C-S bond formation catalyzed by N-heterocylic carbene palladium phosphine complexes

Article abstract of DOI:10.1016/j.tet.2010.01.081

N-Heterocyclic carbenes (NHCs) are known to be useful ligands for palladium-complex catalysis. It was found that [(NHC)Pd(PPh₃)Cl₂] is an effective pre-catalyst in Pd-catalyzed C-S cross coupling reactions to produce the functionalized sulfides in excellent yields. The turn over frequency (TOF) for the coupling of p-CH₃C₆H₄Br with p-CH₃C₆H₄SH reaches to 6.25 (mol of product) (mole of catalyst)^-1 h^-1.

Full text of DOI:10.1016/j.tet.2010.01.081

Tetrahedron 66 (2010) 2119–2122
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
C–S bond formation catalyzed by N-heterocylic carbene palladium
phosphine complexes
*
Ching-Feng Fu, Yi-Hung Liu, Shei-Ming Peng, Shiuh-Tzung Liu
Department of Chemistry, National Taiwan University, 1, Sec.4, Roosevelt Road, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
N-Heterocyclic carbenes (NHCs) are known to be useful ligands for palladium-complex catalysis. It was
found that [(NHC)Pd(PPh3)Cl2] is an effective pre-catalyst in Pd-catalyzed C–S cross coupling reactions to
produce the functionalized sulfides in excellent yields. The turn over frequency (TOF) for the coupling of
Received 2 December 2009
Received in revised form 14 January 2010
Accepted 22 January 2010
Available online 1 February 2010
p-CH3C6H4Br with p-CH3C6H4SH reaches to 6.25 (mol of product) (mole of catalyst)ꢀ1 hꢀ1
.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Palladium
N-Heterocyclic carbene
C–S coupling
Sulfide
1. Introduction
the tungsten carbene complex 1 with Pd(II) ions (Scheme 1).6
Substitution of 1 with triphenylphosphine readily provided the
mono-nuclear palladium species 4 quantitatively. The spectral data
of complexes 2 and 3 are consistent with the reported data.
Organosulfur molecules stand at an important class of com-
pounds in pharmaceutical uses and synthetic intermediates. Thus,
the formation of the C–S bond becomes a crucial indispensable
process in synthetic organic chemistry. However, methodology
leading to C–S bond formation has been a challenge until the recent
development of transition metal (TM)-catalyzed coupling of thiols
with organic halides particularly via the use of palladium-
phosphine catalysts.1,2
N-Heterocyclic carbenes (NHCs) have appeared as an useful
class of ligands for palladium-complex catalysis.3 It is known that
TM–NHC complexes often show better stability and reactivity
compared to their TM–phosphine counterparts.4 Recently,
Ni–NHC complexes have demonstrated that they can serve as the
catalysts for the coupling of thiols with aryl halides.5 In this
context, the use of Pd–NHC complexes as catalysts for C–S
coupling has not been explored. Here, we would like to report the
investigation of catalytic activity of Pd–NHC complexes on this
coupling reaction.
Et
N
Et
N
1 eq
[(COD)PdCl2]
Cl
Cl
Et
N
Pd Cl
PPh3
Pd Cl
PPh3
N
N
Et
N
Cl Pd
Cl
2
Et
Et
N
W(CO)5
Et
4
N
Et
1
Et
Et
N
0.5 eq
[(COD)PdCl2]
Cl
N
Pd
Cl
N
N
Et
Et
3
Scheme 1. Preparation of NHC palladium complexes.
In the 31P NMR spectrum of 4, a single shift appears at 27.3 ppm
(coordination shift dP¼34 ppm), consistent with the coordination of
triphenylphosphine to the metal center. The 1H NMR spectrum of 4
in CDCl3 is illustrative and shows the presence of two signals for the
methylene units of ethyl groups in a 1:1 ratio as well-separated
doublet of quartet (3J¼7 Hz, Jgem¼14 Hz) at 4.14 and 3.17 ppm,
respectively. These diastereotopic hydrogens of methylene units
indicate the asymmetry of the complex, i.e. carbene and phosphine
ligands are in cis arrangement. A crystal structure analysis of 4
provides unambiguous evidence for the anticipated arrangement
(Fig. 1).
