Catalytic desulfitative homocoupling of sodium arylsulfinates in water using PdCl2 as the recyclable catalyst and O2 as the terminal oxidant

Article abstract of DOI:10.1039/c2gc36550b

A green and convenient approach to symmetrical biaryls has been developed. The catalytic desulfitative homocoupling of various inexpensive and readily available sodium arylsulfinates in environment-friendly solvent, water, and under clean oxidant, molecular O₂, afforded symmetrical biaryls in good yields. What's more, water can hold the noble catalyst PdCl₂ for recycling several times easily, and facilitate separating insoluble organic product.

Full text of DOI:10.1039/c2gc36550b

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PAPER
Catalytic desulfitative homocoupling of sodium arylsulfinates in water using
PdCl as the recyclable catalyst and O as the terminal oxidant†
2
2
Bin Rao, Weixi Zhang, Lan Hu and Meiming Luo*
Received 1st October 2012, Accepted 17th October 2012
DOI: 10.1039/c2gc36550b
A green and convenient approach to symmetrical biaryls has been developed. The catalytic desulfitative
homocoupling of various inexpensive and readily available sodium arylsulfinates in environment-friendly
solvent, water, and under clean oxidant, molecular O , afforded symmetrical biaryls in good yields.
2
What’s more, water can hold the noble catalyst PdCl for recycling several times easily, and facilitate
2
separating insoluble organic product.
Introduction
Organic reactions in water without using harmful organic sol-
vents have attracted a great deal of interest in both academic and
1
industrial research because water is not only cheap, readily
available, non-toxic, nonflammable and environment-friendly,
but also may enhance reactivity and selectivity, facilitate the sep-
aration of hydrophobic organic products and hold the water-
2
,3
soluble catalysts for recycling. Conducting a reaction in water
has become the focus of many research activities, and a great
number of reaction processes have been designed and success-
fully developed to be conducted in water in recent years for sus-
1
–3
tainability benefits.
The biaryl scaffold is widely found in natural products, chiral
4
5
6
catalysts and functional materials. Searching for straightfor-
ward and environmentally friendly methods for biaryl synthesis
is a matter of great interest. For symmetrical biaryl synthesis, the
most straight and concise approach is the transition metal-cata-
lyzed homocoupling reactions (Scheme 1), including the reduc-
Scheme 1 Transition-metal catalyzed homocoupling methods for the
synthesis of symmetrical biaryls.
7
tive homocoupling of aryl electrophilic reagent (Scheme 1,
7
b,8
route a), the oxidative dimerization of arylmetal reagent
9
(
(
Scheme 1, route b), dehydrogenative homocoupling of arenes
Scheme 1, route c) and decarboxylative homocoupling of aryl-
reactions together with ready recovery and recycling of the cata-
1
lyst, the transition metal-catalyzed homocoupling for biaryl syn-
10
carboxylic acid (Scheme 1, route d), which provide powerful
protocols to access symmetrical biaryls. However, there are still
limitations and room for improvement on aryl sources: low reac-
tive aryl chlorides, air- and water-sensitive organometallic
reagents, less regio- and chemo-selective arenes except for
special activated and ortho-substituted arylcarboxylic acids.
Moreover, although the conduction of transition metal-catalyzed
transformations in water using a water-soluble catalyst has
emerged as a very efficient approach that enables efficient
11
thesis in water is scarce, and mainly with aryl halides and
3a,12
arylboronic acids
as aryl sources which usually require the
assistance of organic co-solvents, phase transfer catalysts or
large amounts of metallic reductants.
Sodium arylsulfinate is stable, easy to handle and conveniently
prepared from the corresponding inexpensive sulfonyl chloride,
and has recently been used as an alternative arene donor for cata-
13
14
14a,15
lytic coupling with aldehyde, olefin, nitrile,
arylbro-
16
17
mide and heteroarenes with formation of biaryl by-product in
some cases. These features and obviously some water solubility
of sodium arylsulfinate attracted our attention, and we surmised
that sodium arylsulfinate may serve as an ideal alternative aryl
source for the synthesis of symmetrical biaryl compounds in
water via the catalytic desulfitative homocoupling reaction using
a water-soluble transition metal catalyst which may be readily
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry, Sichuan University, Chengdu 610064,
People’s Republic of China. E-mail: luomm@scu.edu.cn;
Fax: +86-28-85462021; Tel: +86-28-85462021
1
†
Electronic supplementary information (ESI) available: H NMR and
C NMR spectra for all products. See DOI: 10.1039/c2gc36550b
1
3
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Table 1 Optimization of the reaction conditions for the catalytic
desulfitative homocoupling of sodium arylsulfinate in water
26% yield (Table 1, entry 3). It was proposed that some additives
facilitated completing the catalytic cycle of a Pd-catalyzed reac-
tion by promoting the oxidation of Pd(0) which was produced in
a
1
4c,17
the reductive elimination step of an arylpalladium(II).
