Asymmetric copper(I)-catalyzed azide-alkyne cycloaddition to quaternary oxindoles

Article abstract of DOI:10.1021/ja4066656

We report a highly enantioselective Cu(I)-catalyzed azide-alkyne cycloaddition via asymmetric desymmetrization of oxindole-based 1,6-heptadiynes, which furnishes quaternary oxindoles bearing a 1,2,3-triazole-containing moiety with 84-98% ee.

Full text of DOI:10.1021/ja4066656

Communication
pubs.acs.org/JACS
Asymmetric Copper(I)-Catalyzed Azide−Alkyne Cycloaddition to
Quaternary Oxindoles
Feng Zhou,† Chen Tan,† Jing Tang,† Yan-Yan Zhang,‡ Wei-Ming Gao,† Hai-Hong Wu,† Yi-Hua Yu,‡
and Jian Zhou*,†
†Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
3663 North Zhongshan Road, Shanghai 200062, China
‡Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, 3663 North Zhongshan
Road, Shanghai 200062, China
S
* Supporting Information
Scheme 1. Desymmetrization of Dialkynes by CuAAC
ABSTRACT: We report a highly enantioselective Cu(I)-
catalyzed azide−alkyne cycloaddition via asymmetric
desymmetrization of oxindole-based 1,6-heptadiynes,
which furnishes quaternary oxindoles bearing a 1,2,3-
triazole-containing moiety with 84−98% ee.
ince Kolb, Finn, and Sharpless introduced the concept of
S
click chemistry,1 Cu(I)-catalyzed azide−alkyne cyclo-
addition (CuAAC), independently discovered by the Meldal2a
and Sharpless2b laboratories, has been intensively studied and
has found applications in many areas of research.3 Apart from
serving as a powerful strategy for the modular assembly of new
molecular entities, CuAAC provides a facile and selective
synthesis of 1,4-disubstituted 1,2,3-triazoles, which have proven
to be a unit of significant pharmacophoric activity,3a and a
variety of pharmaceutically active compounds featuring a 1,2,3-
triazole-containing moiety at the chiral center have been
identified.3b In view of the versatility of CuAAC in medicinal
chemistry and life and material sciences, it is very important
and urgent to explore the corresponding catalytic asymmetric
studies, as a highly enantioselective CuAAC would facilitate the
synthesis of related biologically active compounds and chiral
materials, provide a better understanding of the mechanism of
this highly useful bond-forming reaction, and helpfully open up
new synthetic opportunities for CuAAC.
carbon quaternary stereogenic center is formed, the catalytic
asymmetric construction of which is a formidable task in the
field of asymmetric catalysis.6
Two major challenges had to be tackled to accomplish the
desired reaction. First, excellent enantiotopic group discrim-
ination in the unprecedented catalytic intermolecular desym-
metrization of prochiral dialkynes had to be realized. It is well-
known that enantioselective catalysis based on functionalization
of C−C triple bonds is very challenging,7 and previous work in
catalytic desymmetrization of dialkynes focused on intra-
molecular transformations.8 Since the pioneering work of
Tanaka and Fu,8a several successful desymmetrizations of
diynes via intramolecular cyclization have been independently
reported by the groups of Yamada,8b Czekelius,8c and
Hennecke,8d but to the best of our knowledge, the
corresponding intermolecular process remains unexplored to
date. Second, the formation of undesired ditriazole products
had to be suppressed. Because of the rapid interaction between
CuI and the alkyne group, the desired chiral monotriazole
product can further react with an azide to form the achiral
ditriazole. This is a severe problem, as evidenced by the
mechanistic studies by Fokin and Finn showing that statistical
mixtures of mono- and ditriazoles were obtained in the CuAAC
of a malonate derived 1,6-heptadiyne.9 In view of these
challenges, we carried out a research program to explore
asymmetric CuAAC. Herein we disclose a highly enantiose-
lective CuAAC involving catalytic desymmetrization of
oxindole-based 1,6-heptadiynes to furnish quaternary oxindoles.
