Rh-Catalyzed Asymmetric Hydrogenation of β-Substituted-β-thio-α,β-unsaturated Esters: Expeditious Access to Chiral Organic Sulfides

Article abstract of DOI:10.1021/acs.orglett.8b02339

Rh/bifunctional bisphosphine-thiourea ligand (ZhaoPhos)-catalyzed asymmetric hydorgenation of both (Z)- and (E)-isomers of β-substituted-β-thio-α,β-unsaturated esters was successfully developed. This new asymmetric catalytic methodology provided highly efficient access to two enantiomers of chiral organic sulfides ethyl β-substituted-β-thio-propanoates with excellent results (up to 99% yield and >99% ee for (Z)-substrates, up to 99% yield and 98% ee for (E)-substrates, TON up to 5000), which are important intermediates in organic synthesis.

Full text of DOI:10.1021/acs.orglett.8b02339

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rh-Catalyzed Asymmetric Hydrogenation of β‑Substituted-β-thio-
α,β-unsaturated Esters: Expeditious Access to Chiral Organic Sulfides
Gang Liu,†,§ Zhengyu Han,†,§ Xiu-Qin Dong,*,† and Xumu Zhang*,‡,†
†Key Laboratory of Biomedical Polymers, Engineering Research Center of Organosilicon Compounds & Materials, Ministry of
Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China
‡Department of Chemistry and Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen, Guangdong
518055, P. R. China
S
* Supporting Information
ABSTRACT: Rh/bifunctional bisphosphine-thiourea ligand
(ZhaoPhos)-catalyzed asymmetric hydorgenation of both (Z)-
and (E)-isomers of β-substituted-β-thio-α,β-unsaturated esters
was successfully developed. This new asymmetric catalytic
methodology provided highly efficient access to two
enantiomers of chiral organic sulfides ethyl β-substituted-β-
thio-propanoates with excellent results (up to 99% yield and
>99% ee for (Z)-substrates, up to 99% yield and 98% ee for
(E)-substrates, TON up to 5000), which are important
intermediates in organic synthesis.
ptically active chiral organic sulfides are widely distributed
necessary to develop highly efficient catalytic synthetic methods
to prepare chiral organic sulfides. Transition-metal-catalyzed
asymmetric hydrogenation of prochiral unsaturated compounds
has been regarded as a direct, simple manipulation and high
atom economy method in asymmetric synthesis.8 Based on the
long-standing research in this field and the powerful perform-
ance of a bifunctional bisphosphine-thiourea ligand (ZhaoPhos)
in asymmetric hydrogenation utilizing important interactions
between the thiourea scaffold and the functional group of
substrates,9 we envisaged that the asymmetric hydrogenation of
prochiral β-substituted-β-thio-α,β-unsaturated esters could
proceed well with high reactivity and excellent stereoselective
control contributed by the possible H-bonding interactions
between the substrates and catalyst. Herein, our Rh/ZhaoPhos
catalytic system exhibited powerful catalytic ability for the
asymmetric hydrogenation of both (Z)- and (E)-isomers of β-
substituted-β-thio-α,β-unsaturated esters, providing two enan-
tiomers of chiral organic sulfide ethyl β-substituted-β-thio-
propanoates with excellent results (Scheme 1, up to 99% yield,
>99% ee for (Z)-substrates, up to 99% yield, 98% ee for (E)-
substrates). Furthermore, our Rh/ZhaoPhos catalytic system
displayed high catalytic activity (TON up to 5000), which was
not affected by the possible deactivation of the sulfur atom from
the substrates and hydrogenation products. In addition, our
hydrogenation products are the key intermediates to construct
biologically active molecules, and they also can be easily
converted into other chiral sulfur containing compounds.
