Highly efficient and regioselective couplings of aryl halides to olefins catalyzed by a palladium complex with a hybrid phosphorus-sulfur ligand

Article abstract of DOI:10.1016/S0020-1693(01)00659-4

The novel palladium phosphino-thioether P-S chelate complex, [PdCl₂{MeSC₆H₄-2-(CH₂PPh ₂)}] (3) is a highly efficient catalyst for the olefinic coupling of aryl bromo and iodo compounds to olefins under aerobic conditions, leading to more than 10⁶ turnovers for the reactions with bromo and iodo benzenes.

Full text of DOI:10.1016/S0020-1693(01)00659-4

www.elsevier.com/locate/ica
Inorganica Chimica Acta 328 (2002) 39–44
Highly efficient and regioselective couplings of aryl halides to
olefins catalyzed by a palladium complex with a hybrid
phosphorus–sulfur ligand
David Morales-Morales a,*, Roc´ıo Redo´n b, Yifan Zheng c, Jonathan R. Dilworth c
a Instituto de Qu´ımica, Uni6ersidad Nacional Auto´noma de Me´xico, Cd. Uni6ersitaria, Circuito Exterior, Coyoaca´n, 04510 Me´xico
b Department of Chemistry, