Stille coupling for the synthesis of isoflavones by a reusable palladium catalyst in water

Article abstract of DOI:10.1002/jccs.202000478

Isoflavones were synthesized from the reaction of 3-bromochromone derivatives and aryltributylstannanes via Stille coupling catalyzed by a water-soluble and reusable PdCl₂(NH₃)₂/2,2′-cationic bipyridyl system in aqueous solution. For prototype 3-bromochromone, the coupling reaction was performed at 80°C for 24 hr with 2.5 mol% catalyst in water in the presence of tetrabutylammonium fluoride. After the reaction, the aqueous solution could be reused for several runs, indicating that its activity was only slightly decreased. For substituted 3-bromochromones, the addition of NaHCO₃ and a higher reaction temperature (120°C) were required to gain satisfactory outcomes. In addition, naturally occurring products, such as daidzein, could be obtained by this protocol via a one-pot reaction.

Full text of DOI:10.1002/jccs.202000478

Received: 28 October 2020
DOI: 10.1002/jccs.202000478
Revised: 28 December 2020
Accepted: 5 January 2021
S P E C I A L I S S U E P A P E R
Stille coupling for the synthesis of isoflavones by a reusable
palladium catalyst in water
Ya-Ting Chang1 | Ling-Jun Liu1 | Wen-Sheng Peng1 | Lin-Ting Lin2
Yi-Tsu Chan2 | Fu-Yu Tsai1
|
1Institute of Organic and Polymeric
Abstract
Materials, National Taipei University of
Technology, Taipei, Taiwan
Isoflavones were synthesized from the reaction of 3-bromochromone deriva-
2Department of Chemistry, National
Taiwan University, Taipei, Taiwan
tives and aryltributylstannanes via Stille coupling catalyzed by a water-soluble
and reusable PdCl2(NH3)2/2,20-cationic bipyridyl system in aqueous solution.
For prototype 3-bromochromone, the coupling reaction was performed at
80ꢀC for 24 hr with 2.5 mol% catalyst in water in the presence of
tetrabutylammonium fluoride. After the reaction, the aqueous solution could
be reused for several runs, indicating that its activity was only slightly
decreased. For substituted 3-bromochromones, the addition of NaHCO3 and
a higher reaction temperature (120ꢀC) were required to gain satisfactory out-
comes. In addition, naturally occurring products, such as daidzein, could be
obtained by this protocol via a one-pot reaction.
Correspondence
Fu-Yu Tsai, Institute of Organic and
Polymeric Materials, National Taipei
University of Technology, 1, Sec.
3, Chung-Hsiao E. Rd., Taipei 10608,
Taiwan.
Email: fuyutsai@ntut.edu.tw
K E Y W O R D S
isoflavone, natural product, reusable catalyst, Stille coupling, water
1 | INTRODUCTION
Transition-metal-catalyzed reactions toward the for-
mation of isoflavones afford a more convenient protocol
Isoflavones are a type of naturally occurring products,
which have been found in the Leguminosae family.[1]
Biological studies of these compounds reveal that
isoflavones have anti-inflammatory,[2] antifungal,[3]
antiviral,[4] antioxidant,[5] and anticancer[6] properties.
Conventional synthetic methods for the preparation of
isoflavones include deoxybenzoin cyclization of
2-hydroxyphenylbenzylketones,[7–16] oxidative rearrange
ment,[17–22] or epoxidation[23] of a chalcone to form
isoflavones, N-heterocyclic carbene-catalyzed reactions,[24,25]
intramolecular [2+2] ketene cycloadditions,[26] reduction and
cyclization of isoxazole derivatives,[27] and oxidation of
flavanones.[20,28–31] Recently, coupling of 3-iodochromones
with heteroaromatics by a photochemical process,[32] and K10
to fulfill this purpose. Rh(I)-catalyzed oxidative-coupling
of salicylaldehyde with phenylacetylene[34] and Cu(I)-
catalyzed
cyclization
of
3-(2-bromophenyl)-
3-oxopropanals[35] gave isoflavones. Transition-metal-
catalyzed C C bond cross-coupling, such as Negishi cou-
pling by Pd[36] or Ni[37,38] and Suzuki-Miyaura
reactions,[39–55] were widely applied for the synthesis of
isoflavones in recent years. In addition, triarylbismuths
were also employed as a coupling partner of 3-iodo or
3-trifloxychromones for Pd-catalyzed cross-coupling.[56,57]
The Stille coupling is one of the most powerful
palladium-catalyzed reactions to build a new C C
bond.[58–60] Organostannanes are moisture and O2 stable
and have tolerance toward a great variety of functional
groups. Hence, they can be handled under air and harsh
reaction conditions. In several large-scale pharmaceutical
purposes, especially for the synthesis of complex hetero-
cyclic systems, Stille cross-coupling reactions are superior
montmorillonite-catalyzed
one-pot
reactions
of
1-(2-hydroxyphenyl)butane-1,3-diones and 2,5-dimethoxy-
2,5-dihydrofuran[33] to produce 3-heteroarylchromones were
also reported.
J Chin Chem Soc. 2021;68:469–475.
http://www.jccs.wiley-vch.de
© 2021 The Chemical Society Located in Taipei & Wiley-VCH GmbH
469

