Rhodium-catalyzed Asymmetric Arylation of Nitroalkenes Powered by Simple Chiral Sulfur-Olefin Ligands

Article abstract of DOI:10.1002/jccs.201700328

An efficient rhodium-catalyzed enantioselective addition of potassium organotrifluoroborates to nitroalkenes powered by simple chiral sulfur-olefin ligands is reported. This protocol is applicable to a broad range of 2-aryl-, alkyl-, and heteroaryl-substituted nitroalkenes, allowing access to diverse chiral β,β-disubstituted nitroethanes in good to excellent yields with high enantioselectivity under mild conditions.

Full text of DOI:10.1002/jccs.201700328

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Special Issue Paper
Rhodium-catalyzed Asymmetric Arylation of Nitroalkenes Powered by Simple Chiral
Sulfur-Olefin Ligands
*
Zheng Wang, Wen-Wen Chen and Ming-Hua Xu
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences,
Shanghai 201203, China
(Received: September 24, 2017; Accepted: September 28, 2017; Published Online: November 27, 2017; DOI: 10.1002/jccs.201700328)
An efficient rhodium-catalyzed enantioselective addition of potassium organotrifluoroborates to
nitroalkenes powered by simple chiral sulfur-olefin ligands is reported. This protocol is applicable
to a broad range of 2-aryl-, alkyl-, and heteroaryl-substituted nitroalkenes, allowing access to
diverse chiral β,β-disubstituted nitroethanes in good to excellent yields with high enantioselectivity
under mild conditions.
Keywords: Asymmetric catalysis; Rhodium; Chiral sulfur-olefin ligands; Asymmetric arylation;
Nitroalkenes.
Zheng Wang obtained the B.S. degree from Central South University in 2013. In the same year, he joined Prof.
Ming-Hua Xu’s group at Shanghai Institute of Materia Medica, Chinese Academy of Sciences (CAS), for the
Ph.D. study until now. His research mainly focuses on transition-metal-catalyzed asymmetric organoboron addi-
tion reactions.
Wen-Wen Chen was born in Jiangxi, China. She received the Ph.D. degree under the supervision of Prof. Guo-
Qiang Lin and Prof. Ming-Hua Xu from the Shanghai Institute of Organic Chemistry, Chinese Academy of Sci-
ences (CAS) in 2008. After pursuing her postdoctoral studies in McGill University and the University of Illinois
at Chicago, she joined Prof. Ming-Hua Xu’s group at the Shanghai Institute of Materia Medica, CAS, in 2014 as
an Assistant Professor and was promoted to Associate Professor in the same year. Her research interests include
the development of synthetic methodologies and drug discovery.
Ming-Hua Xu was born in Zhejiang, China. He obtained the Ph.D. degree from the Shanghai Institute of Organic
Chemistry (SIOC), Chinese Academy of Sciences (CAS), in 1999. Following postdoctoral research at the Uni-
versity of Virginia and Georgetown University Medical Center, he joined the faculty at SIOC as an Associate Pro-
fessor in 2003. He has been a Professor with the Shanghai Institute of Materia Medica (SIMM), CAS, since
2005. His research interests include asymmetric synthesis and drug discovery.
INTRODUCTION
Rhodium-catalyzed asymmetric conjugate addi-
tion of organoboron reagents to electron-deficient ole-
fins has been recognized as one of the most powerful
and efficient strategies to construct new C─C bonds
and generate diverse chiral β-substituted functional
compounds.1 Among these olefins, α,β-unsaturated car-
bonyl compounds are the most intensively investigated
*Corresponding author. Email: xumh@simm.ac.cn
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Wang et al.
