Palladium-Catalyzed Asymmetric Decarboxylative [4+2] Dipolar Cycloaddition of 4-Vinyl-1,3-dioxan-2-ones with α,β-Disubstituted Nitroalkenes Enabled by a Benzylic Substituted P,N-Ligand

Article abstract of DOI:10.1021/acs.orglett.0c01638

The Pd-catalyzed asymmetric [4+2] cycloaddition reaction of an aliphatic 1,4-dipole with singly activated electron-deficient alkenes is realized for the first time, enabled by using a newly developed benzylic substituted P,N-ligand, affording tetrahydropyrans having three continuous chiral centers in high yields with high diastereo-and enantioselectivities. The rational transition states of the reaction as well as the role of the benzylic chiral center are proposed.

Full text of DOI:10.1021/acs.orglett.0c01638

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Letter
Palladium-Catalyzed Asymmetric Decarboxylative [4+2] Dipolar
Cycloaddition of 4‑Vinyl-1,3-dioxan-2-ones with α,β-Disubstituted
Nitroalkenes Enabled by a Benzylic Substituted P,N-Ligand
Juan Du,∥ Yuan-Da Hua,∥ Yang-Jie Jiang, Shuai Huang, Di Chen, Chang-Hua Ding,*
and Xue-Long Hou*
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ABSTRACT: The Pd-catalyzed asymmetric [4+2] cycloaddition reaction of an aliphatic 1,4-dipole with singly activated electron-
deficient alkenes is realized for the first time, enabled by using a newly developed benzylic substituted P,N-ligand, affording
tetrahydropyrans having three continuous chiral centers in high yields with high diastereo- and enantioselectivities. The rational
transition states of the reaction as well as the role of the benzylic chiral center are proposed.
he transition metal-catalyzed asymmetric [3+2] dipolar
cycloaddition of vinyl three-membered rings and their
How to realize Pd-catalyzed asymmetric [4+2] cycloaddition
of aliphatic 1,4-dipoles with alkenes remains the great
challenge. It could be deduced that the lack of a successful
example of the reaction using aliphatic 1,4-dipoles might be
caused by the fact that they are more flexible than their benzo
analogues so that it should be more difficult to control the
stereochemistry of the reaction when aliphatic dipoles were the
reagent. To address this challenge, the development of new
and efficient chiral ligands as well as the catalyst system should
be pursued. We have designed and synthesized a new type of
benzylic substituted chiral P,N-ligand, with which high
catalytic activity and asymmetric induction as well as the
switch of enantioselectivity have been observed in many
enantioselective reactions.6 Recently, we found that when we
modified this type of ligand by introducing a new chiral center
at the benzylic position and used it in Pd-catalyzed asymmetric
[4+2] cycloaddition of 4-vinyl-1,3-dioxan-2-one with α,β-
disubstituted nitroalkenes, high diastereo- and enantioselectiv-
ities were realized. Herein, we report our preliminary study of
this reaction.
T
analogues with unsaturated compounds through a zwitterionic
Pd-π-allyl intermediate (1,3-dipole) has become an important
strategy for the construction of five-membered rings, which as
basic and important subunits have widely been found in
natural products as well as medicines (Scheme 1a).1 It seems
that a similar zwitterionic Pd-π-allyl complex can also be
formed from 2-vinyl azitidines and 2-vinyl oxetanes or their
analogues, which as a 1,4-dipole could react with unsaturated
compounds under transition metal catalysis via [4+2] dipolar
cycloaddition to afford the corresponding tetrahydropyrans
and piperidines. Larksarp and Alper were the first to report the
Pd-catalyzed [4+2] dipolar cycloaddition of vinyl oxetanes
with isocyanates and carbodiimides to provide racemic 1,3-
oxazines in 1999.2 Since then, some reports of Pd-catalyzed
[4+2] dipolar cycloadditions have appeared by using different
starting materials as a precursor of the 1,4-dipole, activated
alkenes, and some other CX compounds as the dipolar-
ophile (Scheme 1b).3 In addition, benzoxazinanones have been
used as benzo analogues of the 1,4-dipole to react with
different kinds of alkenes to afford dihydroquinolines (Scheme
1c).4 An asymmetric version has also been reported; however,
it was mainly limited to those using benzoxazinanones as the
1,4-dipole with excellent results.4 The reaction of an aliphatic
1,4-dipole from vinyl oxetanes and its analogue with CX,
such as azadienes,5a formaldehyde,5b and isocyanates,5c
afforded the products in good ee, the only exception appearing
in a report by Yamamoto in 2001, in which a few examples
using doubly activated alkenes3a could be found in moderate
yields with unsatisfying diastereo- and enantioselectivities.