470
CHANG ET AL.
SCHEME 1 Pd-catalyzed Stille coupling
for the formation of isoflavones in water
TABLE 1 Optimal condition studies for the Stille coupling of 3-bromo-4H-chromen-4-one (1a) and PhSnBu3 (2a) in water
Entry
1
1a (mmol)
0.5
2a (mmol)
1.0
TBAF (mmol)
Base (mmol)
Temp. (ꢀC)
Yield (%)a
—
—
80
80
12
24
38
89
74
78
88
0
2
0.5
1.0
1.0
—
—
3
1.0
0.5
—
80
4
1.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
—
80
5
0.75
1.0
0.5
—
80
6b
7c
8d
9
0.5
—
80
1.0
0.5
—
80
1.0
0.5
—
80
1.0
0.5
—
120
80
80
76
66
40
45
85
10
11
12
13
14
1.0
0.5
NaHCO3 (0.5)
KOAc (0.5)
K2CO3 (0.5)
K3PO4 (0.5)
NaHCO3 (0.5)
1.0
0.5
80
1.0
0.5
80
1.0
0.5
80
1.0
0.5
120
Note: Reaction conditions: 1a, 2a, TBAF, base, PdCl2(NH3)2/L (2.5 mol%), H2O (2.5 ml) at 80 or 120ꢀC for 24 hr.
aIsolated yields of 3aa.
bReaction time was 12 hr.
c5 mol% of catalyst was used.
dIn the absence of L.
SCHEME 2 The reuse studies of
residual aqueous solution in Stille
coupling of 1a and 2a
to Suzuki-Miyaura and Negishi couplings.[61,62] Therefore,
the Stille reaction is still a promising process in modern
organic synthesis. In comparison with other name reac-
tions, such as Suzuki-Miyaura reactions, their study for
the synthesis of isoflavones is still limited. Known proce-
dures include coupling of 3-(trimethylstannyl)-4H-1-ben-
zopyran-4-ones with 4-iodonitrobenzene in N-Methyl-2-
pyrrolidone[63] and tetrathiophentins with substituted
3-iodochromones in refluxing EtOH.[64] However, both the
reactions use organic solvents as the reaction medium, and
the reuse of the expensive catalyst has not been reported.
Although the coupling of 3-iodo-7-isopropoxychromone
with tetraphenyltin has been performed under LiCl-
promoted conditions to give ipriflavone, harmful N,N-
Dimethylformamide was used as the solvent.[65] Hence,
from the viewpoints of sustainable and green chemistry,