2. Results and discussion
Palladium–NHC complexes 2 and 3 were prepared according to
the previously reported procedure via a carbene transfer reaction of
* Corresponding author.
E-mail address: stliu@ntu.edu.tw (S.-T. Liu).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.01.081

2120
C.-F. Fu et al. / Tetrahedron 66 (2010) 2119–2122
Catalyst 4 was used for screening different substrates. It showed
excellent activities for deactivated both aryl iodides and bromides,
but not chlorides. Quantitative yields were achieved by using
2 mol % of Pd-catalysts in toluene at refluxing temperature (Table 2,
entries 1–2). For aryl bromides with electron-donating sub-
stituents, complex 4 also demonstrates high activities (Table 2,
entry 11). Other aryl bromides give moderate to good yields, in-
dicating the potential application of this catalytic system. However,
poor yield was attained with alkyl thiols (Table 2, entry 16 and 17)
and the catalytic system is inert toward aryl chloride. Thus, reaction
of p-ClC6H4Br with p-toluenethiol yielded p-ClC6H4SC6H4CH3
exclusively. The lower conversion for the alkyl thiols is probably
due to the decomposition of the palladium complex in the presence
of these thiols as evidenced by the formation of precipitates.
Figure 1. ORTEP plot of 4. Pd(1)–C(1) 1.995(2)Å, Pd(1)–P(1) 2.2606(5)Å, Pd(1)–Cl(1)
2.3489(6)Å, Pd(1)–Cl(2) 2.3713(6)Å, C(1)–Pd(1)–P(1) 91.56(6)ꢁ.
The slightly distorted square-planar geometry around palla-
dium is completed by one carbene moiety, phosphine and two
chlorides. The Pd(1)–Cl(2) bond trans to carbene [Pd(1)–Cl(2)], is
slightly longer (ca. 0.03 Å) than the Pd(1)–Cl(1) bond trans to the
phosphine, thus reflecting the high trans influence of the NHC
ligand. The azole ring adopts an almost perpendicular orientation
with respect to the coordination plane of the [PdCl2] metal
fragment, as indicated by the torsion angles Cl(1)–Pd(1)–C(1)–N(1)
and P(1)-Pd(1)–C(1)–N(2) of 88.9ꢁ and 89.2ꢁ, respectively.
In the first stage of the investigation for C–S coupling, we tried to
seek the optimal catalytically conditions for the reaction of the
model substrate p-methylbenzenethiol 6 with p-bromoacetophe-
none 5 to produce the coupling product 7 in the presence of 2–4 as
the catalysts (Eq. 1). Table 1 shows the catalytic results of the model
reaction over palladium catalysts with NHC ligands. It was found
that the activities of the catalystswerequitesensitivetothe base and
solvent used for the reactions. A screening suggested that t-BuOK is
the most suitable base, whereas toluene and ethanol are the best
solvents, but carrying out the reaction in an ethanol solution
requires a longer reaction time to achieve a better yield. Reactions of
thiophenol with weaker bases (e.g., carbonate or phosphate) gave
low conversions. The activity of complex 2 with PPh3 additive is
essentially similar to that of 4 (entry 17). Since the substitution of 2
with phosphine gives 4, the best catalyst for C–S coupling reaction
appears to be the phosphine-substituted complex 4.
Table 2
Results of C–S coupling of various substratesa
Entry
Ar-X
R0SH
t (h)
Yieldb(%)
1
2
3
4
5
6
7
8
p-CH3COC6H4I
p-CH3COC6H4Br
p-CH3COC6H4Cl
p-CH3COC6H4I
p-CH3COC6H4Br
C6H5Br
p-CH3C6H4SH
p-CH3C6H4SH
p-CH3C6H4SH
p-CH3C6H4SH
p-CH3C6H4SH
C6H5SH
8
8
8
4
4
8
8
8
8
8
8
8
8
8
98
99 [94]
0
82
89
92
45
p-CH3C6H4Br
p-ClC6H4Br
p-ClC6H4SH
p-CH3C6H4SH
p-CH3C6H4SH
p-CH3C6H4SH
p-CH3OC6H4SH
C6H5SH
98 [97]
99 [88]
54
91 [75]
48
9
p-NO2C6H4Br
p-CH3OC6H4Br
p-CH3C6H4Br
m-CH3COC6H4Br
o-CH3C6H4Br
p-CH3C6H4Br
10
11
12
13
14
C6H5SH
p-CH3C6H4SH
70
99 [86]
SH
15
C6H5Br
8
85 [77]
16
17
C6H5Br
C6H5Br
C6H5CH2SH
C12H25SH
8
8
<5
27
a
ArX (0.4 mmol),R0SH (0.25 mmol), Pd complex (0.005 mmol), tKOBu (0.4 mmol)
in toluene (0.5 mL) at refluxing temperature under N2 atmosphere.