substrates, and remarkable success has been achieved in
their highly enantioselective addition with the use of
various chiral ligands.2 Nitroalkenes are another essen-
tial type of olefin substrates, as the reaction products
bearing a nitro functionality are valuable chiral build-
ing blocks.3 However, unlike the achievements with
α,β-unsaturated carbonyl compounds, there have been
very few reports of the highly enantioselective addition
of nitroalkenes,4 mainly due to the difficulties encoun-
tered in the enantiocontrol of the reaction. In 2000, the
Hayashi group successfully accomplished the asymmet-
ric arylation of organoboronic acids to α-substituted
nitroalkenes with high enantioselectivity by utilizing
Rh/BINAP as catalyst.5 However, with nitroalkenes
that lack α-substitutents, the reaction generally resulted
in low levels of enantioselectivity (<50% ee).4 In 2010,
we developed a new reaction system and discovered
that high enantiocontrol could be achieved using chiral
bicyclo[3.3.0]diene ligands, leading to synthetically use-
ful chiral β,β-disubstituted nitroalkanes with up to
97% ee.6 Since then, much progress has been made, and
chiral sulfoxide-phosphines,7 sulfoxide-olefins,8 biaryl
phosphites,9 bridged bicyclo[2.2.1]/[2.2.2] dienes,10,11
and phosphorous-olefins12 have been demonstrated to
be potential chiral ligands in the rhodium-catalyzed
asymmetric arylation of 2-aryl nitroalkenes, affording
good to high enantioselectivity. Despite these advances,
only scarce examples of the addition of 2-heteroaryl or
2-alkyl nitroalkenes have been reported,7b,8,12 reflecting
the challenges in the use of these substrates. Therefore,
the development of versatile catalysts for efficient asym-
metric organoboron addition to nitroalkenes with a
broader substrate scope is still highly desirable.
In the past few years, our group has been inter-
ested in developing olefin-based chiral ligands for
transition-metal-catalyzed asymmetric transformations.
As a result, a series of structurally interesting chiral ole-
fin ligands including bisolefins,13 sulfur-olefins,14 and
phosphorous-olefins15 have been designed and success-
fully used in asymmetric catalysis. Among them, readily
available chiral sulfinamide-olefins ligands (SOLs)14l are
particularly fascinating because of their extraordinary
structural simplicity and excellent catalytic performance
in Rh-catalyzed asymmetric additions of organoboron
reagents to α-ketoesters,14d,14e α-diketones,14d,14f
imines,14g–k and α,β-unsaturated carbonyl com-
pounds.14a–c As part of our ongoing exploration of the
Scheme 1. Rh-catalyzed asymmetric arylation of
nitroalkenes powered by
a branched
sulfur-olefin ligand.
sulfur-olefin chemistry, we conceived that the Rh/SOL
catalyst system might also be effective for the asymmet-
ric addition of nitroalkenes. Here we report the enantio-
selective addition of potassium aryltrifluoroborates to
2-substituted
nitroalkenes
to
provide
chiral
β,β-disubstituted nitroethanes in good to excellent yield
with high enantioselectivity (Scheme 1).
RESULTS AND DISCUSSION
Initially, we conducted the reaction of nitrostyrene
1a with 4-chlorophenylboronic acid 2a in the presence of
1.5 mol% of [Rh(COE)2Cl]2 under aqueous KOH
(1.5 M)/toluene at 80ꢀC. With our previously developed
branched sulfur-olefin L1 as the chiral ligand, we were
pleased to find that the reaction proceeded to give the
desired product 3a in 64% yield with promising 82% ee
(Table 1, entry 1). To investigate the role of additives,
K3PO4, K2CO3, and KF were then screened (entries
2–4), However, no better results were obtained. Chang-
ing the solvent from toluene to DCE led to a dramatic
decrease of the reaction yield (entry 5), while no reaction
was observed in water-miscible solvents like dioxane or
THF (entries 6 and 7). Interestingly, switching the orga-
noboron reagent from 4-chlorophenylboronic acid to its
corresponding potassium trifluoroborate resulted in
some increase in enantioselectivity (entry 8). Further
evaluation indicated that the additive had an important
effect on the reaction yield (entries 8–11). With KF as
the additive, the reaction proceeded very smoothly to
give a significantly improved yield (99%) of the corre-
sponding nitroalkane with 86% ee (entry 9). When the
reaction temperature was lowered to 60ꢀC, the same
yield and enantioselectivity were observed (entry 12).