CHANG ET AL.
471
TABLE 2 Stille coupling of 1a and 2 under in water
chromone, which is formed via debromination of 1a
(Entry 1). When tetrabutylammonium floride (TBAF)
was added for accelerating the transmetallation step,[67]
the product yield of 3aa was doubled; however, a signifi-
cant amount of chromone was still found (Entry 2). Thus,
the limiting reagent was changed from 1a to 2a for seek-
ing the optimal conditions. It was found that the condi-
tions of Entry 4 affording the best result (Entries 3–5).
Reducing the reaction time to 12 hr gave 78% yield
(Entry 6). An 88% yield of 3aa was delivered when 5 mol
% of catalyst was employed (Entry 7). Removing the
water-soluble 2,20-cationic bipyridyl, L, from the catalytic
system did not afford 3aa, owing to the poor solubility of
PdCl2(NH3)2 in water, supporting the necessity of this
ligand when water was applied as a solvent (Entry 8).
Elevating the reaction temperature to 120ꢀC furnished
3aa in 80% yield (Entry 9). In our former report, the addi-
tion of a weak inorganic base may help the Stille cou-
pling reaction in aqueous solution.[66] Several inorganic
bases were, therefore, added to obtain the best reaction
conditions (Entries 10–13). Among them, NaHCO3 was
found to be the best base (Entry 10). However, the yield
for the added base was still lower than that in Entry
4. Then, we further increased the reaction temperature to
120ꢀC, which gave a comparable yield of 3aa to Entry
4 (Entry 14). For the pharmaceutical synthesis through
metal-catalyzed reactions, the metal contamination in
the organic products is an issue of critical concern. Both
Pd and Sn contents in 3aa after silica gel chromatography
was estimated to be less than 1 ppb by ICP-MS analysis.
The catalyst reuse study was conducted under the
conditions of Table 1, Entry 4 and the results were shown
in Scheme 2. After completion of the Stille coupling in
the first run, the organic portion was extracted with
EtOAc (3 ml × 3), and the product was purified by a typi-
cal work-up procedure, which provided an 89% yield of
3aa in the first run. The residual aqueous phase was then
recharged with 1a, 2a, and TBAF for the first reuse run.
Only a slight decrease in its activity was found for each
reuse run, revealing this catalyst is stable upon heating in
aqueous solution, and could be separated easily from the
organic phase by a simple extraction procedure.
Yield
(%)a
Entry ArSnBu3
Product
1
73
83
62
31
75
64
83
0
2
3
4
5
6
7
8
Note: Reaction conditions: 1a (1.0 mmol), 2 (0.5 mmol), TBAF (0.5 mmol),
PdCl2(NH3)2/L (2.5 mol%), and H2O (2.5 ml) at 80ꢀC for 24 hr.
aIsolated yields.
the development of a reusable catalytic system, which is
compatible with a green solvent, water, is highly valuable.
On the basis of our recent investigation on the Stille
coupling in water,[66] we report herein an efficient Stille
cross-coupling reaction of 3-bromochromone derivatives
and aryltributylstannanes for the synthesis of isoflavones
by a reusable PdCl2(NH3)2/2,20-cationic bipyridyl cata-
lytic system hiring neat water as the reaction medium
(Scheme 1). Moreover, the combination of this coupling
reaction with demethylation, a natural product, daidzein,
could be achieved through a one-pot synthesis.
The coupling reactions of 1a with various ary-
ltributylstannanes, 2, were evaluated and the results were
summarized in Table 2. Aryltributylstannanes with
electron-rich or slightly electron-deficient groups at
4-position, 2b–2d, coupled with 1a smoothly gave 3ab–
3ad in high yields (Entries 1–3). A strongly electron-
withdrawing group, CF3, at para-position caused C–Sn
bond more covalent, and thus slowed the trans-
metallation rate resulting in a low yield of 3ae (Entry 4).
2 | RESULTS AND DISCUSSION
As described in Table 1, treatment of 3-bromo-4H-
chromen-4-one, 1a (0.5 mmol), with PhSnBu3, 2a
(1.0 mmol), in the presence of 2.5 mol% catalyst at 80ꢀC
for 24 hr resulted in the formation of 3-phenyl-4H-
chromen-4-one, 3aa, in 12% yield along with 85%
When
the
multi-methoxy
substituted
ary-
ltributylstannanes, 2f–2h, were utilized, the high product

472
CHANG ET AL.
TABLE 3 Stille coupling of 1 and 2 under aqueous medium
Entry
3-Bromochromone
ArSnBu3
2a
Product
Yield (%)a
1
0b
40c
76
2
3
1b
1b
2b
2c
61
88
4
5
6
1b
1b
1b
2d
2f
60
59
55
2 g
7
1b
2 h
66
8
2a
2b
2c
2d
91
64
80
61
9
1c
1c
1c
10
11
12
13
1c
1c
2f
92
78
2 g
14
1c
2 h
81
15
16
17
1d
2a
2b
2c
71
55
68
1d
1d