a range of additives were examined. To our delight, addition of
0 mol% of Cu O into the catalytic system did improve the yield
Then
2
2
of the desired symmetrical biaryl 2 to 85% under 1 atm O2
(Table 1, entry 4). Air was proved to be less effective than
oxygen giving the homocoupling product in 60% yield (Table 1,
b
Entry
Oxidant
Additive (mol%)
Yield (%)
1
2
3
4
5
6
7
8
9
K
2
S
2
O
8
(2 equiv.)
—
—
—
Trace
35
26
85
60
70
84
66
56
44
30
41
47
40
34
31
46
52
36
86
81
0
entry 5). The influence of the amount of Cu O on the catalytic
reaction was investigated. The results showed that 20 mol% of
2
TBHP (2 equiv.)
O
O
Air
2
Cu O gave the best yield (Table 1, entry 4), 15 mol% of Cu O
2
Cu
2
O (20)
2
2
Cu
Cu
Cu
2
O (20)
O (15)
O (25)
led to reduction of the yield to 70% (Table 1, entry 6), while
O
O
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
2
2
2
more Cu O (25 mol%) did not improve the reaction yield further
2
(
Table 1, entry 7). Other copper salts such as CuO, CuCl , CuCl,
2
CuO (40)
CuCl (40)
CuCl (40)
CuBr (40)
CuSO ·5H
Cu(NO ·3H
Cu(OAc)
CuBr , CuSO ·5H O, Cu(NO ) ·3H O, Cu(OAc) , and FeCl ,
2
4
2
3 2
2
2
3
2
1
0
MnO and AgOAc could also be applied to this transformation
2
1
1
2
affording the symmetrical biaryl 2 in moderate yield (Table 1,
entries 8–17). CuCl or Cu(OAc) alone as the oxidant in the
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
4
2
O (40)
2
2
3
)
2
2
O (40)
2
(40)
absence of O did not give better results (Table 1, entries 18 and
2
AgOAc (40)
MnO
1
9). Then the catalyst loading of PdCl was evaluated (Table 1,
2
2
(40)
(40)
entries 4, 20 and 21). The best result was obtained in the pres-
ence of 2.5 mol% of catalyst (Table 1, entry 4). While increasing
the catalyst loading did not increase the product yield signifi-
cantly (Table 1, entry 20), reduction of the catalyst amount
decreased the product yield (Table 1, entry 21). Notably, as a
control, no reaction occurred in the absence of palladium salt
FeCl
—
—
Cu
Cu
Cu
Cu
Cu
Cu
Cu
3
CuCl
Cu(OAc)
2
(2 equiv.)
(2 equiv.)
2
c
d
e
f
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O (20)
O (20)
O (20)
O (20)
O (20)
O (20)
O (20)
82
80
45
53
g
h
i
(Table 1, entry 22). Pd(OAc) gave slightly lower yield than
2
PdCl (Table 1, entry 23). The reaction afforded decreased yields
2
at lower reaction temperatures (Table 1, entries 24 and 25). Thus,
the optimized reaction conditions for the catalytic homocoupling
reaction of sodium p-toluenesulfinate in water involved the use
of PdCl (2.5 mol%) as the catalyst, Cu O (20 mol%) as the
a
Reaction conditions: sodium p-toluenesulfinate (0.75 mmol), PdCl
2
b
(2.5 mol%), oxidant, additive, water (2.0 mL), 100 °C, 24 h. Isolated
c d e
yield. 4.0 mol% of PdCl
2
.
2 mol% of PdCl
2
.
Without PdCl
2
.5 mol% of Pd(OAc) as the catalyst. The reaction was performed at
h
2
.
f
g
2
2
2
i
9
0 °C. The reaction was performed at 80 °C. p-Toluenesulfinic acid
additive, 1 atm O as the oxidant, and conducting the reaction at
2
was used as the substrate.
reflux. p-Toluenesulfinic acid also underwent this transformation
giving the desired homocoupling product under the optimized
conditions with lower performance than the corresponding
sodium p-toluenesulfinate (Table 1, entry 26).
separated and reused after simple decantation or extraction of the
water-insoluble biaryl products. We here report a green and
efficient method for the synthesis of a symmetrical biaryl frame-
work from sodium arylsulfinate in water catalyzed by recyclable
aqueous PdCl solution using molecular oxygen as the terminal
oxidant.
