Palladium-catalyzed reductive homocoupling of aryl sulfonates via cleavage of C─O bond at room temperature

Article abstract of DOI:10.1002/aoc.3705

Palladium-catalyzed reductive homocoupling of aryl sulfonates has been successfully achieved under mild conditions. This transformation is a new method for the homocoupling reaction of aryl sulfonates at room temperature via the cleavage of C─O bonds, thus providing an alternative synthesis of symmetric biaryls. The reported reductive homocoupling reaction is tolerant of many common functional groups regardless of electron-donating or electron-withdrawing nature, making this newly developed transformation important for complementing Ullmann coupling. Experimental Section. Typical procedure for the products.

Full text of DOI:10.1002/aoc.3705

Received: 13 July 2016
DOI 10.1002/aoc.3705
Revised: 11 November 2016
Accepted: 12 November 2016
F U L L PA P E R
Palladium‐catalyzed reductive homocoupling of aryl sulfonates via
cleavage of C─O bond at room temperature
Shuangling Zhong | Mengxin Chen | Ge Liu | Chenggang Sun | Wencong Liu
College of Resources and Environment, Jilin
Palladium‐catalyzed reductive homocoupling of aryl sulfonates has been success-
fully achieved under mild conditions. This transformation is a new method for the
Agricultural University, Changchun 130118,
People's Republic of China
Correspondence
Shuangling Zhong and Wencong Liu, College of
homocoupling reaction of aryl sulfonates at room temperature via the cleavage of
C─O bonds, thus providing an alternative synthesis of symmetric biaryls. The
reported reductive homocoupling reaction is tolerant of many common functional
Resources and Environment, Jilin Agricultural
University, Changchun 130118, People's Republic
of China.
groups regardless of electron‐donating or electron‐withdrawing nature, making this
newly developed transformation important for complementing Ullmann coupling.
Experimental Section.
Email: shuangling_zhong@126.com;
liu_wencong@126.com
Typical procedure for the products.
KEYWORDS
aryl sulfonates, Pd‐catalyzed, reductive homocoupling
1
|
INTRODUCTION
been well established for
a
long time[25–28] and
arylhydrazines have been recognized as alternative coupling
partners recently (Scheme 1, C─N cleavage),[29] while the
instability of arenediazonium salts and the toxicity of
arylhydrazine substrates obviously restrict the application of
Palladium‐catalyzed cross‐coupling reactions represent uni-
versal tools in synthetic methodology for the connection of
two different fragments through constructing carbon–carbon
or carbon–heteroatom bonds.[1–4] Typical procedures for the
preparation of symmetric biaryl compounds began with
Ullmann coupling[5–8] more than 100 years ago and have
evolved into some of the most investigated aryl–aryl bond‐
forming reactions. Spectacular achievements have been made
in recent decades. Aryl iodides and bromides are frequently
used as the coupling partners in Ullmann‐type homocoupling
due to their relatively high reactivities, while inexpensive but
unreactive aryl chlorides were realized through the develop-
ment of novel ligands (Scheme 1, C─X cleavage).[9–13] How-
ever, organohalides are generally not environmentally
friendly, and the coupling produces environmentally pollut-
ing halide byproducts. Also, the preparation of aryl halides
often involves tedious steps, waste production and harsh reac-
tion conditions. Thus, much effort has been directed to search
for alternative electrophiles to organohalides. These coupling
partners can be classified according to different types of bond
cleavage. Homocoupling of arylmetal reagents[14–16] and
dimerization of organoborane reagents[17–24] can be achieved
under oxidative Pd‐catalyzed coupling conditions (Scheme 1,
C─M cleavage). Self‐coupling of arenediazonium salts has
these
methods.
Decarboxylative
dimerization
of
arylcarboxylic acids[30,31] was reported by Cornella et al.
with limited substrate universality under harsh conditions
(Scheme 1, C─C cleavage). Desulfitative homocoupling of
arylsulfonyl compounds has drawn much attention for their
higher reactivities under milder conditions than correspond-
ing arylcarboxylic acids (Scheme 1, C─S cleavage).[32–37]
However, the generation of environmentally detrimental
sulfur dioxide was unavoidable. Among these X‐, N‐, S‐,
M‐ and C‐based leaving groups, O‐based coupling partners
are particularly attractive due to the ubiquitous presence of
the O‐based starting materials in both the natural world and
synthetic systems.[38–51] Phenol compounds are typically
protected with various groups to enhance their leaving abili-
ties in cross‐coupling reactions. The use of phenol‐based
electrophiles gives various advantages. First, these substrates,
including aryl ethers, carboxylates and sulfonates, can be eas-
ily synthesized from abundant and inexpensive phenol deriv-
atives; they are cheaper and more environmentally benign.
Second, the use of phenol derivatives instead of halides
reduces the production of halide waste, thus lowering the
Appl Organometal Chem 2017; e3705;
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DOI 10.1002/aoc.3705

