Cu(I)-Catalyzed Enantioselective Alkynylation of Thiochromones

Article abstract of DOI:10.1021/acs.orglett.0c00005

A highly efficient asymmetric synthesis of chiral thiochromanones is developed via Cu(I)/phosphoramidite catalyzed asymmetric alkynylation of thiochromones under mild reaction conditions. The catalyst system is tolerant of various thiochromone precursors and terminal alkynes. The established asymmetric transformation provides different enatiomeric-enriched thiochromanones with more molecular complexity and enables access to chiral thioflavanones, a subgroup of flavonoid by further functionalization.

Full text of DOI:10.1021/acs.orglett.0c00005

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Cu(I)-Catalyzed Enantioselective Alkynylation of Thiochromones
Ling Meng, Ka Yan Ngai, Xiaoyong Chang, Zhenyang Lin,* and Jun Wang*
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ABSTRACT: A highly efficient asymmetric synthesis of chiral
thiochromanones is developed via Cu(I)/phosphoramidite cata-
lyzed asymmetric alkynylation of thiochromones under mild
reaction conditions. The catalyst system is tolerant of various
thiochromone precursors and terminal alkynes. The established
asymmetric transformation provides different enatiomeric-enriched
thiochromanones with more molecular complexity and enables access to chiral thioflavanones, a subgroup of flavonoid by further
functionalization.
Scheme 1. Asymmetric Synthesis of Thiochromanones
ulfur-containing heterocyclic systems have been found in
Snumerous natural products and pharmaceuticals and
received extensive research from scientists.1 Thioflavonoids,
the sulfur analogues of ubiquitous and useful flavanoids, are an
important and indispensible class of heterocyclic compounds.
Although thioflavanoids have not been found in natural
products yet, they present various interesting biological
activities, including antihuman breast cancer, antidepressive,
antiallergic, anti-inflammatory, antiarthritic, antifungal, blood
lipid lowering, etc.2 More interestingly, some thioflavanoids
have showed improved biological activity compared to
flavanoids, the sure victors in the battle of bioactive potency.3
Apart from pharmaceutical application, thioflavonoids are also
important in the area of materials science, in which
thioflavonoids and their derivatives can be served as photo-
labile protecting groups,4 photoinitiators,5 etc. Some of them
exhibit a great liquid crystal (LC) property6 and thermally
activated delayed fluorescence which may promote exciton
utilization efficiency in organic light-emitting diodes
(OLEDs).7 Therefore, investigations of this kind of hetero-
cycles in the fields of chemical synthesis, bioactivity, and
optical materials have grown continuously in recent decades
and have gained increasing attention from scientists around the
world.
selective conjugated reduction of thiochromones to give 2-
alkylthiochromanones (Scheme 1b).11e Recently, Cu-catalyzed
formal conjugate addition to thiochromones with Grignard
reagents was reported but only in moderate yield and
enantioselectivity (Scheme 1c).11f Furthermore, the alkyl
groups on the 2-position of thiochromanones are difficult for
further functionalization. Herein, alternative access to optically
active thiochromanones remains desirable and in high need.
Even though alkynylation to chromone has been reported very
recently,12 the reactivities of chromone and thiochromone are
quite different. The latter is more sluggish than the former, as
there is a higher degree of delocalization of the sulfur lone pair
compared to that of oxygen, which would result in higher
electron density in the unsaturated system, hence lowering the
The approaches to racemic thiochromanes8 and thiochro-
manones9 have been developed extensively. There are also
several elegant examples that showed enantioselective synthesis
of thiochromanes in recent years.10 However, only limited
examples showed the asymmetric version of the synthesis of
thiochromanones. Our group has a long-standing interest in
the synthesis and bioactive research of chiral flavanoids and
thioflavanoids.11 In 2016, an asymmetric conjugate addition of
arylzinc chlorides to thiochromones was disclosed, producing
the 2-arylthiochromanones (thioflavanones) in high yields with
excellent enantioselectivities (Scheme 1a).11c It was the first
example that the thiochromones could react via formal
conjugate addition. Later, we reported Cu-catalyzed enantio-
Received: January 1, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00005
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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reactivity. Also, the affinity of sulfur with transition metals
invariably makes the catalytic reaction complicated.13 Besides,
asymmetric alkynylation of thichromones can offer a new
method to synthesize various functionalized thiochromanones
for further research in bioactivity and materials science.14
Therefore, we conducted the investigation of formal conjugate
addition to thiochromones with terminal alkynes.