b
Determined by the integration of 1H NMR. Isolated yield given in square
brackets.
The reaction mechanism of Pd-phosphine catalysts in coupling
of C–S bonds has been well studied.7 Here, the palladium carbene
complex 4 is believed to be undergoing the similar pathway
(Scheme 2). It is known that the donating ability of coordinating
ligands could affect the activity of catalysts dramatically. Under the
same reaction conditions, complexes 2–4 show various catalytic
activities on the C–S coupling reaction. In the absence of phosphine,
the mono-carbene complex 2 displays no catalytic activity at all
(Table 1, entry 7). Also, the bis-carbene complex 3 exhibits poor
activity (Table 1, entry 3). On the contrary, complex 4 reveals its
excellent activity, suggesting that the combination of phosphine
O
O
[Pd]
+
Br
SH
S
ð1Þ
5
6
7
Table 1
Optimal conditions for the C–S couplinga
Entry
Complex
Base
Solvent
Additive
Time (h)
Yieldb
1
2
3
4
5
6
7
8
3
3
3
2
2
2
2
2
2
2
2
2
2
2
2
2
4
4
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
EtONa
K3PO4
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
toluene
Toluene
Toluene
Toluene
CH3CN
t-Bu3P
Ph3P
d
24
24
24
24
8
70%
75%
23%
54%
99%
55%
0
0
4%
9%
31%
2
99%
38%
0
10%
99%
55%
t-Bu3P
Ph3P
Ph3P
d
Cl
(II)
(NHC) Pd Cl
4
24
24
24
24
24
24
24
8
PPh3
d
ArS-
9
t-Bu3P
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
d
10
11
12
13
14
15
16
17
18
K2CO3
ArS-SAr
(0)
Et3N
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
EtOH
EtOH
(NHC) Pd
Ar'X
PPh3
Ar-S-Ar'
Cl(CH2)2Cl
THF
Toluene
Toluene
24
24
8
[Pd]
Ar'
(II)
Ph3P
8
[Pd]
X
Cl-
Ar'
(II)
[Pd]
a
p-CH3COC6H4Br
(0.4 mmol),
CH3C6H4SH
(0.25 mmol),
Pd
complex
ArS-
(0.005 mmol), base (0.4 mmol), additive (0.005 mmol) in toluene (0.5 mL), reflux
24 h under N2 atmosphere.
SAr
Determined by the integration of 1H NMR with internal standard.
Scheme 2. Catalytic cycle for C–S coupling reaction.
b

C.-F. Fu et al. / Tetrahedron 66 (2010) 2119–2122
2121
and HNC ligands on the palladium ions could create the most active
catalyst for the coupling reaction. The turn over frequency (TOF) for
the coupling of p-CH3C6H4Br with p-CH3C6H4SH reaches to 6.25
[(mol of product) (mole of catalyst)ꢀ1 hꢀ1], which is better than that
of Pd(PPh3)4 (TOFw2).1c It is noted that the use of tri(t-Bu)phos-
phine as the auxiliary ligand (Table 1, entry 1) decreases the yields
on the coupling reactions. Thus, fine-tuning the steric hindrance
and electron-donating properties of ligands is important for
catalyst development and this study illustrates a good example.
In summary, we have described an efficient palladium catalyst
for C–S cross coupling of aryl bromides and thiols. The new catalyst
is easy to synthesize and could be an excellent candidate for large
scale reactions. In this study, it is also found that ligands around the
metal center affect the catalytic activities dramatically. Fine-tuning
of ligand effect to improve the catalytically performance is cur-
rently under investigation.
