Copper-catalyzed desymmetric intramolecular Ullmann C-N coupling: An enantioselective preparation of indolines

Article abstract of DOI:10.1021/ja306631z

The first highly enantioselective copper-catalyzed intramolecular Ullmann C-N coupling reaction has been developed. The asymmetric desymmetrization of 1,3-bis(2-iodoaryl)propan-2-amines catalyzed by CuI/(R)-BINOL-derived ligands led to the enantioselectiv

Full text of DOI:10.1021/ja306631z

Communication
pubs.acs.org/JACS
Copper-Catalyzed Desymmetric Intramolecular Ullmann C−N
Coupling: An Enantioselective Preparation of Indolines
Fengtao Zhou,† Jiajia Guo,‡ Jianguang Liu,† Ke Ding,† Shouyun Yu,*,‡ and Qian Cai*,†
†Key Laboratory of Regenerative Biology and Institute of Chemical Biology, Guangzhou Institutes of Biomedicine and Health,
Chinese Academy of Sciences, No. 190 Kaiyuan Avenue, Guangzhou Science Park, Guangzhou 510530, China
‡State Key Lab of Analytical Chemistry for Life Science, Institute of Chemical Biology and Drug Innovation, School of Chemistry and
Chemical Engineering, Nanjing University, No. 22 Hankou Road, Nanjing 210093, China
S
* Supporting Information
high yields and excellent enantioselectivity, through a
desymmetrization strategy.
ABSTRACT: The first highly enantioselective copper-
catalyzed intramolecular Ullmann C−N coupling reaction
has been developed. The asymmetric desymmetrization of
1,3-bis(2-iodoaryl)propan-2-amines catalyzed by CuI/(R)-
BINOL-derived ligands led to the enantioselective
formation of indolines in high yields and excellent
enantiomeric excesses. This method was also applied to
the formation of 1,2,3,4-tetrahydroquinolines in high yields
and excellent enantioselectivity.
We envisioned that, with the assistance of a chiral ligand, the
copper-catalyzed desymmetric intramolecular Ullmann C−N
coupling reaction of 1 would lead to the enantioselective
formation of product 2 bearing a chiral quaternary carbon
center (Scheme 1).
Scheme 1. Asymmetric Ullmann C−N Coupling Reaction via
the Desymmetrization Strategy
opper-catalyzed Ullmann-type coupling reactions have
Cbeen extensively applied in both academia and industry,
especially in recent years when great progress in the
development of mild reaction conditions has been made.1
However, achieving enantioselectivity in copper-catalyzed
Ullmann-type coupling reactions remains a significant chal-
lenge.2,3 So far, only one example for catalytic asymmetric
Ullmann-type coupling reaction was reported by Ma et al.3 in
2006. In their reaction, a quaternary stereochemical center was
directly formed by coupling 2-halotrifluoroacetanilides with 2-
methylacetoacetates. Satisfactory yields and good enantiosele-
tivity were achieved at −45 °C. However, since other Ullmann-
type couplings such as C−N, C−O, and C−S couplings did not
involve direct formation of new stereochemical centers, little
attention was focused on the asymmetric pattern of these
copper-catalyzed coupling reactions.