Initial investigation was commenced by using unsubstituted
thiochromone 1a and phenylacetylene 2a as model substrates
with CuI as the transition metal catalyst (Table 1). Without
ligand, the reaction offered the desired product 3a in 66% yield
(entry 1), which illustrated that the background reaction exists
to some extent and the stereoselectivity of the product is not
easily controlled. To address the problem, a large variety of
chiral ligands were tested extensively for the addition (see
Supporting Information (SI)). Ligands with different skeletons
that were generally utilized in a copper catalyzed trans-
formation or 1,4-addition were selected and are shown in
Table 1. Considering the feasibility of modification of ligand,
we first carried out the asymmetric investigation using the
chiral Box ligand. Disappointedly, ligand L1 did not give
satisfactory enantioselectivity (22% ee, entry 2). Other Box
ligands with different R groups were also tested, and 4% to
47% ee values were obtained (see SI). Bisphosphorus ligand
L2 offered the targeted adduct in 70% yield with no
enantioselectivity (entry 3). L3 and L4, two types of N, P-
ligand, gave the corresponding products in moderate yields
with 29% and trace ee, respectively (entries 4 and 5). Next, a
number of phosphoramidite ligands were investigated. The
commercially available ones with different substituted groups
on the N atom provided the products with 7% to 36%
enantioselectivities (see SI). By analogy, we found that L6, a
ligand with methyl groups on the 3,3′-positions of the
binaphthol backbone, led to a 15% higher enantioselectivity
than L5 (entries 6 and 7). Thus, changing the substituted R1
groups on the binaphthol was a probable way to attain higher
enantioselectivity. Therefore, various phosphoramidite ligands
with substituents possessing different steric hindrance and
electronic property were synthesized according to the
literature. L7 led to a breakthrough in progress, affording the
desired product in 99% yield with 73% ee (entry 8). However,
more bulky L8 did not provide a better result but instead a
sharp decrease in yield and ee (entry 9). L9, L10, and L11
resulted in 91% to 55% yield with 76% to 6% ee (entries 10−
12). To our delight, the product 3a was obtained in 90% yield
with 86% ee via replacing the group on the 3,3′-positions of
binaphthol by an electron-withdrawing group (entry 13).
Changing the substitution on the N atom from Me to Et did
not improve the enantioselectivity but did slightly improve the
reactivity (97% yield with 86% ee, entry 14). More bulky
ligand L14 with isopropyl substituents offered extremely
inferior reactivity and stereoselectivity (38% yield with 4%
ee, entry 15). The phosphoramidite ligands with the
pentafluorophenyl group were tested as follows. L15 provided
the adduct in 85% yield with 78% ee while more hindered
ligand L16 gave the highest enantioselectivity and an excellent
yield (95% yield with 92% ee, entry 17).
With the optimized reaction conditions in hand, the scope of
terminal alkynes was explored. As shown in Table 2, various
alkynes can provide the corresponding products in moderate to
high yield with good to high enantioselectivities. A series of
aryl alkynes bearing either an electron-rich group (e.g., 3-MeO,
3-Me) or electron-poor group (e.g., 3-F, 3-Br, 3-I) on the aryl
ring were well tolerated, offering the corresponding products in
91% to 98% yields with 92% to 94% ee (Table 2, entries 2, 4,
and 6−8). The reactions still proceeded successfully when aryl
alkynes have strong electron-withdrawing groups such as 3-CF3
(91% yield with 92% ee) and 3-NO2 (80% yield with 82% ee)
at the benzene ring (entries 9 and 10). Moreover, an aryl
alkyne with active hydrogen was also compatible in this
addition, conferring the product in 62% yield with 73% ee
(entry 11). The position of the substituted group on the
benzene ring of the alkyne was also examined. Both 4-Me and
3-Me substituted aryl alkynes can result in the corresponding
adducts in high yield with high enantiomeric excess (entries 3
and 4). When there was a methyl group at the ortho-position of
the alkyne, the reactivity and stereoselectivity were slightly
reduced (79% yield and 80% ee, entry 5). Apart from aryl
a
Table 1. Optimization
b
c
entry
ligand
yield (%)
ee (%)
1
2
3
4
5
6
7
8
−
L1
L2
L3
L4
L5
L6
L7
66
63
70
86
64
81
77
99
50
91
86
55
90
97
38
85
95
0
22
4
29
1
33
48
73
10
76
44
6
86
86
4
9
L8
L9
10
11
12
13
14
15
16
17
L10
L11
L12
L13
L14
L15
L16
78
92
a
The reactions were carried out with 10 mol % CuI and 11 mol %
ligand in toluene (2.0 mL) which were stirred at rt under Ar. After 30
min, 0.15 mmol of 1a, 0.2 mmol of 2a, and 0.24 mmol of i-Pr2NEt
were added at room temperature. Then the reaction temperature was
cooled down to 0 °C, and 0.2 mmol of TMSOTf was added dropwise.
b
c
Isolated yields. Determined by HPLC analyses.