We began our initial study to investigate the solvent effect for
the Rh-catalyzed asymmetric hydrogenation of model substrate
O
in natural products1 and also have a broad range of
applications in the fields of organic chemistry, biology, and
pharmaceuticals, for example, working as chiral building blocks,
chiral auxiliaries, chiral ligands, chiral organocatalysts, and
bioactive molecules.2 As shown in Figure 1, Montelukast is a
Figure 1. Examples of some chiral organic sulfides with bioactivity.
potent antagonist of cysteinyl leukotriene (cysLT) receptor used
for the treatment of asthma,2b,c and Diltiazem is mainly used for
the treatment of hypertension and angina pectoris,2d−f while
Thiazesim displays antidepressant activity.2g−k
Owing to the remarkable importance of chiral organic sulfides,
considerable attention has been focused on the development of
efficient asymmetric catalytic methodologies to construct them
in recent decades.3−7 Among the existing methods, both metal-
and organic-catalyst-promoted asymmetric sulfa-Michael addi-
tion (SMA) of thiol nucleophiles to a variety of activated
electron-deficient alkenes,4−7 such as α,β-unsaturated alde-
hydes/ketones,4 α,β-unsaturated esters,4c,5 α,β-unsaturated
imides,6 and nitroolefins,7 have been a reliable and powerful
route for the generation of carbon−sulfur bonds in organic
synthesis. Despite great efforts toward achieving significant
progress, most of these catalytic methodologies have always
required a relatively high catalyst loading. Therefore, it is
Received: July 24, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02339
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Rh/ZhaoPhos-Catalyzed Asymmetric
Hydrogenation for the Construction of Chiral Organic
Sulfides
Table 2. Screening Ligands for the Rh-Catalyzed Asymmetric
Hydrogenation of Ethyl (Z)-3-Phenyl-3-(phenylthio)acrylate
a
(1a)
b
c
entry
ligand
conv (%)
ee (%)
1
2
3
4
5
6
7
8
ZhaoPhos L1
L2
L3
L4
>99
78
50
>99
>99
>99
>99
NA
NA
NA
NA
ethyl (Z)-3-phenyl-3-(phenylthio)acrylate (1a)10 under 50 atm
of H2 at 30 °C with the catalyst generated in situ by mixing
Rh(NBD)2BF4 with ZhaoPhos L1. As shown in Table 1,
11
L5
NR
NR
NR
NR
(S)-SegPhos
(Rc,Sp)-DuanPhos
(S)-BINAP
Table 1. Screening Solvents and Metal Precursors for the
Asymmetric Hydrogenation of Ethyl (Z)-3-Phenyl-3-
a
(phenylthio)acrylate (1a)
a
Unless otherwise noted, all reactions were carried out with a
Rh(NBD)2BF4/ligand/1a (0.1 mmol) ratio of 1:1.1:100 in 1.0 mL of
b
CH2Cl2 under 50 atm of H2 at 30 °C. Conversion was determined
c
1
by H NMR analysis. Ee was determined by chiral HPLC analysis.
NR = no reaction, NA = not available.
b
c
entry
metal precursor
solvent
MeOH
THF
1,4-dioxane
toluene
TFE
conv (%)
ee (%)
1
2
3
4
5
6
7
8
9
Rh(NBD)2BF4
Rh(NBD)2BF4
Rh(NBD)2BF4
Rh(NBD)2BF4
Rh(NBD)2BF4
Rh(NBD)2BF4
Rh(NBD)2BF4
Rh(COD)2BF4
[Ir(COD)Cl]2
28
11
46
77
92
93
>99
98
NR
>99
>99
>99
>99
>99
>99
>99
>99
NA
DCE
CH2Cl2
CH2Cl2
CH2Cl2
a
Unless otherwise noted, all reactions were carried out with a metal
precursor/ZhaoPhos L1/1a (0.1 mmol) ratio of 1:1.1:100 in 1.0 mL
of solvent under 50 atm of H2 at 30 °C. The catalyst was
b
precomplexed in CH2Cl2 (0.1 mL for each reaction vial). Conversion
c
1
SegPhos, (Rc, Sp)-DuanPhos, and (S)-BINAP, did not work in
this asymmetric transformation (Table 2, entries 6−8).
was determined by H NMR analysis. Ee was determined by chiral
HPLC analysis. The configuration of 2a was determined by
comparing the optical rotation data with those reported in the
literature.2j,4c,l THF is tetrahydrofuran. TFE is trifluoroethanol. DCE
is dichloroethane. NR = no reaction, NA = not available.