Initially, several 1,4-dipole precursors were adopted to react
with substituted nitroalkenes under Pd catalysis. It was found
that tetrahydropyrans 3a and 4a were afforded in 92% yield in
Received: May 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01638
© XXXX American Chemical Society
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Scheme 1. Pd-Catalyzed Dipolar Cycloaddition through a
Zwitterionic Pd-π-Allyl Intermediate
Table 1. Influence of Ligands on Pd-Catalyzed [4+2]
Cycloaddition of 4-Vinyl-1,3-dioxan-2-one 1a and
a
Nitroalkene 2a
b
c
d
entry
L
yield (%)
dr
ee (%)
e
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11
(S,S)-L12
(S,S)-L13
(S,S)-L14
(S,S)-L15
(R,S)-L16
(R,S)-L17
(R,S)-L18
(R,S)-L19
(S,S)-L13
nr
−
−
−
−
−
−
−
−
−
40
84
0
20
59
86
65
94
93
90
71
98
94
90
96
e
nr
e
nr
e
nr
92
96
92
96
92
33
62
92
83
92
70
64
21
8
3/1
2/1
2/1
6/1
1/1
2/1
4/1
6/1
11/1
3/1
4/1
6/1
2/1
4/1
3/1
17/1
9
10
11
12
13
14
15
16
17
18
19
12
92
f
a 3/1 ratio when 4-vinyl-1,3-dioxan-2-one (1a) reacted with
(E)-1-methyl-2-phenyl nitroethene (2a) in the presence of
Pd2(dba)3·CHCl3 and achiral PHOX L5 as the catalyst (entry
5, Table 1) by trial and error (vide infra), while the reaction
with 2-phenyl nitroethene provided more diastereomers of the
corresponding [4+2] cycloadduct in 65% yields with poor
selectivity. These results are in accord with our previous
findings that the yields and the diastereo- and enantioselectiv-
ities are better for the reaction of α,β-disubstituted nitro-
alkenes than that of β-substituted nitroalkenes.7 The low yield
of the cycloadduct with low selectivity was also realized for the
reaction of (E)-5-hydroxypent-2-en-1-yl methyl carbonate with
2-phenyl nitroethene by using [Pd(C3H5)Cl]2/PPh3 as the
catalyst. On the basis of the results described above, the
investigation of the reaction with α,β-disubstituted nitro-
alkenes continued.
20
a
At a 5.0/10.0/200/100 Pd2dba3·CHCl3/L/1a/2a molar ratio and 25
b
c
°C. Isolated yield of diastereomers 3a and 4a. dr (3a/4a ratio)
d
e
determined by 1H NMR. ee of 3a determined by chiral HPLC. nr =
f
no reaction. At a 5.0/5.0/150/100 Pd(dba)2/L13/1a/2a molar ratio
and 0 °C.
From the preliminary screen of different types of ligands
(Figure 1), it was found that only by using a P,N-ligand were
the desired cycloadducts 3a and 4a afforded while no reaction
occurred by using ligands L1−L4 (entries 1−4, respectively,
Table 1). The reaction using P,N-ligands L6−L10 produced
3a in high yield with varied enantioselectivity, but with very
low diastereoselectivity (entries 6−10, respectively, Table 1).
However, both the yield and diastereo- and enantioselectivities
were improved if benzylic substituted P,N-ligands L11 with
two methyls as substituents at the benzylic position were used
(entry 11). Previously, we observed a dramatic switch in
enantioselectivity in Pd-catalyzed asymmetric Heck reaction
using ligands L10 and L11 with and without substituents at the
benzylic position and provided a rational explanation for such
observations.6d Therefore, we envisioned that the diastereo-
and enantioselectivities would be improved further if a new
chiral center were introduced at the benzylic position of the
ligand. Indeed, the diastereo- and enantioselectivities increased
greatly by using these newly designed and synthesized benzylic
Figure 1. Various chiral ligands screened for the asymmetric
cycloaddition.
substituted P,N-ligands (Figure 1; entries 12−19) (for detailed
synthesis of ligands L12−L19, see the Supporting Informa-
tion). The best results were afforded when (S,S)-L13 was the
ligand, the yield being 83% with the dr being 11/1 and the ee
being 94% for 3a (entry 13). With L13 as the ligand, the
influence of other reaction conditions, including the palladium
source, solvent, and temperature, on the cycloaddition was
investigated (for details, see the Supporting Information).