B
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a
a
Table 2. Scope of Terminal Aryl Alkynes
Scheme 2. Scope of Thiochromones
b
c
entry
R
3
yield (%)
ee (%)
1
2
3
4
5
6
7
8
PhC6H4
3da
3db
3dc
3dd
3de
3df
3dg
3dh
3di
97
91
96
93
79
98
97
97
91
80
62
91
94
94
92
93
80
92
93
94
92
82
73
94
3-MeOC6H4
4-MeC6H4
3-MeC6H4
2-MeC6H4
3-FC6H4
3-ClC6H4
3-BrC6H4
3-CF3C6H4
3-NO2C6H4
4-CHOC6H4
3-thienyl
9
10
11
12
3dj
3dk
3dl
a
The reactions were carried out with 10 mol % CuI and 11 mol %
ligand in toluene (2.0 mL) which were stirred at rt under Ar. After 30
min, 0.15 of mmol 1a, 0.2 mmol of 2a, and 0.24 mmol of iPr2NEt
were added at room temperature. Then the reaction temperature was
cooled down to 0 °C, and 0.2 mmol of TMSOTf was added dropwise.
b
c
Isolated yields. Determined by HPLC analyses.
alkynes, a heterocyclic alkyne was tested as well and gave the
product in good yield with high enantioselectivities (91% yield,
94% ee) (entry 12).
a
Reaction conditions are same as those for Table 2.
To further investigate the substrate scope, a variety of
substituted thiochromones was examined as follows (Scheme
2). The electronic property of substituted groups had no
significant effect on the reactivity and stereoselectivity of the
reaction, offering the products in 93% to 97% yields and 86%
to 95% ee (3ba−3fa). Even the substrate bearing a CF3 group
can provide the product in 90% yield with 83% ee (3ma).
When the substituted group on the substrate was a methyl
group, the position of the methyl group affected the
enantioselectivity more than the reactivity (see 3ca, 3ga,
3ha, and 3ia). The opposite results were observed with F as
the substituent on thiochromone, which provided the product
in only 66% yield but still with high ee when F is located in the
8-position (see 3da, 3ja, and 3ka). Substrates containing two
substituents and a naphthalene nucleus also gave the desired
products in high yields with moderate to high ee values (3la,
3na). We also tried chromone under the same reaction
conditions, and the corresponding product was obtained in
72% yield with 69% enantioselectivity (3oa). Finally, the
absolute configuration of the product 3ea was confirmed as S
by X-ray crystallography (CCDC 1967411).
Scheme 3. Plausible Mechanism
triflate (OTf) produced the highest reactivity and enantiose-
lectivity in the catalyst system of CuI/L (see SI).
To evaluate the efficacy of the reaction, a scaled-up reaction
was conducted by using 1 mmol of thiochromone 1e. The
reaction was completed in 12 h under the standard reaction
conditions, and the product 3ea was obtained in 90% yield
(269 mg) with 95% ee (Scheme 4a). And the chiral 3ea can
easily undergo oxidation and reduction to generate the
corresponding sulfone and 2-alkylthiochromanone in good
yield without sacrificing enantioselectivity (Scheme 4b).
In summary, we have successfully developed a Cu(I)/
phosphoramidite catalyzed asymmetric formal conjugate
addition of thiochromones with terminal alkynes, from which
various 2-alkynylthiochromanones were obtained in high
reactivities (up to 98% yield) and high enantioselectivities
(up to 95% ee). TMSOTf proved uniquely efficient for the
transformation. It might serve as not only a Lewis acid to
activate thiochromones but also an appropriate counteranion
to stabilize the complex formed in the transformation. Further
Based on the related literature15 and the NMR experiment
(see SI), we proposed a plausible mechanism for the formal
conjugate addition to thiochromones with alkynes (Scheme 3).
The reaction likely takes place via the initial formation of
copper acetylide A with the help of a base. Thiochromone is
activated by addition of TMSOTf to form 4-((trimethylsilyl)-
oxy)thiochromenylium which would coordinate to the copper
acetylide to generate complex B. Then, acetylide proceeds in
migratory insertion to form complex C which subsequently
releases the coordinated silyl enol ether D to give the final
product. In this reaction, a remarkable anion effect from the
trimethylsilyl salt was observed and the less coordinating
C
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(KQJSCX20180319114439973) and the Research Grants
Council of Hong Kong (HKUST 16304416) for financial
support.
Scheme 4. Scale-up Reaction and Functionalizations
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00005.
Experimental procedures and characterization/HPLC
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Accession Codes
CCDC 1967411 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-
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AUTHOR INFORMATION
Corresponding Authors
■
Zhenyang Lin − The Hong Kong University of Science &
Technology, Kowloon, Hong Kong; orcid.org/0000-
0003-4104-8767; Email: chzlin@ust.hk
Jun Wang − Southern University of Science and
Technology, Shenzhen, China; orcid.org/0000-0001-
9723-4054; Email: wang.j@sustech.edu.cn
Other Authors
Ling Meng − Southern University of Science and
Technology, Shenzhen, China, and The Hong Kong
University of Science & Technology, Kowloon, Hong Kong
Ka Yan Ngai − Southern University of Science and
Technology, Shenzhen, China; orcid.org/0000-0002-
7785-8166
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge Shenzhen Overseas High-level
Talents Innovation Plan of Technical Innovation Project
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