Having established the optimized reaction conditions, we
started to inspect the substrate generality of this Rh-catalyzed
asymmetric hydrogenation of ethyl (Z)-3-substituted-3-thio-
acrylates. As shown in Scheme 2, a series of ethyl (Z)-3-
substituted-3-thio-acrylates proceeded well to provide the
desired hydrogenation products with excellent results (up to
>99% conversion, 99% yield, >99% ee). We found that the
electron-neutral (1a), electron-rich (1b−1e), or electron-
deficient (1f−1g) substituents on the phenyl ring of the arylthio
group can be hydrogenated smoothly to prepare hydrogenation
products (2a−2g) with full conversions, 96%−99% yields, and
>99% ee. In addition, the electric properties and positions of the
substituents on the 3-aryl ring substituted group also exhibited
little influence on the reactivity and enantioselectivity, affording
hydrogenation products (2h−2l) with excellent results (>99%
conversion, 98%−99% yields, 94% − >99% ee). The 2-naphthyl
substituted substrate (1m) also worked well with 99% yield and
96% ee. The challenging alkyl substrate ethyl (Z)-3-cyclohexyl-
3-(phenylthio)acrylate (1n) and heteroaromatic substrate ethyl
(Z)-3-(phenylthio)-3-(pyridin-3-yl)acrylate (1o) also displayed
good to excellent results (80% − >99% conversions, 77%−98%
yields, 97% − >99% ee).
although excellent enantioselectivity can be obtained in various
solvents, the solvents have great influence on the reactivity (11%
− >99% conversions, > 99% ee, Table 1, entries 1−7). Full
conversion and >99% ee can be obtained in CH2Cl2 (Table 1,
entry 7). Other metal precursors, such as Rh(COD)2BF4 and
[Ir(COD)Cl]2, were also inspected in this transformation
(Table 1, entries 8−9). Excellent results were afforded with
Rh(COD)2BF4 working as the metal precursor (98% con-
version, >99% ee, Table 1, entry 8). No product was detected
with [Ir(COD)Cl]2 as the metal precursor (Table 1, entry 9).
Subsequently, various chiral bisphosphine ligands were
applied to this Rh(NBD)2BF4-catalyzed asymmetric hydro-
genation of ethyl (Z)-3-phenyl-3-(phenylthio)acrylate (1a) in
DCM. These results are summarized in Table 2. A series of
bifunctional bisphosphine-(thio)urea ligands L1−L4 provided
poor to excellent reactivities and >99% ee (11% − >99%
conversions, >99% ee, Table 2, entries 1−4). ZhaoPhos L1
achieved the best results with full conversion and >99% ee
(Table 2, entry 1). In addition, no reaction was obtained in the
presence of ligand L5 without a thiourea group (Table 2, entry
5). Other important chiral bisphosphine ligands, such as (S)-
Having succeeded in the highly enantioselective hydro-
genation of ethyl (Z)-3-substituted-3-thio-acrylates, we turned
our attention to investigating the reactivity and enantioselectiv-
B
DOI: 10.1021/acs.orglett.8b02339
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Substrate Scope of Rh-Catalyzed Asymmetric
Hydrogenation of Ethyl (Z)-3-Substituted-3-thio-acrylates
Scheme 3. Substrate Scope of Rh-Catalyzed Asymmetric
a
a
Hydrogenation of Ethyl (E)-3-Substituted-3-thio-acrylates
a
Unless otherwise noted, all reactions were carried out with a
Rh(NBD)2BF4/ZhaoPhos L1/3 (0.1 mmol) ratio of 1:1.1:100 in 1.0
mL of TFE under 50 atm of H2 for 18 h at 60 °C. Conversion was
1
determined by H NMR analysis. Yield was isolated yield. Ee was
determined by chiral HPLC analysis.
