination (TS5), suggesting that
oxidative addition (TS4) is the enantioselectivity-determining
step.20 The most stable transition states for oxidative addition
First, different substituents such as methyl, ethyl, allyl, and
benzyl group on the N atom of amides were explored, and all
delivered the corresponding spiro bis-oxindoles in high yields
and with excellent enantioselectivity. A variety of diamide
substrates with symmetric substituents on both aryl rings were
explored. Both electron-donating and electron-withdrawing
groups, including alkyl, methoxy, halide, ester, and carbonyl
groups, were well-tolerated in the reactions and all afforded the
desired double C-arylation products in high yields and with
excellent enantioselectivity. In some cases, such as products 2g,
2j, 2k, and 2l, the enantioselectivities even reached >99% ee.
An exception was compound 1f, with two methyl groups ortho
to the two coupling iodo sites. The coupling reaction of 1f at
60 °C with Cs2CO3 as the base was slow, presumably because
of steric hindrance. However, such a problem could be solved
by switching the base to K3PO4, with which the spirocyclic
product 2f was produced in moderate yield and with excellent
enantioselectivity (48% yield, 91% ee). The absolute
configuration of product 2a was assigned as S by comparison
with reported data.14 The absolute configurations of other
products were assigned by analogy.
C
DOI: 10.1021/acs.orglett.9b01373
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In summary, we have developed a copper-catalyzed double
C-arylation process for the enantioselective formation of spiro
bis-oxindoles. This method provides a highly efficient and
enantioselective approach to the asymmetric synthesis of spiro
bis-oxindoles. Further explorations and applications of this
method are in progress in our laboratory.
Scheme 4. Proposed Reaction Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01373.
Full experimental and characterization data, including
1H and 13C NMR for all the new compounds, chiral
HPLC spectra for the products (PDF)
Accession Codes
CCDC 1912332 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.
(TS4-R and TS4-S) leading to two enantiomers are depicted
in Figure 1. The relative free energy of TS4-R is higher than
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: guangxzh@Sina.com (G. Zhou).
*E-mail: zhangxinhao@pku.edu.cn (X. Zhang).
*E-mail: caiqian@jnu.edu.cn (Q. Cai).
ORCID
Xinhao Zhang: 0000-0002-8210-2531
Qian Cai: 0000-0002-5700-3275
Author Contributions
∇These authors contributed equally.
Notes
The authors declare no competing financial interest.
Figure 1. Structures of TS4-R and TS4-S. Relative free energies and
enthalpies (shown in parentheses) are given in units of kcal/mol, and
dihedral angles and distances are presented in units of degrees and Å,
respectively.
ACKNOWLEDGMENTS
■
The authors are grateful to the National Natural Science
Foundation (Grant Nos. 21772066, 21572229), Guangdong
Special Support Program (No. 2017TX04R059) and Shenzhen
STIC (No. JCYJ20170412150343516) for their financial
support.
that of TS4-S, which is in good agreement with the
experimental observed reference on the S-product. In TS4,
CuI adopts a tetrahedral geometry. The substrate INT2
complex with CuI has two orientations and the main difference
between them is the orientation of the methyl group (C2) of
the amine. The dihedral angle ψ(C1−Cu−N−C2) in TS4-R is
10.1°, indicating that the methyl group and the incoming aryl
group are eclipsed, and repulsion between the methyl group
and the aryl group destabilizes TS4-R. On the other hand, in
TS4-S, the corresponding methyl group and aryl group adopt a
comfortable staggered conformation, with a dihedral angle of
ψ(C1−Cu−N−C2) = 50.5°. The repulsion in TS4-R can also
be seen in the fact that the Cu−N bond in TS4-R is longer that
in TS4-S. Another effect of discrimination is the repulsion
between the indole with the 1,2-diamine ligand. As shown
from the top view of the transition states, the indole ring of
TS4-R occupies a more crowded position which suffers from
the repulsion of those H atoms of 1,2-diamine ligand. By
switching the face, the indole in TS4-S can be accommodated
at a sterically relaxed position.
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DOI: 10.1021/acs.orglett.9b01373
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