Very surprisingly, despite the significant achievements in
click chemistry, asymmetric CuAAC is largely unexplored. To
date, only one example has been reported, in which Fokin and
Finn pioneered the kinetic resolution of α-azides and the
catalytic desymmetrization of gem-diazides with modest
selectivity.4 In the latter case, the desired monotriazole
products were obtained in up to 25% yield with 59% ee, as
the formation of achiral ditriazoles dominated. Distinct from
their research, we envisioned that enantioselective desymmet-
rization of prochiral dialkynes would be a promising strategy for
the development of a highly enantioselective CuAAC (Scheme
1). The attractive features of this approach include ready access
to various types of prochiral dialkynes and, more importantly,
the installation of a versatile alkyne-containing moiety on the
resulting stereogenic carbon centers5 after the selective
conversion of a terminal alkyne group to a 1,2,3-triazole
moiety. If both R1 and R2 are carbon-based substituents, an all-
Received: July 1, 2013
Published: July 15, 2013
© 2013 American Chemical Society
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The need for privileged scaffolds in medicinal research has
recently provided an impetus to develop diverse syntheses of
3,3-disubstituted oxindoles,10 which are widely present in
natural products, drugs, and pharmaceutically active com-
pounds. In particular, the development of catalytic asymmetric
methods has received considerable attention,11 as both the
substituent and the configuration of the C3 position
significantly influence the biological activity. During our
continuous efforts devoted to the synthesis of quaternary
oxindoles,12 we designed oxindole-based 1,6-heptadiyne 1 to
initiate the study of asymmetric CuAAC, as the resulting
quaternary oxindole 3 featuring a 1,2,3-triazole-containing
substituent at C3 would be interesting for medicinal research.13
Accordingly, the reaction of dialkyne 1a and benzyl azide (2a)
was chosen for optimization, and some typical results are
summarized in Table 1.
the steric and electronic properties of PYBOX-type ligands to
inhibit the side reaction and improve the ee of 3a ended in vain
(entries 4−7).
We then carefully optimized other reaction parameters to
improve the reaction outcome. The reaction solvent was found
to play a pivotal role, and using acetone as the solvent not only
noticeably improved the ee of the desired product 3a to 75%
but also enhanced the 3a/4a ratio to 1:2 (entry 8). This result
encouraged us to try other ketone solvents (entries 9−13).
Remarkably, when 2,5-hexanedione was used as the solvent, the
enantioselectivity and yield of 3a improved to 90% ee and 32%,
respectively, with a 3a/4a ratio of 1:1 (entry 13). Varying the
1a/2a ratio from 1:1 to 1.2:1 resulted in an increase in the 3a/
4a ratio to 2:1, although the ee of 3a slightly decreased to 85%
(entry 14). On the basis of this variation, further lowering the
reaction temperature to 0 °C and using a 15 mol % loading of
the chiral Cu(I) catalyst afforded 3a in 77% yield with 90% ee
and a 3a/4a ratio of 7:1 (entry 15). It is worth mentioning that
to our knowledge the use of 2,5-hexanedione as the solvent to
improve the enantioselectivity of an asymmetric reaction is
unprecedented to date.
Table 1. Optimization of the Conditions
The scope of this reaction was then evaluated with respect to
substituted oxindole-derived dialkynes 1 and azides 2 under the
optimized conditions (Table 2). The reactions were carried out
in 2,5-hexanedione at 0 °C using 15 mol % chiral Cu(I) catalyst
and a 1/2 ratio of 1.2:1.0. The effects of the N-protecting group
on the oxindole were evaluated first. The products 3a−d were
obtained with excellent enantioselectivity irrespective of
whether the N-protecting group was an alkyl or acetyl group
(entries 1−4). However, the electron-withdrawing acetyl
protecting group seemed to be less favorable, resulting in
increased formation of the undesired ditriazole (entry 4 vs
entries 1−3). Both alkyl- and aryl-substituted azides proved to
be viable substrates in the reaction with N-benzyloxindole 1b.
The reactions of benzyl-type azides 2b−i generally worked well,
giving the desired products 3e−l in 70−82% yield with 88−
95% ee and good 3/4 ratios (entries 5−12). Good to excellent
ee (86−98%) was also achieved in the reactions of function-
alized alkyl azides 2j−l, although the yield and/or the 3/4 ratio
were somewhat less satisfactory (entries 13−15). In the case of
phenyl azide (2m), the desired product 3p was obtained with
84% ee in a modest yield of 35%, but the 3p/4p ratio was
moderate (entry 16). Finally, it was found that the presence of
either electron-withdrawing or electron-donating substituents
on the oxindole framework had no big influence on the
enantioselectivity, as dialkynes 1e−k reacted with 2-
(azidomethyl)phenol (2h) to give products 3q−w in
reasonable yields with 93−95% ee and good 3/4 ratios (entries
17−23). The absolute configuration of product 3v was
determined to be S by X-ray diffraction analysis.
With an alkyne group as a synthetically versatile handle, the
thus-obtained quaternary oxindoles 3 could be readily
elaborated. For example, oxindole 3b was readily converted
without loss of enantioselectivity to 5−8, quaternary oxindoles
of considerable interest in medicinal research, via trans-
formations based on the alkyne group, including [3 + 2]
cycloaddition, Sonogashira coupling, and full or partial
hydrogenation, respectively (Scheme 2).
