Enantioselective [3 + 2] Cycloaddition Reaction of Ethynylethylene Carbonates with Malononitrile Enabled by Organo/Metal Cooperative Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b03454

The first catalytic asymmetric decarboxylative [3 + 2] cycloaddition reaction of ethynylethylene carbonates with malononitrile has been developed successfully by an organo/copper cooperative system. This strategy led to a series of optically active polysu

Full text of DOI:10.1021/acs.orglett.8b03454

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective [3 + 2] Cycloaddition Reaction of Ethynylethylene
Carbonates with Malononitrile Enabled by Organo/Metal
Cooperative Catalysis
Yu-Chen Zhang, Bo-Wen Zhang, Rui-Long Geng, and Jin Song*
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
S
* Supporting Information
ABSTRACT: The first catalytic asymmetric decarboxylative [3
+ 2] cycloaddition reaction of ethynylethylene carbonates with
malononitrile has been developed successfully by an organo/
copper cooperative system. This strategy led to a series of
optically active polysubstituted dihydrofurans in good yields with
high levels of enantioselectivities (up to 99% yield, 97% ee). The
presence of the terminal alkynyl and the cyano group in the
dihydrofuran products provides a wide scope for further
structural transformations. More importantly, this organo/metal cooperative catalytic system will broaden the substrate
scope and enable fundamentally new reactions.
Scheme 1. Design of Catalytic Enantioselective [3 + 2]
Cycloaddition Reaction of EECs
ransition-metal-catalyzed asymmetric cycloaddition reac-
tions have become powerful strategies for the con-
T
struction of structurally diverse chiral heterocyclic com-
pounds.1 Particularly, vinylethylene carbonates (VECs) have
been employed as alternatives for the generation of
zwitterionic π-allyl-Pd intermediate I and were applied
successfully in palladium-catalyzed asymmetric decarboxylative
cycloaddition reactions. Moreover, this strategy provides
valuable access to diverse oxygen-containing heterocycles
(Scheme 1a).2 Over the past decades, copper-catalyzed
propargylic substitution and cycloaddition reactions via
copper−allenylidene complexes have emerged as the most
powerful strategy for the construction of complex molecules.3,4
The role of the ethynylethylene carbonates (EECs) serving as
synthons in asymmetric reactions for the construction of
quaternary stereocenters has never been investigated until
most recently. The group of Zhang reported an elegant and
efficient copper-catalyzed asymmetric decarboxylative prop-
argylic substitution reaction via copper−allenylidene complex
II (II′) for the enantioselective construction of β-amino
alcohols (Scheme 1b).5 The development of methods for the
cycloaddition reactions of EECs would be of significant value,
providing access to distinct chiral oxygen-containing hetero-
cycles with terminal alkynes for further transformations. To
realize the application of EECs as 1,3-dipole precursors in
copper-catalyzed asymmetric cycloaddition reactions, a suitable
reaction partner possessing both the nucleophilic and electro-
philic sites is highly desired. In this context, malononitrile
attracted our attention because of its high reactivity as a 1,2-
dipole surrogate in the presence of amine catalysts.6
logical activities (e.g., aflatoxin B1 and clerodin).7 Cyclo-
addition approaches that assemble the heterocyclic core are
particularly attractive for dihydrofuran synthesis. Developing
catalytic asymmetric methods to access these frameworks is
2,3-Dihydrofurans have emerged as useful synthetic
intermediates for organic synthesis and one of the most
commonly observed classes of structural units in natural and
synthetic products with diverse and well-documented bio-
Received: October 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03454
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
therefore a topic of continued interest for synthetic chemists.8
Based on recent advances on the copper-catalyzed cyclo-
addition reaction via copper−allenylidene intermediates and
our ongoing efforts toward the development of cooperative
catalysis systems,4,5,9 we designed the asymmetric [3 + 2]
cycloaddition reactions of EECs with malononitrile in the
cooperative catalysis of copper and amine10 for the
enantioselective construction of polysubstituted dihydrofurans
(Scheme 1c). As proposed in Scheme 1c, copper-mediated
decarboxylation of EECs 1 would lead to the copper−
allenylidene intermediate II. Meanwhile, the nucleophilic
intermediate III was generated from the chiral amine catalyst
and malononitrile 2. The asymmetric propargylic substitution
reaction between intermediates II and III will afford
intermediate IV. The final chemoselective intramolecular
cyclization step would furnish 2-amino-3-cyano-dihydrofurans
3. Herein, we will report the first asymmetric [3 + 2]
cycloaddition reaction of EECs with malononitrile enabled by
a cooperatively catalytic copper/amine system.