Asymmetric desymmetrization has offered a general and
powerful method for the enantioselective synthesis of chiral
molecules.4 This strategy has been successfully implemented
through organocatalytic asymmetric reactions5 such as the
Stetter reactions and Michael additions as well as transition-
metal-catalyzed asymmetric reactions,6−8 including Pd-cata-
lyzed coupling reactions,6 copper-catalyzed reactions,7 and so
forth. Recently, Pd-catalyzed asymmetric desymmetric Buch-
wald−Hartwig reactions for enantioselective N-arylation9,10
have been reported, which, however, afforded the desired
products in only moderate enantiomeric excess (ee) values.9d,e
To the best of our knowledge, no example of asymmetric
desymmetrization was reported in copper-catalyzed Ullmann-
type coupling reactions. In this paper, we would like to report
the first example of copper-catalyzed asymmetric Ullmann C−
N coupling reaction, which afforded chiral indolines in both
The investigation was started with the desymmetric reaction
of ethyl 2-(2-iodobenzyl)-2-amino-3-(2-iodophenyl)propanoate
1a. In a brief screening of the chiral ligands and reaction
conditions, we found that the reaction proceeded smoothly
under the catalysis of 10 mol% CuI and 20 mol% (R)-BINOL
(L1) in 1,4-dioxane at room temperature, with 150 mol%
K3PO4 as the base.11,12 The desired product 2a was afforded in
54% yield and 40% ee (Table 1, entry 1). This result prompted
us to further test several (R)-BINOL-derived ligands. The
reactions were repeated under the same conditions by utilizing
ligands L2−L4 with bulky aryl substituents in the 3,3′-positions
of the BINOL backbone, and quenched after 10 h, disregarding
the conversion ratio. As shown in Table 1, a remarkable
improvement in the enantioselectivity was observed, albeit in
lower yields. In all these new tests, the ee values of the products
reached about 70% (Table 1, entries 2−4). It was also observed
that ligand L4, which bears electron-withdrawing trifluor-
omethyl groups in the aryl rings, accelerated the reaction and
afforded the desired product in relatively higher yield. This was
perhaps caused by the enhancement of the acidity of the ligand
which made it easier to be deprotonated and coordinate with
CuI. Based on these observations, we then tested two other
ligands, L5 and L6, which bear even more bulky substituents in
the 3,3′-positions than L2−L4. As expected, the CuI-L5-
catalyzed reaction proceeded very smoothly and afforded the
desired product in both high yield and very good
Received: July 7, 2012
Published: August 22, 2012
© 2012 American Chemical Society
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a
a
Table 1. Screening Reaction Conditions
Table 2. Scope of the Substrates
b
c
entry ligand base or additive
solvent
yield (%) ee (%)
1
L1
L2
L3
L4
L5
L6
L5
L5
L5
L5
L5
L6
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2CO3
Cs2CO3
Cs2CO3
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
MeCN
51
20
16
34
78
12
92
90
61
53
95
91
40
68
70
71
89
98
77
73
85
87
87
96
2
3
4
5
6
7
8
toluene
9
THF
10
11
12
1,4-dioxane
1,4-dioxane
1,4-dioxane
a
Reagents and reaction conditions: 1 (0.25 mmol, 1.0 equiv), CuI
(0.025 mmol, 10 mol%), L6 (0.05 mmol, 20 mol%), Cs2CO3, (0.375
mmol, 1.5 equiv), 1,4-dioxane (1 mL), 10 h. Yields given are isolated
yields. Enantiomeric excesses were determined by HPLC analysis
(Chirapak AD-H or OD-H column).
a
Reagents and reaction conditions: 1a (0.25 mmol, 1.0 equiv), CuI
(0.025 mmol, 10 mol%), ligand (0.05 mmol, 20 mol%), base, (0.375
b
c
mmol, 1.5 equiv), solvent (1 mL), 10 h. Isolated yields. Determined
by HPLC analysis (Chirapak AD-H column).