Using Pd(dba)2 and ligand L13 as the catalyst and performing
the reaction in THF at 0 °C, the [4+2] cycloaddition afforded
tetrahydropyran 3a in 92% yield with a 17/1 dr and a 95% ee
B
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(entry 20). It was found that better yields and diastereose-
lectivity were obtained by using (S,S)-ligands L12−L15 than
by using (R,S)-L16−L19, though the ee values were slightly
higher when using (R,S)-ligands (entries 12−15 vs entries 16−
19), suggesting the chiralities in the former are matched. More
interestingly, the reaction with both ligands (S,S)-L12−L15
and (R,S)-L16−L19 delivered the same enantiomer for
product 3a as the major stereoisomer, which was identified
by their HPLC spectra (for details, see the Supporting
Information). These results suggest the absolute configuration
of product 3a is controlled by the chiral center on the
oxazoline ring of the ligand, while the chiral center on the
benzylic position mainly affects the yield and the diaster-
eoselectivity of the reaction.
product was also carried out to demonstrate the utility of the
protocol (eqs 1 and 2). Product 3a was easily converted into
the corresponding epoxide 5 through oxidation using m-CPBA
in CH2Cl2 in 57% yield (eq 1). Moreover, a sequential Pd/C-
catalyzed reduction and amide formation of 3a delivered
compound 6 in 73% yield in two steps (eq 2).
Under the optimized conditions, the substrate scope was
examined. A range of disubstituted nitroalkenes 2a−e
containing both electron-donating and -withdrawing substitu-
ents at the para or meta position of the phenyl ring (R2) were
well-tolerated to produce the corresponding tetrahydropyrans
3a−e, respectively, in high yields with high diastereoselectivity
and excellent enantioselectivity (entries 1−5), while 2f and 2g
with ortho substituents on the phenyl ring as well as a β-
naphthyl or β-furanyl substituent with α-methyl nitroethene 2h
or 2i, respectively, delivered products 3f−i in moderate yields
with reduced ee (entries 6−9, respectively). β-Pyridyl-
substituted nitroalkene 2j was also a suitable substrate for
the cycloaddition to give a high dr and excellent ee (entry 10).
The cycloaddition with nitroalkenes 2k−2m bearing an α-
phenyl, isopropyl, or ethyl substituent worked well to afford
the corresponding products in high yields with excellent
enantioselectivity, but the dr ratio decreased when the steric
hindrance of the α-substituent became bulkier (entries 11−
13). Notably, the cycloaddition with a cyclic nitroalkene 2n
proceeded well to form 1,8-dioxadecalin 3n, which is the core
structure of the bioactive natural product,8 with a 12/1 dr and
a 96% ee (entry 14). Furthermore, substituted vinyl-
dioxanones 1b and 1c also reacted smoothly with nitroalkene
2a to provide adducts 3o bearing two vicinal chiral quaternary
carbon centers with excellent dr and ee (entry 15) and 3p
(entry 16). In addition, nitroalkene 2o bearing a BnO-ether
functional group was tested, leading to the corresponding
product 3q with 83% ee but in a low yield and dr (entry 17).
Unfortunately, no reaction was observed in the case of using
(E)-4-(prop-1-en-1-yl)-1,3-dioxan-2-one (1d) and 4-(prop-1-
en-2-yl)-1,3-dioxan-2-one (1e) as the reactant (entries 18 and
19, respectively).
To improve our understanding of this cycloaddition, (Z)-2a
with a Z/E ratio of 60/1 reacted with 4-vinyl-1,3-dioxan-2-one
(1a) under the optimized reaction conditions (eq 3).
Cycloadduct 3a was obtained in 12% yield with a 11/1 dr
and a 78% ee, while 2a was recovered in a 87% yield with a Z/
E ratio of 7/1. Comparison of the Z/E ratio of recovered 2a
with that of starting material 2a indicated that (Z)-2a was
isomerized to (E)-2a during the reaction, probably through a
reversible oxo-Michael addition of Pd alkoxide generated via
the decarboxylative oxidative addition of the Pd(0) catalyst to
cyclic carbonate 1a with (Z)-2a.10 Upon comparison of the
reaction with (Z)-2a (eq 3) and (E)-2a (entry 1, Table 2),
these results suggested that the geometry of nitroalkene has a
significant effect on the cycloaddition.
To explain the stereochemistry of the reaction, the plausible
transition states were proposed (Figure 3). The reaction
should proceed through a stepwise mechanism involving
Michael addition followed by intramolecular allylic alkylatio-
n.3a For the reaction with (S,S)-L13 as the ligand, the
isopropyl substituent at the benzyl site is oriented away from
the Pd center as shown from the crystal structure of the PdCl2-
(S,S)-L13 complex (Figure 4).11 Intermediate A is dominant
for the Michael addition step, because intermediate B has both
nitro and phenyl groups in axial positions and should be less
stable. In the second intramolecular allylic alkylation step,
intermediate E should be favorable due to the bulky nitro
group occupying an axial position in intermediate F and thus
leading to product 3 with a (2S,3R,4R) configuration.
Similarly, in the case of the reaction using (R,S)-L17,
intermediates C and G should be favored, also affording
(2S,3R,4R)-3. However, the isopropyl substituent at the benzyl
site is oriented toward the Pd center as indicated by X-ray
crystal structure of the PdCl2-(R,S)-L17 complex12 (Figure 4).