produced the corresponding polysubstituted dihydrofuran 3a
in moderate yield and enantioselectivity (50% yield, 80% ee,
entry 1). Encouraged by this result, we next evaluated a series
of chiral ligands L2−L5 and an achiral ligand L6 and identified
chiral ligand L1 as a uniquely effective catalyst delivering the
desired product 3a (entries 2−6 vs entry 1). Furthermore, a
survey of urea-cinchona catalysts 4b and 4c (entries 7 and 8)
revealed that 4c was the optimal organocatalyst in terms of
reactivity and enantioselection when employed in concert with
a copper catalyst. Further improvements in reaction efficiency
were realized via the change of the stoichiometric ratio of
reactants in combination with the addition of MgSO4 as a
reaction supplement to deliver the desired adduct 3a in 90%
yield and 95% ee (entry 9). All other deviations from the
optimal conditions led to a decrease of the enantioselectivity,
and in some cases even the yield (see Supporting Information,
Tables S1 and S2) and the standard conditions were obtained
as Cu(CH3CN)4PF6 (1 mol %), box ligand L1 (2 mol %),
urea-cinchona catalyst 4c (20 mol %), EEC 1a (0.6 mmol),
malononitrile 2 (0.3 mmol), and MgSO4 (100 mg) in DCM
(2.0 mL)/m-xylene (4.0 mL) at −10 °C for 48 h (entry 9). A
survey of control experiments were carried out to gain more
insight into the cooperative catalytic system (entries 10−15).
While the use of Cu(CH3CN)4PF6 without ligands resulted in
diminished yield and enantioselectivity, the box ligand L1 was
found to be a competent catalyst (entry 10 vs 9). The use of
the individual catalysts completely failed to promote the
cycloaddition reaction, thus suggesting that the cooperative
catalytic system plays a crucial role in this reaction (entries 11
and 12). The use of box ligand ent-L1, achiral ligand L6, and 4-
dimethylaminopyridine (DMAP) would dramatically affect
both the efficiency and enantioselectivity of this reaction
(entries 13−15), and all these results would imply that the
matched chirality of the ligand and amine catalyst plays an
irreplaceable role in the asymmetric [3 + 2] cycloaddition
reactions.
The feasibility of our design was initially examined by the
reaction of EEC 1a and malononitrile 2 under the cooperative
catalysis of ligand-bound copper(I) complex and urea-
cinchona alkaloid 4 (Table 1). We were delighted to find
that the designed [3 + 2] cycloaddition reaction, indeed,
occurred in the presence of 1 mol % Cu(CH3CN)4PF6, 2 mol
% bis(oxazoline) (box) ligand L1, and 20 mol % 4a and
a
Table 1. Optimization of Reaction Conditions
Under the optimal reaction conditions, we evaluated the
substrate scope of the EECs 1 in the catalytic asymmetric [3 +
2] cycloaddition reactions. As shown in Figure 1, this protocol
was applicable to a wide range of EECs 1 with either electron-
withdrawing or electron-donating substituents on the benzene
ring (1a−1q), delivering chiral 2-amino-3-cyano-dihydrofurans
3a−3q in generally high yields (up to 96% yield) and excellent
enantioselectivities (up to 97% ee). In short, the substrates 1b,
1i, and 1n, which, respectively, possess a methoxy substituent
at the 4-, 3-, and 2-positions, all participated in this
transformation successfully and resulted in excellent enantio-
selectivities (3b, 3i, and 3n, 90−96% ee). Moreover, the
presence of halogen atom functionalities on the benzene ring
does not have a deleterious impact on the reaction perform-
ance (3e−g, 3j−l). Disubstituted substrates were also tolerated
to provide the desired products with excellent enantioselectiv-
ities (3p and 3q). Notably, substrates 1r−1u bearing naphthyl
and heteroaryl groups could also participate in the [3 + 2]
cycloaddition reaction to give the corresponding product 3r−
3u in excellent stereoselectivity (91%−96% ee). Nevertheless,
this method can also be extended to aliphatic-substituted EEC
1v and ethynyl oxazolidinone 1w, affording products 3v and
3w with moderate stereoselectivity. The absolute configuration
of the cycloadduct 3i (>99% ee after recrystallization) was
assigned by single-crystal X-ray diffraction analysis.
b
c
entry
[Cu]
L
4
yield (%)
ee (%)
1
2
3
4
5
6
7
8
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
L1
4a
4a
4a
4a
4a
4a
4b
4c
4c
4c
4c
50
50
90
10
95
35
38
62
90
75
n.d.
n.d.