In an attempt to improve the enantioselectivity, we
next turned to the evaluation of different sulfur-olefin
ligands. A series of branched SOLs (L2–L5) with vary-
ing R substituents were synthesized and examined under
similar reaction conditions. The incorporation of a
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Rh-catalyzed Asymmetric Arylation of Nitroalkenes
Table 1. Optimization of reaction conditionsa
Entry
4-ClC6H4[B]
L
Additive
Yield (%)b
ee (%)c
1
2
3
4
4-ClC6H4B(OH)2
4-ClC6H4B(OH)2
4-ClC6H4B(OH)2
4-ClC6H4B(OH)2
4-ClC6H4B(OH)2
4-ClC6H4B(OH)2
4-ClC6H4B(OH)2
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
4-ClC6H4BF3K
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
KOH
K3PO4
K2CO3
KF
KOH
KOH
KOH
KOH
KF
KHF2
K3PO4
KF
KF
KF
64
37
59
24
82
75
74
86
5d
6e
29
73
N.R.
N.R.
61
99
76
29
99
99
74
N.D.
N.D.
86
86
86
82
87
90
83
7f
8
9
10
11
12g
13g
14g
15g
16g
17g
KF
KF
KF
99
99
N.R.
82
82
N.D.
a Reaction conditions: 1a (0.20 mmol), 2 (0.40 mmol, 2.0 equiv), [Rh(COE)2Cl]2 (1.5 mol%), ligand (3.3 mol%), and additive
(1.5 M, 0.20 mmol, 1.0 equiv) in 2.0 mL of solvent at 80ꢀC for 12 h unless otherwise noted.
b Isolated yield.
c Determined by chiral HPLC analysis.
d DCE as solvent.
e THF as solvent.
f Dioxane as solvent.
g 60ꢀC.
bulky t-Bu group, 1-admantyl group, or the electron-
deficient 3,5-bis(trifluoromethyl)phenyl group led to a
slight decrease in enantiocontrol (82–83% ee) (entries
14–16), while a closely structurally related analogue of
L1 bearing a 9-anthryl substituent (L2) delivered the
product in quantitative yield (99%) and improved enan-
tioselectivity (90% ee) (entry 13). Following our previous
procedure, this ligand can be easily prepared with the
use of 3-(9-anthracenyl)propanal as the starting material
via a three-step sequence involving organocatalyzed
Mannich reaction, condensation, and reduction
(Scheme 2). It is worth noting that the linear SOL L6
was ineffective in this addition reaction (entry 17), indi-
cating that the ligand structure has a crucial impact on
the catalyst activity.
Scheme 2. Synthesis of ligand L2.
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Wang et al.
3f vs. 3e, 3i vs. 3h, 3k vs. 3j, 3l0 vs. 3a0, 3p vs. 3o). It is
noteworthy that sterically encumbered nitroalkene sub-
strates possessing bulky aryl groups such as 1-naphthyl
and o-tolyl performed well in the reaction, yielding the
products with high ee values (up to 93%) (3g–3i). It
appears that the electronic nature of the R substitutents of
nitroalkenes has little effect on the enantioselectivity of
the reaction (3a vs. 3l, 3b vs. 3l0, 3k vs. 3l). Interestingly,
both enantiomers of the product can be readily obtained
by simply switching the acceptor and donor aryl substitu-
ents under the same catalytic conditions (3a vs. 3a0, 3l
vs. 3l0). Gratifyingly, the challenging 2-heteroaryl-
substituted nitroalkenes were also suitable substrates. For
example, a high level of enantiomeric control (92–93% ee)
was attained when 2-(2-furyl)nitroethene and 2-(2-thienyl)
nitroethene were employed (3m–3p). To our knowledge,
these results are among the best in the asymmetric addi-
tion of arylboron reagents to 2-heteroaryl nitroalke-
nes.7b,8,9,12 Moreover, 3-thienyl substituted nitroethene
could also be applied in this reaction, affording the addi-
tion product 3q in 90% yield with 80% ee. Notably, the
catalytic system is compatible with 2-aliphatic substituted
nitroalkenes. The reaction could be carried out with 2-
nitrovinylcyclohexane to form the corresponding product
3r with 86% ee, though the yield was somewhat less satis-
factory (52%). The stereochemistry of the newly formed
carbon stereocenter of product 3a was determined to be
R by comparing the [α]D with known literature data.6
Accordingly, the absolute configuration of the other prod-
ucts was assigned by assuming an analogous stereochemi-
cal pathway.