7.39 (d, J¼8.1 Hz, 2H, Ar-H), 7.21 (d, J¼8.1 Hz, 2H, Ar-H), 7.14 (d,
J¼8.6 Hz, 2H, Ar-H), 2.52 (s, 3H, –COCH3), 2.38 (s, 3H, –CH3).These
data are similar to those reported.
3.3.2. Diphenyl sulfide9. Liquids, 1H NMR (CDCl3, 400 MHz):
7.33 (m, 10H, Ph).
d 7.24–
3.3.3. p-Chlorophenyl p-tolyl sulfide10. White solids, mp: 71–73 ꢁC,
1H NMR (400 MHz, CDCl3):
d
7.27 (d, J¼8.1 Hz, 2H, Ar-H), 7.16–7.24
(m, 4H, Ar-H), 7.13 (d, J¼8.1 Hz, 2H, Ar-H), 2.40 (s, 3H, CH3).
3.3.4. 4-Nitrophenyl p-tolyl sulfide11. Light yellow solids., mp: 86–
87 ꢁC, 1H NMR (400 MHz, CDCl3):
d
8.03 (d, J¼8.9 Hz, 2H, Ar-H), 7.42
(d, J¼8.1 Hz, 2H, Ar-H), 7.25 (d, J¼8.1 Hz, 2H, Ar-H), 7.12 (d, J¼8.9 Hz,
2H, Ar-H), 2.40 (s, 3H, CH3).
3.3.5. 4-Methoxyphenyl p-tolyl sulfide10. Viscous liquids, 1H NMR
3. Experimental
3.1. General
(400 MHz, CDCl3):
2H, Ar-H), 7.05 (d, J¼8.1 Hz, 2H, Ar-H), 6.85 (d, J¼8.9 Hz, 2H, Ar-H),
3.79 (s, 3H, OCH3), 2.28 (s, 3H, CH3).
d
7.34 (d, J¼8.9 Hz, 2H, Ar-H), 7.11 (d, J¼8.1 Hz,
All reactions, manipulations and purifications steps were
performed under a dry nitrogen atmosphere. Tungsten carbene
complexes 2 and 3 were prepared accordingly to the method
reported previously.6a Nuclear magnetic resonance spectra were
recorded in CDCl3 or acetone-d6 on either a Bruker AVANCE 400
spectrometer. Chemical shifts are given in parts per million relative
to Me4Si for 1H and 13C NMR. Chemicals and solvents were of
analytical grade and used as received unless otherwise stated.
3.3.6. m-Acetylphenyl phenyl sulfide12. Viscous liquids, 1H NMR
(400 MHz, CDCl3):
d 7.87–7.88 (m, 1H, Ar-H), 7.76–7.79 (m, 1H, Ar-
H), 7.43–7.50 (m, 1H, Ar-H), 7.29–7.36 (m, 6H, Ar-H), 2.53 (s, 3H,
CH3).
3.3.7. Phenyl o-tolyl sulfide13. Viscous liquids, 1H NMR (400 MHz,
CDCl3):
d 7.17–7.27 (m, 9H, –Ar), 2.35 (s, 3H, –CH3).
3.3.8. Di(p-tolyl) sulfide10. Light brown solid, mp: 54–56 ꢁC, 1H
3.2. Preparation of complex 4
NMR (400 MHz, CDCl3):
J¼8.1 Hz, 4H, Ar-H), 2.30 (s, 6H, –CH3).
d
7.21 (d, J¼8.1 Hz, 4H, Ar-H), 7.08 (d,
Triphenylphosphine (13.2 mg, 5.0ꢂ10ꢀ2 mmol) was added to
a solution of 2 (15 mg, 2.5ꢂ10ꢀ2 mmol) in acetonitrile (2 mL). The
resulting mixture was stirred at room temperature for 48 h. Upon
concentration, the residue was re-crystallized from CH2Cl2/hexane to
give 4 as white crystalline solids (22 mg, 80%). Mp: 250–255 ꢁC (dec);
3.3.9. 2-Naphthyl phenyl sulfide13. Light brown solid, mp: 51–52 ꢁC,
1H NMR (400 MHz, CDCl3):
d 7.70–8.03 (m, 4H, Ar-H), 7.25–7.48 (m,
8H, Ar-H).