With the optimized conditions in hand, the scope of substrates
was further investigated. As shown in Table 2, most of the
sodium arylsulfinates with a variety of substituents afforded the
corresponding symmetrical biaryls in good yields under the opti-
mized reaction conditions. Functional groups including fluoride,
chloride, bromide, methoxyl, ester, nitryl and thienyl were all
well tolerated. Substrates with either electron-withdrawing or
electron-donating substituents underwent this transformation
smoothly giving the desired biaryls in moderate to good yields.
Notably, formation of nitrobenzene as a side product was
observed with sodium 3-nitrobenzenesulfinate (Table 2, entry 6).
Sodium o-toluenesulfinate gave a lower product yield of 51%
than sodium p-toluenesulfinate, probably owing to the steric hin-
drance (Table 2, entry 8). Sodium benzylsulfinate as an alkyl-
sulfinate also underwent this transformation giving the desired
homocoupling product in 35% yield (Table 2, entry 14). The coup-
ling between two different sodium arylsulfinates of different and
similar electron density was also tested under the optimum reaction
conditions, and no significant selectivity was observed (see ESI†).
The compatibility of this reaction with various functional
groups provides potential to be incorporated into a synthetic
2
Results and discussion
The homocoupling of sodium p-toluenesulfinate in water was
initially explored in the presence of PdCl (2.5 mol%) to opti-
mize the reaction conditions with various oxidants. As shown in
Table 1, K S O was first used as the oxidant which gave only
traces of the homocoupling product (Table 1, entry 1). Replace-
ment of K S O with tert-butyl hydroperoxide (TBHP) afforded
the desired symmetrical biaryl 2 in 35% yield (Table 1, entry 2).
To obtain a substantial gain in the environmental impact with an
oxidative coupling reaction, the choice of oxidant is crucial, and
in particular the use of clean and inexpensive molecular oxygen
2
2
2 8
2
2 8
1
h
is highly desirable. Then atmospheric molecular oxygen was
employed as the oxidant which gave the symmetrical biaryl 2 in
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a
2
Table 2 PdCl -catalyzed homocoupling of various sodium arylsulfinates in water
b
Entry
1
Sodium arylsulfinate
Product
Time (h)
24
Yield (%)
1a
1b
1c
2a
2b
2c
85
90
80
2
3
20
22
4
1d
2d
24
71
5
6
1e
1f
2e
2f
40
28
74
c
60
7
1g
1h
2g
2h
30
30
62
51
8
9
1i
2i
24
24
24
30
26
79
75
81
77
55
1
0
1j
2j
11
1k
1l
2k
2l
1
1
2
3
1m
2m
d
1
4
1n
2n
40
35
a
c
b
Reaction conditions: sodium arylsulfinate (0.75 mmol), PdCl
A trace amount of nitrobenzene was detected by GC analysis. GC yield.
2
(2.5 mol%), Cu
2
O (20 mol%), H
2
O (2 mL), under 1 atm O
2
, 100 °C. Isolated yield.
d
1
9
protocol combined with other processes for further decoration.
This is illustrated in a newly designed one-pot synthesis of func-
tionalized quaterphenylenes which are versatile building blocks
for generating aligned fiber-like nanostructures. For example as
shown in Scheme 2, sodium 4-bromobenzenesulfinate was suc-
cessfully applied to prepare 1,4′′′-dimethoxyquaterphenylene
4-methoxyphenylboronate. It was noted that the simple and
single PdCl efficiently promoted the transformations in water
without isolation of the intermediate.
As transition metal catalysts are often expensive and display
toxicity, from both an economical and environmental point of
view, the recovery and recycling of the catalyst is a major
concern with a transition-metal catalyzed reaction for sustainabil-
2
1
8
(MOP4) through a consecutive desulfitative homocoupling
and the Suzuki–Miyaura cross-coupling with sodium
ity. Taking advantage of the good solubility of PdCl and the
2
3438 | Green Chem., 2012, 14, 3436–3440
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Scheme 2 Application of the Pd-catalyzed homocoupling of sodium arylsulfinate in the one-pot synthesis of MOP4.