Table 2. Rh-catalyzed asymmetric arylation of
nitroalkenesa,b,c
a Reaction conditions: 1 (0.20 mmol), 2 (0.20 mmol, 2.0
equiv), [Rh(COE)2Cl]2 (1.5 mol%), ligand (3.3 mol%), and
KOH (1.5 M, 3.0 equiv) in 2.0 mL of toluene at 60ꢀC
for 12 h.
On the basis of the observed stereochemical out-
come of the reaction, we proposed an empirical
transition-state model to rationalize the origin of enan-
tioselectivity. As shown in Figure 1, a favored confor-
mation of the arylrhodium species is assumed in which
the aryl group coordinated to the rhodium center
adopts a trans geometry16 with the olefin moiety of the
ligand, and the tert-butyl group is staggered. To avoid
b Isolated yield.
c Determined by chiral HPLC analysis.
Under the optimal conditions, we then investigated
the substrate scope of the reaction (Table 2). To our
delight, a variety of potassium aryltrifluoroborates with
diverse electronic and steric properties could be employed.
In most cases, the reaction proceeded smoothly to afford
the corresponding β,β-diarylnitroethane products in excel-
lent yields with good enantioselectivities. In comparison
with the use of electron-rich aryltrifluoroborates and phe-
nyltrifluoroborate, higher enantiomeric excesses were
attained with aryltrifluoroborates bearing electron-
withdrawing substitutents on the phenyl ring (3a,3b vs. 3c,
A
B
Fig. 1. Proposed transition state model.
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JOURNAL OF THE CHINESE
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Rh-catalyzed Asymmetric Arylation of Nitroalkenes
unfavorable steric interaction, the nitro group of sub-
strate is oriented away from the bulky R substituent of
the olefin ligand. Therefore, the intramolecular transfer
of aryl group takes place from the Si face of the C=C
bond, giving the observed major enantiomer.
petroleum ether/ethyl acetate to afford the correspond-
ing addition product 3.
ACKNOWLEDGMENTS
This work was supported by the National Natural
Science Foundation of China (21325209, 21472205)
and the Shanghai Municipal Committee of Science and
Technology (Program of Shanghai Academic Research
Leader, 14XD1404400).
CONCLUSIONS
In summary, a new, readily available chiral sulfur-
olefin L2 was discovered to be an effective ligand for
rhodium-catalyzed asymmetric addition of aryltrifluorobo-
rates to nitroalkenes. The simple catalytic system tolerates
a broad variety of substrates and enables access to a wide
range of β,β-diaryl substituted nitroethanes in good to
excellent yields with high enantioselectivities under mild
reaction conditions. Efforts to expand the applicability of
this method for the efficient preparation of related bioactive
compounds are currently under way in our laboratory.
Supporting information
Additional supporting information (experimental
procedures, characterization data, and copies of NMR
and HPLC spectra) is available in the online version of
this article.
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General
All anaerobic manipulations were carried out with
standard Schlenk techniques under argon. Solvents
were dried and distilled by standard procedures. NMR
spectra were recorded on a Mercury 300 spectrometer
(300 MHz for 1H), and
(125 MHz for 13C). Chemical shifts are reported in δ
a Varian spectrometer
1
ppm referenced to an internal SiMe4 standard for H
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resolution mass spectra (HRMS) were recorded on a Q-
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