1H NMR (CDCl3, 400 MHz):
d
7.69–7.74 (m, 6H, Ar-H), 7.38–7.47 (m,
3.3.10. Benzyl phenyl sulfide13. Viscous liquids, 1H NMR (400 MHz,
CDCl3): 7.13–7.35 (m, 10H, Ar), 4.06 (s, 2H, –CH2–).
9H, Ar-H), 4.09–4.16 (dq, J¼14, 7 Hz, 2H, –CHH–), 3.41 (t, 2H, imi-H,
d
J¼9 Hz), 4.10–4.24 (dq, J¼14, 7 Hz, 2H, –CHH–), 2.83 (t, 2H, imi-H,
3
3JHH¼9 Hz), 1.07 (t, 6H, –CH3, JHH¼7.2 Hz); 31P {1H} NMR (CDCl3,
3.3.11. n-Dodecylthiobenzene14. Viscous liquids, 1H NMR (400 MHz,
CDCl3):
162 MHz):
d
27.3; 13C NMR (CDCl3, 100.6 MHz):
d
191.4 (M]C), 134.4
d
7.13–7.33 (m, 5H, Ar-H), 2.94 (t, J¼7.4 Hz, 2H, S–CH2–),
(d, JP–C¼11.2 Hz), 131.2 (d, JP–C¼2.3 Hz), 130.2 (d, JP–C¼53.3 Hz), 128.4
(d, JP–C¼11.1 Hz), 47.0, 45.1, 12.5. ESIMS calcd for C25H29N2ClPPd
[MꢀCl]:m/z:529.13;found:(529.08). Anal. CalcdforC25H29Cl2N2PPd:
C, 53.07; H, 5.17; N, 4.95. Found: C, 52.87; H, 5.32; N, 5.05.
1.55–1.68 (m, 4H, –CH2–), 1.24–1.40 (m, 16H, –CH2–), 0.84–0.88 (m,
3H, –CH3).
3.4. Crystallography
3.3. Catalysis-general procedure
Crystals suitable for X-ray determination were obtained for 4 by
recrystallization at room temperature. Cell parameters were
determined either by a Siemens SMART CCD diffractometer. The
structure was solved using the SHELXS-97 program and refined
using the SHELXL-97 program by full-matrix least-squares on F2
values. Crystal data of the complex 4: C25H29Cl2N2PPd, Mw¼565.77,
Orthorhombic, P2(1)2(1)2(1), a¼9.7720(1) Å, b¼13.1533(1) Å,
c¼18.8063(2) Å, V¼2417.25(4) Å3, Z¼4, Dcalcd¼1.555 Mg/m3,
A mixture of palladium complex (2.5ꢂ10ꢀ3 mmol), aryl halide
(0.4 mmol), thiol (0.25 mmol), and potassium tert-butoxide
(0.4 mmol) in solvent (0.5 mL) was placed in flask under nitrogen
atmosphere. The mixture was stirred at room temperature for
10 min, and then heated to reflux for a period of time. The reaction
was then monitored by 1H NMR. After the completion of the
reaction, brine (3 mL) and CH2Cl2(5 mL) were added. The organic
layer was separated and the aqueous layer was extracted with
CH2Cl2 (5 mLꢂ2). The combined organic extracts were dried over
magnesium sulfate and concentrated. The residue was chromato-
graphed on the silica gel with the elution of a mixture of hexane
and ethyl acetate. Products obtained in this work were character-
ized by spectral methods particularly with 1H and 13C NMR, and the
data were consistent with those reported.
F(000)¼1152, 0.25ꢂ0.20ꢂ0.15 mm3, 2
q
¼2.60–27.49ꢁ, 23927 reflns
collected, 5547 independent reflns [R(int)¼0.0219], Full-matrix
least-squares on F2, R1¼0.0199, wR2¼0.0508 [I>2
erence number for 4: 755885.
s(I)]. CCDC ref-
Acknowledgements
This work was partially supported by the National Science
Council, Taiwan, ROC for the financial support (NSC97-2113-M002-
013-MY3).
3.3.1. p-Acetylphenyl p-tolyl sulfide8. Light yellow solids, mp:
89–92 ꢁC, 1H NMR(400 MHz, CDCl3):
d
7.78 (d, J¼8.6 Hz, 2H, Ar-H),

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C.-F. Fu et al. / Tetrahedron 66 (2010) 2119–2122
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