A mild and efficient copper-catalyzed coupling of aryl iodides and thiols using an oxime-phosphine oxide ligand

Article abstract of DOI:10.1016/j.tetlet.2006.05.178

A mild and efficient copper-catalyzed system for the coupling of aryl iodides and thiols was developed using a readily prepared and highly stable oxime-phosphine oxide ligand. Good to excellent yields were obtained.

Full text of DOI:10.1016/j.tetlet.2006.05.178

Tetrahedron Letters 47 (2006) 5781–5784
A mild and efficient copper-catalyzed coupling of aryl iodides
and thiols using an oxime–phosphine oxide ligand
Di Zhu, Lei Xu, Fan Wu and Boshun Wan*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
Received 5 April 2006; revised 30 May 2006; accepted 31 May 2006
Available online 27 June 2006
Abstract—A mild and efficient copper-catalyzed system for the coupling of aryl iodides and thiols was developed using a readily
prepared and highly stable oxime–phosphine oxide ligand. Good to excellent yields were obtained.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Aryl sulfides are valuable intermediates in organic syn-
thesis and are of great importance to the pharmaceutical
industry.1 Transition metal-catalyzed or -mediated aryl-
ation of thiols is a direct and powerful method to form
these products. Although palladium-catalyzed C–S bond
formation has achieved some good results,2 the use of
copper catalysts for this transformation is still attractive
from an industrial perspective. Traditional copper-medi-
ated couplings between thiols and aryl halides have
required the use of copper salts in greater than stoichio-
metric amounts, polar solvents such as HMPA, and high
temperatures around 200 ꢁC.3 Recently, along with the
development of the research on mild copper-catalyzed
aromatic carbon–heteroatom bond formations, copper-
catalyzed C–S bond formation has received attention3b,4
and some efficient ligands have been developed for this
transformation, including phosphazene P2-Et base,5 neo-
cuproine,6 ethene glycol,7 and amino acids such as
N-methylglycine and N,N-dimethylglycine.8 Though
some progress has been made, copper-catalyzed thioether
formations are less fruitful than the formations of aryl-
nitrogen and aryl-oxygen.4 Therefore, the development
for more efficient and mild catalyst systems for copper-
catalyzed arylation of thiols is still desirable.
Our group has embarked on a programme aiming at the
development of ligands that are of low-cost and easily
prepared9,10 and have found that Cu2O/oxime–phos-
phine oxide10 can efficiently catalyze the coupling of aryl
iodides with alkyl amines or N–H heterocycles. After
these discoveries, we found that the copper-mediated sys-
tem based on the oxime–phosphine oxide ligand could
also be used to catalyze the C–S bond formation. Herein,
we reported a mild and highly efficient copper-catalyzed
coupling of aryl iodides and thiols using a readily pre-
pared and highly stable oxime–phosphine oxide ligand.
The synthesis of oxime–phosphine oxide 5 was some-
what modified based on our previous procedure.10,11
Intermediate 4 was easily prepared from commercially
available and low-cost crude material 1 by conventional
methods.10,12 Ligand 5 was conveniently obtained with-
out the need of an additional step to isolate the hydro-
lysis product of 4 (Scheme 1).
Ph
Ph
Ph
Ph
Br
O
PPh2
P
Br
P
a
c
b
O
d
O
O
O
O
N
CHO
OH
O
O
1
3
5
4
2
Scheme 1. Reagents and conditions: (a) TsOH, ethylene glycol, toluene, reflux; (b) n-BuLi, Et2O, À78 ꢁC, 3–4 h, Ph2PCl, RT, 14 h; (c) H2O2,
CH2Cl2, RT, 1 h; (d) TsOH, C2H5OH, H2O, NaHCO3, NH2OHÆHCl.
*
Corresponding author. Fax: +86 411 84379260; e-mail: bswan@dicp.ac.cn
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.178