a
Scheme 4. Gram-Scale Experiment with High TON and
Transformations of Hydrogenation Product
Unless otherwise noted, all reactions were carried out with a
Rh(NBD)2BF4/ZhaoPhos L1/1 (0.1 mmol) ratio of 1:1.1:100 in 1.0
mL of CH2Cl2 under 50 atm of H2 for 12 h at 30 °C. Conversion was
1
determined by H NMR analysis. Yield was isolated yield. Ee was
determined by chiral HPLC analysis. The reaction was carried out at
40 °C for 18 h. The reaction was carried out at 60 °C for 48 h.
b
c
ity of (E)-3-substituted-3-thio-acrylates10a catalyzed by the Rh/
ZhaoPhos system, which can provide the hydrogenation
products with opposite configuration. The optimization of
reaction conditions was summarized in Table S1 in the
Supporting Information. As shown in Scheme 3, we found
that the substituents either on the phenyl ring of the arylthio
group or on the 3-aryl ring group have little effect on this
asymmetric hydrogenation. The asymmetric hydrogenation of
substrates (3a−3j) proceeded well, affording the desired
products with full conversions, 97%−99% yields, and 94%−
98% ee. In addition, the 2-naphthyl substituted substrate (3k)
also worked well with excellent results (>99% conversion, 99%
yield, and 97% ee).
To our delight, the gram-scale asymmetric hydrogenation of
ethyl (Z)-3-phenyl-3-(phenylthio)acrylate (1a) proceeded
smoothly to obtain product (2a) with 98% yield and 99% ee,
even only in the presence of 0.02 mol % catalyst (S/C = 5000),
which displayed that our Rh/ZhaoPhos system showed high
activity in this transformation (Scheme 4). Moreover, the
hydrogenation product (2a) was an important synthetic
intermediate for the construction of biologically active molecule
Thiazesim.2j In addition, it was easily reduced by LiAlH4 to
generate (R)-3-phenyl-3-(phenylthio)propan-1-ol (5a) in 85%
yield and nearly without loss of ee (98% ee).11 The carboxylate
group of compound 2a was efficiently hydrolyzed to carboxylic
acid (R)-3-phenyl-3-(phenylthio)propanoic acid (6a) in 89%
yield and 99% ee.2j,12
In summary, we successfully developed a highly enantiose-
lective Rh/ZhaoPhos-catalyzed asymmetric hydrogenation of
both ethyl (Z)- and (E)-3-substituted-3-thio-acrylates, provid-
ing two enantiomers of chiral organic sulfides ethyl β-
substituted-β-thio-propanoates with excellent results (up to
99% yield and >99% ee for (Z)-substrates, up to 99% yield and
98% ee for (E)-substrates, TON up to 5000). In addition, the
hydrogenation products are important synthetic intermediates,
which can be efficiently converted to other useful sulfur-
containing compounds.
C
DOI: 10.1021/acs.orglett.8b02339
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02339.
■
S
Optimization reaction conditions; procedures; NMR and
HPLC spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
́
(k) Marigo, M.; Schulte, T.; Franzen, J.; Jørgensen, K. A. J. Am. Chem.
*E-mail: zhangxm@sustc.edu.cn.
*E-mail: xiuqindong@whu.edu.cn.
ORCID
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Xumu Zhang: 0000-0001-5700-0608
Author Contributions
§G.L. and Z.H. contributed equally.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (Grant No. 21432007,
21502145), the Natural Science Foundation of Hubei Province
(Grant No. 2016CFB449), the Wuhan Morning Light Plan of
Y o ut h S c i e n c e a n d T e c h n o lo g y ( G r a n t N o .
2017050304010307), Shenzhen Nobel Prize Scientists Labo-
ratory Project (Grant No. C17213101), and the Fundamental
Research Funds for Central Universities (Grant No.
2042018kf0202). The Program of Introducing Talents of
Discipline to Universities of China (111 Project) is also
appreciated.
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Org. Lett. XXXX, XXX, XXX−XXX