Table 3 Catalyst recycling experiments for the homocoupling reaction
of sodium p-toluenesulfinate
a
b
b
b
b
b
Cycle
Yield (%)
1
2
3
4
5
6
c
85
82
83
80
77
65
a
Reaction conditions: sodium p-toluenesulfinate (0.75 mmol), PdCl
O (20 mol%), H O (2 mL), under 1 atm O , 100 °C,
4 h. The aqueous phase from the previous run was reused instead of a
2
(2.5 mol%), Cu
2
2
2
b
2
c
recharge of catalyst and water. Isolated yield.
insolubility of homocoupling products in water, a simple fil-
tration was sufficient to separate the aqueous catalyst solution
from the products. The recyclability of the aqueous PdCl solu-
2
tion was then studied in the homocoupling reaction of sodium
p-toluenesulfinate. The results of the recycling experiments are
shown in Table 3. In the recycling experiment, the separated
Scheme 3 Proposed mechanism for the Pd-catalyzed homocoupling
reaction of sodium arylsulfinate.
aqueous PdCl solution was recharged with fresh substrate and
2
Cu O for the next run under the same reaction conditions. It was
2
Conclusions
notable that the aqueous PdCl solution still remained catalyti-
2
cally active after being reused five times. The homocoupling
reaction at the 4th and 5th runs gave the desired biaryl product in
In summary, we have successfully developed a simple and
efficient palladium-catalyzed homocoupling of sodium arylsulfi-
nate for the synthesis of symmetrical biaryls. This synthetic pro-
tocol displays attractive features including using economical and
8
0% and 77% yields. However, an obvious decrease of the
product yield to 65% at the 6th run was observed. Notably, the
homocoupling of sodium p-toluenesulfinate in the presence of
environment-friendly water as the solvent, using 1 atm O as the
2
14a
a stoichiometric amount of a Pd(II) salt has been reported,
terminal oxidant, employing inexpensive and readily available
starting materials, and a wide range of functional group toler-
ance, as well as the recyclability of the catalytic system. In par-
ticular, this catalytic system can be reused five times without
significant loss of catalytic performance.
yet our catalytic version is much more efficient with
additional advantages of recyclable catalyst and 1 atm O as the
2
oxidant.
Although the exact mechanism of this desulfitative homocou-
pling reaction is not clear at this stage, on the basis of our obser-
vation and the suggested pathways for the reactions of sodium
arylsulfinate with alkene, nitrile, and heteroarenes in the
14–17
Experimental section
literature,
a plausible mechanism for the Pd-catalyzed
homocoupling reaction of sodium arylsulfinate in the presence of
copper salt leading to symmetrical biaryls is proposed and
shown in Scheme 3 involving the following steps: (1) Pd(II)–
sulfinate species A is formed by coordination of sodium aryl-
sulfinate to Pd(II) salt; (2) aryl–Pd(II) complex B is formed from
A in the presence of O and H O with the release of NaHSO
4
General
Unless otherwise noted, all reagents were obtained from com-
mercial suppliers and used without further purification. Column
chromatography was performed with silica gel (200–300 mesh).
Thin layer chromatography was carried out using Merck silica
gel GF254 plates. Mass spectra (GC-MS) experiments were con-
ducted on an Agilent-6890. H NMR and C NMR spectra
were recorded on a Bruker 400 and 600 MHz instrument.
Spectra were reported relative to Me Si (δ 0.0 ppm) and CDCl
2
2
(see ESI†); (3) following the same progress, the reaction of
1
13
sodium arylsulfinate with aryl–Pd(II) complex B produces the
intermediate C; (4) reductive elimination of intermediate C gen-
erates the desired homocoupling product and Pd(0); (5) oxidation
of Pd(0) by Cu(II) regenerates Pd(II) and Cu(I) which is then
4
3
13
(δ 7.26 ppm). C NMR were reported relative to CDCl3 (δ
reoxidized to Cu(II) by O completing the catalytic cycle.
77.0 ppm). Splitting patterns are designated as follows: s,
2
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singlet; d, doublet; t, triplet; m, multiplet. Products were charac-
terized by comparison of H and C NMR spectroscopic data
with those available in the literature.
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allowed to react in a sealed tube at 100 °C under an atmosphere
5
6
of 1 atm O for 20–40 h. After cooling to room temperature, the
2
mixture was extracted with ether (3 × 10 mL), then dried and
concentrated in vacuo. The residue was further purified by a
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pure product.
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5, 1272.
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8
9
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2
2
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2
ture, the reaction mixture was filtered. The filtrate containing
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2
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(
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Synthesis of MOP4
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(
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lected by filtration, washed with 1 M dilute hydrochloric acid (3
(
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×
5 mL), H O (3 × 5 mL) and CH Cl (3 × 5 mL) to give
2 2 2
1
MOP4 (137 mg, 71%). The H NMR spectroscopic data of
MOP4 are in accordance with those reported in ref. 20.
1
701.
1
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Acknowledgements
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3440 | Green Chem., 2012, 14, 3436–3440
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