5782
D. Zhu et al. / Tetrahedron Letters 47 (2006) 5781–5784
Table 1. Screening of the reaction conditionsa
10 mol% [Cu]
20 mol% L
I
S
+
SH
2 .1 eq. Base, Solvent
90 oC, 12 h
Entry
[Cu]
Solvent
Base
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
CuI
Toluene
DMF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
K3PO4
K3PO4Æ3H2O
KOH
KOAc
60
96
89
77
97
88
93
95
85
3
CH3CN
Dioxane
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
DMFc
NaOt-Bu
Et3N
63
4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
96
97
98
87
86
CuBr
CuCl
CuCN
Cu Powder
a Reaction conditions: 0.5 mmol PhI, 0.6 mmol n-butanethiol, 10 mol % [Cu], 20% ligand 5, 1.05 mmol base, 0.5 mL solvent, 90 ꢁC, 12 h.
b GC yield using n-dodecane as the internal standard.
c Reagent-grade DMF was degassed.
With the ligand 5 in hand, iodobenzene and n-butaneth-
iol were used as model substrates for the subsequent
screening experiments (Table 1).13 Polar solvents were
better than less polar solvents (entries 1–4). Reagent-
grade DMF (degassed) gave similar result to newly dis-
tilled DMF (entry 5). K3PO4, K3PO4Æ3H2O, K2CO3 and
Cs2CO3 were found to be effective bases for this cou-
pling, while strong bases KOH and NaOt-Bu and organ-
ic base Et3N gave bad results (entries 6–12). In addition,
copper sources played a somewhat small role in the effi-
ciency (entries 13–17). Although CuI provided some-
what lower yield than CuCl, we chose to focus on the
use of CuI due to its stability to air. Therefore, Cs2CO3,
CuI and degassed DMF were used in the following
experiment.
To expand the scope of our catalyst system, a number of
aryl iodides and thiols were employed (Table 2).13 Good
Table 2. Coupling of aryl iodides with alkyl thiols or aryl thiolsa
10 mol% CuI
20 mol% L
I
+
S
R-SH
R'
2.1 equiv Cs2CO3
DMF, 90 oC, 24 h
R'
R
Entry
1
ArI
R–SH
Product
Yieldb (%)
98c
I
S
SH
I
S
2
3
86
92
SH
SH
O
O
I
S
H3CO
H3CO
I
S
4
5
80d
98c
SH
SH
Br
Br
I
S
CH3
CH3

D. Zhu et al. / Tetrahedron Letters 47 (2006) 5781–5784
5783
Table 2 (continued)
Entry
ArI
R–SH
Product
Yieldb (%)
I
I
S
6
7
94e
SH
SH
CF3
CF3
S
98
I
I
S
S
8
SH
87
98
89
89
Ph
S
9
Ph
Ph
Ph
SH
SH
SH
I
Ph
10
11
O
O
I
S
Ph
H3CO
H3CO
I
S
Ph
12
13
14
84f
86
50
Ph
Ph
SH
CN
Br
CN
Br
I
S
Ph
SH
SH
I
S
H3CO
H3CO
SH
S
I
15
89
Cl
Cl
I
S
SH
SH
SH
16
17
18
88
98
94
Cl
Cl
Cl
Cl
O
O
I
S
H3CO
H3CO
Cl
I
S
Br
Br
Cl
I
I
S
SH
SH
SH
19
20
21
85
98
88f
Cl
Cl
Cl
Cl
Cl
CH3
S
OH
OH
I
S
Cl
a Reaction conditions: 1.0 mmol ArI, 1.2 mmol RSH, 0.1 mmol CuI, 0.2 mmol Ligand 5, 2.1 equiv Cs2CO3, 1.0 mL DMF (degassed), 90 ꢁC.
b Isolated yield.
c DMF was distilled from CaCl2.
d K3PO4 as the base.
e Yield of the corresponding sulfone obtained after oxidation of sulfide with m-CPBA.
f Toluene as solvent, 100 ꢁC, 29 h.