50
65
63
80
62
68
66
57
8
76
89
95
92
L2
L3
L4
L5
L6
L1
L1
L1
d
9
d
10
11
12
13
14
15
d
d
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
L1
ent-L1
L6
d
4c
4c
53
22
4
d
d
L1
DMAP
a
Reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol), Cu-
(CH3CN)4PF6 (1 mol %), L (2 mol %), and 4 (20 mol %) in
DCM (1.0 mL)/ toluene (2.0 mL) at −20 °C for 48 h. Isolated
yield. The ee value was determined by HPLC. With 1a (0.6 mmol),
2 (0.3 mmol), Cu(CH3CN)4PF6 (1 mol %), L1 (2 mol %), 4c (20
mol %), and MgSO4 (100 mg) in DCM (2.0 mL)/ m-xylene (4.0 mL)
at −10 °C for 48 h.
b
c
d
In order to demonstrate the utility of this catalytic
asymmetric [3 + 2] cycloaddition reaction, a gram-scale
B
DOI: 10.1021/acs.orglett.8b03454
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 1. Expansion of substrate scope for the reaction of various ethynylethylene carbonates (EECs) 1 with malononitrile 2. Reaction conditions:
1 (0.6 mmol), 2 (0.3 mmol), Cu(CH3CN)4PF6 (1 mol %), L1 (2 mol %), 4c (20 mol %), MgSO4 (100 mg) in DCM (2.0 mL)/m-xylene (4.0 mL)
at −10 °C for 48 h. Isolated yield. The ee value was determined by HPLC. aFor 5 days. bCu(CH3CN)4PF6 (3 mol %), L1 (6 mol %), 4c (20 mol
%), for 5 days. cCu(CH3CN)4PF6 (5 mol %), L1 (10 mol %), 4c (20 mol %) at 0 °C. dCu(CH3CN)4PF6 (10 mol %), 4c (20 mol %) at 30 °C for 7
e
days. On the average of 10−20% of starting materials was recovered, and trace amounts of unknown products were obtained.
synthesis of dihydrofuran 3a was conducted under the
standard reaction conditions (Scheme 2). To our great
pleasure, the reaction proceeded well, and the desired product
was isolated in a very good yield of 87% (1.10 g) and excellent
stereoselectivity (93% ee).
group using a Pd/C catalyst leads to alkyl-substituted product
5 with maintained enantioselectivity. A copper-catalyzed click
reaction of 3a with BnN3 produced 1,2,3-triazole-substituted
chiral dihydrofuran 6 in 83% yield, again without loss of optical
purity. Taking advantage of the Sonagashira coupling of 3a
with iodobenzene, we obtained compound 7 in 87% yield and
95% ee. The hydrolysis of the cyano group of 3a occurred to
afford the corresponding amide 8 with excellent stereo-
selectivity. Treatment of 3a with 3-aminopyrazole in HOAc
gave 6,7-dihydro-5H-cyclopenta[d]pyrazolo[1,5-a]pyrimidine
9 in 63% yield.
Scheme 2. Gram-Scale Reaction
In conclusion, a convenient and efficient catalytic asym-
metric [3 + 2] cycloaddition reaction of EECs with
malononitrile has been established for the synthesis of 2-
amino-3-cyano-dihydrofuran derivatives in good yields and
enantioselectivities (up to 99% yield, 97% ee) under mild
conditions. Control experiments suggest that chiral copper
complexes and the urea-cinchona organocatalyst, respectively,
play a crucial role to exhibit the high reactivity and
enantioselectivity of this [3 + 2] cycloaddition reaction.
Moreover, the reaction is compatible with a variety of
functionalities and is amenable to be scaled up to a gram
scale. Further investigations into this organo/metal cooperative
catalytic process are currently being pursued in our laboratory.
Synthetic transformations starting from dihydrofuran 3a
were performed to demonstrate the applicability of this
reaction. As shown in Scheme 3, hydrogenation of the alkyne
Scheme 3. Synthetic Transformations of 3a
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03454.
Experimental details and characterization data (PDF)
C
DOI: 10.1021/acs.orglett.8b03454
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Accession Codes
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CCDC 1871659 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.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jill@ustc.edu.cn
ORCID
Jin Song: 0000-0003-0449-1727
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (Grant
No. 21702199), the Anhui Provincial Natural Science
Foundation (Grant No. 1808085QB30) and the China
Postdoctoral Science Foundation (Grant No. 2017M612075
and 2018T110619).
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