Since the pioneering kinetic studies by Fokin and Finn to
elucidate the mechanism,9a the possible involvement of
dinuclear copper intermediates as the catalytically active species
in CuAAC has been supported by both experimental evidence
and theoretical calculations.9b−e We envisioned that the highly
a
b
a
entry
L
solvent
CH2Cl2
yield of 3a (%) ee of 3a (%) 3a:4a
1
2
−
10
17
11
11
5
−
−6
67
64
2
1:3
1:1
1:4
1:4
1:9
1:9
1:5
1:2
1:2
1:2
1:2
1:2
1:1
2:1
7:1
L1
L2
L3
L4
L5
L6
L2
L2
L2
L2
L2
L2
L2
L2
CH2Cl2
3
CH2Cl2
4
CH2Cl2
5
CH2Cl2
6
CH2Cl2
6
23
0
7
CH2Cl2
8
8
acetone
20
21
22
21
24
32
50
77
75
77
75
84
60
90
85
90
9
2-butanone
2-pentanone
3-pentanone
cyclopentanone
2,5-hexanedione
2,5-hexanedione
10
11
12
13
c
c
c
d
14
e
15
2,5-hexanedione
1
a
Determined by H NMR spectroscopy using CH2Br2 as an internal
standard. Determined by HPLC analysis. Isolated yield. 0.24 mmol
of 1a was used. 0.24 mmol of 1a, 15 mol % CuCl, and 18 mol % L2
b
c
d
e
were used at 0 °C for 96 h.
Of all the cuprous salts screened under ligand-free
conditions, CuCl proved to be the best in terms of reactivity.
However, even in the presence of 10 mol % CuCl, the reaction
of equal amounts of 1a and 2a proceeded slowly in CH2Cl2 at
25 °C and gave the desired product 3a in only 10% yield (Table
1, entry 1). Not unexpectedly, the unwanted ditriazole 4a was
obtained as the major product. Next, various chiral ligands in
combination with CuCl were examined. To our disappoint-
ment, all of the ligands accelerated the formation of 4a except
for trisoxazoline L1,14a which improved the 3a/4a ratio from
1:3 to 1:1; however, 3a was obtained with only 6% ee (entry 2).
PYBOX ligand L214b proved to be the most promising in terms
of enantioselectivity, affording 3a in 11% yield with 67% ee and
a 3a/4a ratio of 1:4 (entry 3). Unfortunately, endeavors to tune
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Communication
a
Table 2. Substrate Scope of the Asymmetric CuAAC
Scheme 2. Synthetic Elaboration of 3b
a
Conditions: (a) N-Hydroxy-4-methylbenzimidoyl chloride (2.5
equiv), Et3N (2.5 equiv), CH2Cl2, 10 h. (b) PhI (1.5 equiv),
Pd(PPh3)4 (5 mol %), CuI (5 mol %), Et3N (10.0 equiv), DMF, 4 h.
(c) Raney Ni, MeOH, H2, 4 h. (d) Lindlar’s catalyst, quinine (10.0
equiv), MeOH, H2, 5 h.
Figure 1. Negative NLE in the reaction of 1a and 2a using 15 mol %
CuCl/L2.
medicinal research. Our reaction also features an unprece-
dented highly enantioselective intermolecular desymmetriza-
tion of dialkynes, which demonstrates the great potential of this
strategy in the construction of stereogenic carbon centers with
an alkyne group. The results further indicate the power of the
desymmetrization strategy in the construction of quaternary
carbon stereogenic centers.16 The extension of this method-
ology to other classes of prochiral dialkynes for the develop-
ment of highly enantioselective CuAAC reactions, together
with the catalytic asymmetric intermolecular desymmetrization
of dialkynes by other types of reactions, is ongoing in our
laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
a
b
c
Isolated yields. Determined by chiral HPLC analysis. Determined
Experimental procedures, characterization data, NMR spectra,
and HPLC traces for all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
d
from the isolated yields of 3 and 4. At 25 °C.
enantioselective CuAAC developed in the present work might
offer a useful probe for mechanistic investigations of this
important bond-forming reaction by means of a nonlinear effect
(NLE) study.15 Indeed, a strong negative NLE was observed in
the reaction of 1a and 2a (Figure 1). Such asymmetric
depletion might be rationalized by the hypothesis that the
homochiral dimeric species is less reactive than the
corresponding heterodimer. Studies of the exact nature of the
catalytic species are currently underway.
In conclusion, we have developed the first highly
enantioselective CuAAC via desymmetrization of oxindole-
based 1,6-heptadiynes to furnish chiral quaternary oxindoles
bearing a 1,2,3-triazole moiety, which are interesting targets for
AUTHOR INFORMATION
Corresponding Author
jzhou@chem.ecnu.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the 973 Program (2011CB808600), the NNSFC
(21172075, 21222204), the Ministry of Education (NCET-11-
0147), the SSCS Program (13XD1401600), and the SMEC
Innovation Program (12ZZ046) for financial support.
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R.; Zhang, J. Org. Lett. 2013, 15, 2266.
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