5784
D. Zhu et al. / Tetrahedron Letters 47 (2006) 5781–5784
6. (a) Bates, C. G.; Gujadhur, R. K.; Venkataraman, D. Org.
Lett. 2002, 4, 2803; (b) Bates, C. G.; Saejueng, P.;
Doherty, M. Q.; Venkataraman, D. Org. Lett. 2004, 6,
5005.
7. Kwong, F. Y.; Buchwald, S. L. Org. Lett. 2002, 4, 3517.
8. Deng, W.; Zou, Y.; Wang, Y.-F.; Liu, L.; Guo, Q.-X.
Synlett 2004, 1254.
9. (a) Mao, J. C.; Wan, B. S.; Wang, R. L.; Wu, F.; Lu, S. W.
J. Org. Chem. 2004, 69, 9123; (b) Zhang, Z. J.; Mao, J. C.;
Wang, R. L.; Wu, F.; Chen, H. L.; Wan, B. S. J. Mol.
Catal. A Chem. 2006, 243, 239; (c) Zhang, Z. J.; Mao, J.
C.; Zhu, D.; Wu, F.; Chen, H. L.; Wan, B. S. Catal.
Commun. 2005, 6, 784; (d) Zhang, Z. J.; Mao, J. C.; Zhu,
D.; Wu, F.; Chen, H. L.; Wan, B. S. Tetrahedron 2006, 62,
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to excellent yields were observed with primary alkyl thi-
ols (entries 1–13). Methoxy, trifluoromethyl, cyano,
methyl, bromo, acetyl and hydroxyl groups were all tol-
erated on the aryl iodide component. When secondary
alkyl thiol was employed as substrate, low yield was ob-
tained (entry 14). 4-Chlorothiophenol was also found to
be an effective nucleophile under these reaction condi-
tions (entries 15–21).
In summary, we have developed a mild and efficient cop-
per-catalyzed cross-coupling reaction between aryl iod-
ides and thiols using a readily prepared and highly
stable oxime-functionalized phosphine oxide ligand.
Good to excellent yields were obtained. Many func-
tional groups, especially the hydroxyl group, were toler-
ated. Although this method is restricted to the coupling
of aryl iodides, the readily available ligand with excellent
stability and high efficiency makes this protocol of
potentially practical utility in many cases. Further stud-
ies to expand the application of this method to other cat-
alytic reactions are currently under way.
10. Xu, L.; Zhu, D.; Wu, F.; Wang, R. L.; Wan, B. S.
Tetrahedron 2005, 61, 6553.
11. Procedure for the preparation of ligand 5: Intermediate 3
was prepared from crude material 1 according to the
literature.12 2-[2-(diphenylphosphinyl)phenyl]-1,3-dioxo-
lane 4 was synthesized according to the method based
on our previously reported procedure.10 Intermediate 4
(2.37 g, 6.77 mmol) was dissolved in EtOH (60 mL). TsOH
(36.2 mg, 0.21 mmol) was dissolved in H2O (20 mL) and
was added into the above solution of 4. After the reaction
mixture was refluxed for 12 h, NaHCO3 (17.67 mg,
0.21 mmol) was added and the mixture was cooled to
room temperature. Hydroxylamine hydrochloride
(848.68 mg, 12.3 mmol) was dissolved in H2O (20 mL),
and NaHCO3 (1.23 g, 14.64 mmol) added with stirring.
The resulting aqueous solution of hydroxylamine was
added into the hydrolysis solution of 4. After stirring for
15 min, many white solids precipitated from the solution.
The mixture was continued to be stirred overnight. The
white solid was filtered, washed with water and CH3OH
and recrystallized from CH3OH to give ligand 5 (1.75 g) in
Acknowledgements
Financial support from the National Key Project
for Basic Research (2003CB114402) is gratefully
acknowledged.
References and notes
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80% yield. H NMR (400 MHz, DMSO): d 7.04–7.09 (m,
1H), 7.44–7.64 (m, 12H), 8.01–8.07 (m, 1H), 8.80 (s, 1H),
11.49 (s, 1H). 13C NMR (100 MHz, DMSO): d 126.2,
126.3, 128.7, 128.9, 129.0, 130.0, 131.0, 131.4, 131.5, 131.9,
132.3, 132.9, 133.0, 136.7, 146.3. 31P NMR: d 29.7. HRMS
(APCI) calcd for C19H17NO2P (M+H+): 322.0991, found:
322.0975.
12. Schenkel, L. B.; Ellman, J. A. Org. Lett. 2003, 5, 545.
13. General procedure for copper-catalyzed arylation of
thiols: CuI (19.6 mg, 0.10 mmol), ligand (64.4 mg,
0.20 mmol), Cs2CO3 (692.0 mg, 2.1 mmol) and aryl iod-
ides (if solid, 1.0 mmol) were weighed in air and trans-
ferred into a dried Schlenk tube. The tube was evacuated
and backfilled with argon (3 cycles). Aryl iodides (if liquid,
1.0 mmol), degassed reagent-grade DMF (1.0 mL) and
thiols (1.2 mmol) were injected to the tube successively by
micro-syringe at RT. The tube was sealed and stirred in an
oil bath (preheated to 90 ꢁC) for the required time period.
After cooling to RT, H2O (3 mL) and Et2O (5 mL) were
added. The aqueous layer was extracted by Et2O
(10 mL · 4). The combined organic layers were washed
with saturated brine, dried over Na2SO4. Solvent was
removed in vacuo, and the residue was further purified by
flash column chromatography on silica to afford the
desired product.
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