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ZHONG ET AL.
TABLE 1 Influence of ligands in homocoupling of phenyl
4‐methylbenzenesulfonatea
Entry
Ligand
Yield (%)b
Entry
Ligand
Yield (%)b
1
2
3
4
5
6
—
38
49
62
54
71
75
7
8
dppp
81
86
74
55
41
93
PPh3
dppb
P(o‐tolyl)3
TFP
9
P(t‐Bu)3
PCy3
10
11
12
P(p‐tolyl)3
dppe
rac‐BINAP
dppf
aReaction conditions: phenyl 4‐methylbenzenesulfonate (1 mmol), Pd(acac)2
(1 mol%), ligand (2 mol%), Zn (0.6 mmol), THF (1.0 ml) at 25 °C for 6 h unless
otherwise indicated.
bIsolated yield.
SCHEME 1 Palladium‐catalyzed homocoupling via various types of bond
cleavage
the use of a ligand is essential to obtain synthetically useful
yields of biphenyl from the homocoupling reaction of phenyl
4‐methylbenzenesulfonate. Triphenylphosphine was first
tested and a slightly higher yield was obtained (Table 1, entry
2). Further screening of mono‐phosphine ligands, such as
P(o‐tolyl)3, [P(2‐furyl)3] (TFP) and P(p‐tolyl)3, indicated that
P(o‐tolyl)3 was more effective in affording the biphenyl
product (Table 1, entries 3–5). When the reaction was
performed with 1,2‐bis(diphenylphosphino)ethane (dppe), the
yieldofbiphenylwasimprovedto75%andtheyieldofbiphenyl
was increased to 81% when 1,3‐bis(diphenylphosphino)pro-
pane (dppp) was utilized (Table 1, entries 6 and 7). When using
1,4‐bis(diphenylphosphino)butane(dppb)astheligand, ayield
of 86% of the desired product was obtained (Table 1, entry 8).
As evident from Table 1, P(t‐Bu)3 is more effective than PCy3 for
thesereactions(Table1,entries9and10).However,theuseofother
bisphosphine ligand, such as 2,2′‐bis(diphenylphosphino)‐1,1′‐
binaphthyl (rac‐BINAP), was relatively ineffective in the
homocoupling (Table 1, entry 11). Among the ligands screened,
1,1′‐bis(diphenylphosphino)ferrocene (dppf) afforded the highest
yield of the desired product (Table 1, entry 12).
environmental impact of the cross‐coupling process. Further-
more, phenol derivatives may exhibit orthogonal reactivity to
aryl halides. The direct application of phenol in coupling
reactions is a highly appealing process but remains as an elu-
sive goal. However, the Pd‐catalyzed homocoupling of aryl
sulfonates has not been well established yet. As early as
1992, the Pd(II)‐catalyzed homocoupling of relatively reac-
tive aryl triflates was reported by Jutand et al. in yields of
0–85% with high loading of PdCl2(PPh3)3.[52] More challeng-
ing O‐based coupling partners, such as ArOTs and ArOMs,
have not been applied in Pd‐catalyzed homocoupling. Con-
sidering our continued interest in Pd‐catalyzed coupling reac-
tions,[53] we report here the reductive homocoupling version
of Ullmann‐type homocoupling with aryl sulfonates under
mild conditions.
2
| RESULTS AND DISCUSSION
Our next investigation focused on the effect of various
transition metal catalysts and solvents on the model reac-
tion. The results are summarized in Table 2. Several Pd(0)
salts were screened in the presence of dppf as ligand in tet-
rahydrofuran (THF), with not all exhibiting catalytic activi-
ties (Table 2, entries 1 and 2). Reactions catalyzed by other
Pd salts, such as PdI2 and PdCl2, either did not proceed or
gave poor yields (Table 2, entries 3 and 4). The use of
Pd(OAc)2 and Pd(TFA)2 showed higher efficiency, and
67–75% yields of biphenyl were obtained (Table 2, entries
5 and 6). The reaction catalyzed by P‐chelated palladium
catalysts (PdCl2(PPh3)3 and PdCl2(dppf)) revealed better
Our studies commenced with the investigation of reaction
parameters for the homocoupling of phenyl 4‐
methylbenzenesulfonate. The use of Pd(acac)2 (acac = ace-
tylacetonate) was explored in the presence of phosphine
ligands used in analogous Pd‐catalyzed transformations.
Unfortunately, biphenyl was not obtained in the absence of
any ligands and additives. Cu‐ and Ni‐catalyzed ArOT and
ArOM homocoupling has been well understood for a few
decades already. As shown in previously reported Ni‐cata-
lyzed homocoupling reactions of aryl sulfonates,[54–56] reduc-
tant Zn(0) or Mn(0) is required to initiate the reaction to form
symmetric biaryl products (reduce Ni(II) pre‐catalyst to Ni(0)
active catalyst) as well as to turn over the catalyst after the
homocoupling. So we added zinc powder to smooth the pro-
cess of this transformation, and the desired biphenyl was
obtained in a yield of 38% (Table 1, entry 1). In addition,
reactivities
over
N‐chelated
palladium
catalysts
(Pd(CH3CN)2Cl2 and Pd(PhCN)2Cl2) (Table 2, entries 7–
10). We observed that Pd(acac)2 had higher activity than
other palladium catalysts, providing biphenyl in 93% yield
(Table 2, entry 11) in the presence of dppf in THF. And