CHANG ET AL.
473
TABLE 3 (Continued)
Entry
3-Bromochromone
ArSnBu3
2d
Product
Yield (%)a
18
1d
52
19
20
1d
1d
2f
72
56
2 g
21
1d
2 h
64
Note: Reaction conditions: 1 (1.0 mmol), 2 (0.5 mmol), TBAF (0.5 mmol), NaHCO3 (0.5 mmol), PdCl2(NH3)2/L (2.5 mol%), and H2O (2.5 ml) at 120ꢀC for 24 hr.
aIsolated yields.
bAt 80ꢀC in the absence of NaHCO3.
cAt 120ꢀC in the absence of NaHCO3.
SCHEME 3 One-pot reaction
for the synthesis of daidzein
yields of 3af–3ah could be achieved (Entries 5–7). How-
ever, the use of 2i did not provide the corresponding
coupling product, presumably because the trans-
metallation was retarded by sterically hindered
organotin (Entry 8).
As shown in Table 3, several substituted
3-bromochromones, 1b–1d, were examined in combina-
tion with 2a–2d and 2f–2h. In contrast to 1a, substituted
3-bromochromone, 1b, could not be coupled with 2a
under the conditions of Entry 4 in Table 1, leading to the
recovery of 1b. Elevating the reaction temperature to
120ꢀC provided 3ba in only 40% yield. To our delight,
76% of 3ba was obtained when the reaction was con-
ducted under the conditions of Table 1, Entry 14 (Entry
1). Therefore, a variety of isoflavones, 3bb–3dh, can be
readily synthesized in good to high yields by the coupling
of substituted 3-bromochromones with ary-
ltributylstannanes (Entries 2–21).
As an application of this protocol, we synthesized the
naturally occurring isoflavone, daidzein (4), starting from
the coupling reaction of 1d and 2c in one pot. After per-
forming the Stille coupling based on the conditions of
Entry 17 in Table 3, the aqueous phase was evaporated to
dryness under vacuum. The residual mixture was then
treated with an excess amount of pyridine hydrochloride
and heated at 180ꢀC under inert atmosphere for demeth-
ylation.[68] After cooling the reaction to room tempera-
ture, precipitation of the crude product and purification
by column chromatography provided daidzein 4 in 63%
yield (Scheme 3).

474
CHANG ET AL.
3 | EXPERIMENTAL
3.1 | General
reaction tube was evaporated under vacuum to dryness.
Pyridine hydrochloride (5.0 mmol) was added and the
mixture was then heated at 180ꢀC for 12 hr under N2.
After cooling to room temperature, 1N HCl aqueous solu-
tion was added to produce deep brown precipitates. The
crude was purified by column chromatography (SiO2,
hexane/THF = 1/1, vol/vol) to afford daidzein 4 in 63%
yield.
Chemicals were utilized as received from the commercial
source. The water-soluble cationic 2,20-bipyridyl ligand, L,[69]
1,[70] and 2[71] were prepared according to published proce-
1
13
dures. H- and C-NMR spectra were acquired at 25ꢀC in
CDCl3 or DMSO-d6 solution on a Bruker Biospin AG
300 NMR spectrometer (Bruker Co., Faellanden, Switzerland),
where the chemical shifts (δ in ppm) were established with
respect to CHCl3 and the non-deuterated DMSO, which was
employed as a reference (1H-NMR: CHCl3 at 7.24 and non-
deuterated DMSO at 2.49 ppm; 13C-NMR: CDCl3 at 77.0 and
DMSO-d6 at 39.5 ppm). High-resolution mass spectra (HRMS)
for new compounds were acquired on a Waters Synapt High
Definition Mass Spectrometry (HDMS) G2 instrument with a
LockSpray ESI source (see Supporting information for the spec-
tral data of all Stille coupling products).
4 | CONCLUSIONS
In conclusion, we have successfully applied the
PdCl2(NH3)2/cationic 2,20-bipyridyl system for the Stille
coupling to prepare isoflavones in water. This water-
soluble catalyst enables the reaction to be operationally-
simple and easy separation of the catalyst from the organic
layer for further reuse. Moreover, a naturally occurring iso-
flavone, daidzein, could be readily obtained by this simple
protocol via a one-pot reaction.
3.2 | Typical procedure for the Stille
coupling reaction
ACKNOWLEDGMENTS
This research was financially supported by the Ministry
of Science and Technology of Taiwan (MOST
108-2113-M-027-003).
A PdCl2(NH3)2/L aqueous solution (0.0125 mmol, 2.5 mol%,
in 2.5 ml H2O) in a 25 ml sealable tube was charged with 1
(1.0 mmol), 2 (0.5 mmol), and TBAF (0.5 mmol). After stir-
ring the mixture at room temperature for 30 min, the reaction
tube was sealed and stirred at 80ꢀC for 24 hr (In the cases of
Table 3 and Scheme 3, addition of 0.5 mmol NaHCO3 and
stirred at 120ꢀC are required). After cooling the mixture to
room temperature, the organic portion was obtained by
extracting the aqueous phase with EtOAc (3 × 3 ml). The col-
lected organic layer was then dried over MgSO4, evaporated
the solvent under reduced pressure, and purified by column
chromatography to provide isoflavones 3.
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SUPPORTING INFORMATION
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article.
How to cite this article: Chang Y-T, Liu L-J,
Peng W-S, Lin L-T, Chan Y-T, Tsai F-Y. Stille
coupling for the synthesis of isoflavones by a
reusable palladium catalyst in water. J Chin Chem
Soc. 2021;68:469–475. https://doi.org/10.1002/jccs.
202000478
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