configurations of these products were assigned to be S in
analogy to that of 2g, whose absolute configuration was
ambiguously determined through the X-ray crystal diffraction
analysis.13 Further exploration of substrate scope revealed that
the reactions of 1j−l, which bear a hydrogen atom at the
tertiary prochiral center, also proceeded well and afforded the
corresponding products 2j−l in high yields and excellent
enantioselectivity, while other substrates like 1m−o furnished
the desired products 2m−o in high yields but slightly lower
enantioselectivity.14
Encouraged by the success of enantioselective preparation of
the chiral indoline products, we then explored a similar
intramolecular desymmetric Ullmann C−N coupling reaction
for the enantioselective synthesis of 1,2,3,4-tetrahydroquinoline
derivatives. As shown in Scheme 2, although they were less
reactive than their one-carbon-shorter counterparts, 3a and 3b
could undergo desymmetrization smoothly under the catalysis
of 20 mol% CuI and 40 mol% L5, to afford the corresponding
enantioselectivity (78% yield and 89% ee, Table 1, entry 5),
while the CuI-L6-catalyzed reaction proceeded very slowly and
afforded the desired product in only 12% yield, but in excellent
enantioselectivity (98% ee, Table 1, entry 6). With L5 as the
ligand, further screening of the solvents were performed and
the results revealed that the reaction also worked well under
several tested solvents such as MeCN, THF, and toluene
(Table 1, entries 7−9), while 1,4-dioxane turned out to be the
best solvent for the high enantioselectivity. It seemed that the
base played an important role in affecting the rate of the
reaction. Cs2CO3 was found to accelerate the reaction most
significantly and the product was obtained in higher yield and
with only a slightly loss of enantioselectivity as compared with
that of K3PO4 (Table 1, entries 11 and 5). Similar result was
observed with L6 as the ligand and the desired product was
afforded in both excellent yield and excellent enantioselectivity
(91% yield, 96% ee) in 1,4-dioxane at room temperature with
Cs2CO3 as the base (Table 1, entry 12).
With the optimized conditions in hand, we then explored the
reaction scope with a series of substrates and the results are
shown in Table 2. First, the substrates bearing different ester
groups at the quaternary prochiral center were tested and all
delivered the corresponding products in high yields and
excellent enantioselectivity (Table 2, 2a−c). Both electron-
withdrawing and electron-donating substituents in the aryl rings
of the substrates were well tolerated (Table 2, 2d−i ). In some
cases, the ee values of the desired products even reached more
than 99% (Table 2, 2e and 2g). Furthermore, the absolute
Scheme 2. Enantioselective Formation of 1,2,3,4-
Tetrahydroquinoline Derivatives via Intramolecular
Desymmetric Ullmann C−N Coupling Reaction
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1,2,3,4-tetrahydroquinolines 4a and 4b with quaternary chiral
centers in high yields and excellent enantioselectivity. Under
the same condition, the reaction of 1,5-bis(2-iodophenyl)-
pentan-3-amine 3c also proceeded smoothly and provided the
desired product 4c with a tertiary chiral center in high yield and
good enantioselectivity.
To further elucidate the application of such desymmetric
reactions in organic synthesis, we chose the synthesis of the
chiral spirocyclic compound 7 as an example. As shown in
Scheme 3, compound 7 was synthesized through a simple
three-step transformation from the desymmetric product 2a.
with Cs2CO3 as the base. This method was also applied to the
enantioselective synthesis of 1,2,3,4-tetrahydroquinoline deriv-
atives. Further exploration and application of this reaction in
organic synthesis is underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
Full experimental and characterization data, including H and
■
S
1
13C NMR for all the new compounds, chiral HPLC spectra for
the products, and crystal structure (CIF) of 2g. This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 3. Example for the Synthesis of Chiral Spirocyclic
Compounds
AUTHOR INFORMATION
Corresponding Author
yushouyun@nju.edu.cn; cai_qian@gibh.ac.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the 100-talent program of CAS,
National Natural Science Foundation (Grants 21002102 and
21102072), National S&T Major Special Project on Major New
Drug Innovation (Grant 2011ZX09102-010), and Guangdong
Natural Science Foundation (Grant S2011010003705) for
financial support. We also thank Prof. Jingsong Liu in
Guangzhou Institutes of Biomedicine and Health (GIBH),
Chinese Academy of Sciences, for the X-ray experiments, and
Prof. Fayang Qiu (GIBH) for the helpful discussions.
Based on the literature reports8b,15 and our experimental
observations, we propose a plausible mechanism to account for
the chirality induction. As shown in Figure 1, the CuI may
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products, see Supporting Information.
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