ZHONG ET AL.
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TABLE 2 Influence of catalysts and solvents in homocoupling of phenyl
4‐methylbenzenesulfonatea
TABLE 3 Homocoupling of various phenyl sulfonatesa
Entry
R
Yield (%)b
Entry
Catalyst
Solvent
Yield (%)b
1
C6H5
95
93
86
83
76
91
1
Pd(PPh3)4
Pd2(dba)3
PdCl2
THF
—
—
18
—
67
75
78
85
54
45
93
66
52
31
55
74
79
86
‐
2
3
4
5
6
4‐CH3C6H4
4‐BrC6H4
4‐NO2C6H4
CH3
2
THF
3
THF
4
PdI2
THF
5
Pd(OAc)2
Pd(TFA)2
PdCl2(PPh3)3
PdCl2(dppf)
PdCl2(MeCN)2
PdCl2(PhCN)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
THF
CF3
6
THF
aReaction conditions: phenyl sulfonate (1 mmol), Pd(acac)2 (1 mol%), dppf
(2 mol%), Zn (0.6 mmol), THF (1.0 ml) at 25 °C for 6 h unless otherwise
indicated.
7
THF
8
THF
9
THF
bIsolated yield.
10
11
12
13
14
15
16
17
18
19
THF
THF
sulfonyl. Benzenesulfonate with p‐Me, p‐Br and p‐NO2 sub-
stituents could be well tolerated, affording the corresponding
homocoupling products in 93, 86 and 83% yield (Table 3,
entries 2–4). Triflate (OTf) (Table 3, entry 6) gave a good
result, but lower yield was obtained when mesylate (OMs)
(Table 3, entry 5) was used.
CH3CN
DMSO
DMF
NMP
1,4‐Dioxane
DME
MTBE
toluene
Having explored the preliminary scope for the
homocoupling of phenyl sulfonates, we next examined the
scope of the reaction using a broad range of substituted aryl
tosylates (Table 4). Electron‐neutral, electron‐deficient and
electron‐rich tosylates afforded the homocoupling products
in modest to good yields (Table 4, entries 1–12). Notably, this
reaction has a good tolerance of various functional groups on
aryl rings. Aryl tosylates with methoxy, alkyl, ester, cyano,
trifluoromethyl and nitro functions were well tolerated under
these reaction conditions (Table 4, entries 1–8). Also, haloge-
nated phenyl tosylates such as p‐FPhOTs and p‐ClPhOTs
underwent homocoupling smoothly in good yields (Table 4,
entries 9 and 10). It is very important that p‐BrPhOTs and
p‐IPhOTs, respectively, afforded 4,4′‐dibromobiphenyl and
4,4′‐diiodobiphenyl in high yields (Table 4, entries 11 and
12). The excellent selectivity makes this transformation very
attractive as an alternative for traditional Ullmann coupling.
The ortho and meta substituents on phenyl ring did not have
significant influence on the yield of the reaction (69–89%
yields for entries 13–20), except for ortho‐methyl‐substituted
(73% yield) and ortho‐methoxy substituted (69% yield) aryl
tosylates. Notably, aldehyde and hydroxyl groups were well
tolerated in this coupling method, which are generally inert
in Pd‐catalyzed coupling reactions (Table 4, entries 21 and
22). Heterocyclic tosylates also participated effectively in
these homocouplings, such as 2‐naphthyl, 9‐phenanthryl,
2‐furyl and 3‐pyridyl (Table 4, entries 23–26). The tolerance
of the styryl group is especially significant, as successive
functional group transformations are possible (Table 4,
entry 27).
aReaction conditions: phenyl 4‐methylbenzenesulfonate (1 mmol), catalyst
(1 mol%), dppf (2 mol%), Zn (0.6 mmol), solvent (1.0 ml) at 25 °C for 6 h unless
otherwise indicated.
bIsolated yield.
a control experiment revealed that palladium catalyst was
indispensable to this reaction. Increased temperature and
duration of reaction did not increase the efficiency of this
homocoupling. In the next stage, we examined the effect
of the solvent on the reaction (Table 2, entries 12–20).
Among the solvents (CH3CN, dimethylsulfoxide (DMSO),
dimethylformamide (DMF) and N‐methyl‐2‐pyrrolidone
(NMP)) tested, CH3CN gave the best result (Table 2, entries
12–15). Three additional ethereal solvents (dimethyl ether
(DME), methyl t‐butyl ether (MTBE) and 1,4‐dioxane) were
surveyed without leading to any improvement (Table 2,
entries 16–18). However, the reaction did not proceed in tol-
uene (Table 2, entry 19). Further optimization did not lead to
increased yields, such as increasing the temperature of the
reaction and doubling the reaction time. Thus, the optimized
conditions were Pd(acac)2, dppf as the ligand, in THF at
25 °C for 6 h.
Furthermore, considerable efforts were made to test the
substrate scope of this reaction under the optimized reaction
conditions. As detailed in Table 3, the optimal conditions
for the homocoupling can be applied towards the use of other
phenyl sulfonates. Phenyl benzenesulfonate was reacted effi-
ciently to produce biphenyl in 96% yield via selective cleav-
age of C─O bond (Table 3, entry 1). Notably, this reaction
has a good tolerance for various groups on phenyl rings of
Although the exact mechanism of this reductive
homocoupling reaction is not clear at this stage, on the