The orientation of the isopropyl substituent at the benzyl
position may decrease the energy gap between transition states
G and H, thus decreasing the diastereoselectivity of the
reaction. These plausible transition states provided some
explanations for why the major enantiomer was same for the
reactions by using ligands (S,S)-L13 and (R,S)-L17 and
The absolute configurations of reaction product 3c were
determined to be (2S,3R,4R) by X-ray diffraction analysis of its
single crystal (Figure 2).9 The transformation of the reaction
Figure 2. ORTEP drawing of product 3c.
C
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Table 2. Substrate Scope of the Pd-Catalyzed Asymmetric
[4+2] Cycloaddition of 4-Vinyl-1,3-dioxan-2-ones 1 with
a
Nitroalkenes 2
Figure 3. Plausible transition states of the cycloaddition.
Figure 4. ORTEP drawings of X-ray crystal structures of PdCl2-(S,S)-
L13 and PdCl2-(R,S)-L17 complexes.
results provided some support for these plausible transition
states (vide supra). Further investigations are needed to
improve our understanding of the reaction mechanism.
In conclusion, a new type of benzylic substituted P,N-ligand
has been designed and synthesized. It enables a Pd-catalyzed
asymmetric [4+2] cycloaddition of an aliphatic 1,4-dipole with
singly activated electron-deficient alkenes with high enantio-
selectivity for the first time. Further exploration of this type of
[4+2] cycloaddition as well as more applications in asymmetric
catalysis by using these benzylic substituted P,N-ligands is
ongoing.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01638.
Screening data, experimental procedures, preparation of
ligands L12−L19, crystallographic data of 3c, the PdCl2-
(S,S)-L13 complex, and the PdCl2-(R,S)-L17 complex,
and NMR and HPLC spectra (PDF)
a
At a 5.0/5.0/150/100 Pd(dba)2/L13/1/2 molar ratio with a
b
reaction time of 4.0 days. Isolated yield of diastereomers 3 and 4.
c
d
1
dr (ratio of 3/4) determined by H NMR. ee of 3a determined by
e
Accession Codes
chiral HPLC. With 0.1 mmol of 1 added in 1 day via a syringe pump.
f
g
Reaction temperature of 10 °C. nr = no reaction.
CCDC 1999956, 1999962, and 1999970 contain 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 Cambridge Crystallographic Data
implied that the chiral center on the benzylic position did
influence the stereoselectivities (dr and ee). The experimental
D
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Letter
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AUTHOR INFORMATION
Corresponding Authors
■
Chang-Hua Ding − Department of Chemistry, Innovative Drug
Research Center, Shanghai University, Shanghai 200444,
China; orcid.org/0000-0002-4628-4016;
Email: dingchanghua@shu.edu.cn
Xue-Long Hou − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis and
Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China; orcid.org/0000-0003-
4396-3184; Email: xlhou@sioc.ac.cn
Authors
Juan Du − State Key Laboratory of Organometallic Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai
200032, China
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Lan, Y.; Xiao, W. J. Angew. Chem., Int. Ed. 2019, 58, 11013−11017.
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Chem. 2018, 83, 9291−9299.
Yuan-Da Hua − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China
Yang-Jie Jiang − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China
Shuai Huang − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China
Di Chen − State Key Laboratory of Organometallic Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai
200032, China
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2019, 21, 9452−9456.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01638
(6) (a) Hou, X. L.; Wu, X. W.; Dai, L. X.; Cao, B. X.; Sun, J. Chem.
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Author Contributions
∥J.D. and Y.-D.H. contributed equally.
Notes
The authors declare no competing financial interest.
(7) Du, J.; Jiang, Y.-J.; Suo, J.-J.; Wu, W.-Q.; Liu, X.-Y.; Chen, D.;
Ding, C.-H.; Wei, Y.; Hou, X.-L. Chem. Commun. 2018, 54, 13143−
13146.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (21772215 and 21532010), the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB20030100), the Chinese Academy of Sciences, the
Technology Commission of Shanghai Municipality, and the
Croucher Foundation of Hong Kong for financial support.
This work is dedicated to Professor Henry N. C. Wong on the
occasion of his 70th birthday.
(8) Wang, X.-J.; Zhang, J.; Wang, J.-D.; Qian, P.-T.; Liu, C.-X.;
Xiang, W.-S. Nat. Prod. Res. 2013, 27, 1863−1869.
(9) CCDC 1999956 contains the supplementary crystallographic
data for this paper. The data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
̈
(10) Nising, C. F.; Brase, S. Chem. Soc. Rev. 2008, 37, 1218−1228.
(11) CCDC 1999962 contains the supplementary crystallographic
data for this paper. The data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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(12) CCDC 1999970 contains the supplementary crystallographic
data for this paper. The data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
E
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