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ZHONG ET AL.
TABLE 4 Homocoupling of various aryl tosylatesa
Entry
R
Yield (%)b
1
4‐CH3OC6H4
89
91
84
92
87
86
81
83
85
90
87
83
88
78
85
80
67
71
82
85
83
76
91
81
84
79
66
2
4‐CH3C6H4
4‐t‐BuC6H4
C6H5
3
4
5
4‐CH3OOCC6H4
4‐CNC6H4
4‐CF3C6H4
4‐NO2C6H4
4‐FC6H4
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
4‐ClC6H4
SCHEME 2 Proposed mechanism
4‐BrC6H4
4‐IC6H4
3,4‐(CH3O)2C6H3
3‐NO2C6H4
3‐CH3OOCC6H4
3‐CNC6H4
2‐CH3OC6H4
2‐CH3C6H4
2‐ClC6H4
3
| CONCLUSIONS
We have developed a base‐free Pd‐catalyzed reductive
homocoupling reaction under mild reaction conditions. The
transformation is compatible with a number of synthetically
versatile functional groups including phenols, nitriles,
aldehydes and esters. Various aryl triflates underwent the
homocoupling reaction in good to high yields, and heterocy-
clic and styryl tosylates were efficient coupling partners.
Additionally, many phenyl sulfonates afford products in
yields comparable to those obtained using the corresponding
aryl tosylates at room temperature. The excellent selectivity
makes this transformation attractive for complementing
traditional Ullmann coupling.
2‐NO2C6H4
4‐CHOC6H4
4‐OHC6H4
2‐Naphthyl
9‐Phenanthryl─
2‐Furyl
3‐Pyridyl
Styryl
ACKNOWLEDGMENTS
aReaction conditions: aryl tosylate (1 mmol), Pd(acac)2 (1 mol%), dppf (2 mol%),
Zn (0.6 mmol), THF (1.0 ml) at 25 °C for 6 h unless otherwise indicated.
bIsolated yield.
This work was financially supported by the 12th Five‐Year
Science and Technology Research Program for Department
of Education of Jilin Province and National Training Programs
of Innovation and Entrepreneurship for Undergraduates.
basis of our observation and the suggested pathways for the
reactions of aryl sulfonates with other metals in the litera-
ture,[54–56] a plausible mechanism for the Pd‐catalyzed
homocoupling reaction of aryl sulfonates in the presence
of zinc powder leading to symmetric biaryls is proposed
and shown in Scheme 2. This involves the following steps:
(1) Pd(0)–acac species A is formed by reduction of zinc
powder to Pd(II)(acac)2; (2) aryl–Pd(II)(acac) complex B
is formed from A in the presence of aryl sulfonates; (3) fol-
lowing the same process, the reduction of complex B with
Zn(0) generated aryl–Pd(0) species C; (4) the reaction of
aryl–Pd(0) species C with ArOTs produces the aryl–
Pd(II)–aryl complex D; and (5) reductive elimination of
complex D generates the desired homocoupling product
and Pd(0) species A, thus completing the catalytic cycle
(Scheme 2).
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How to cite this article: Zhong S, Chen M, Liu G,
Sun C, Liu W. Palladium‐catalyzed reductive
homocoupling of aryl sulfonates via cleavage of
C─O bond at room temperature. Appl Organometal
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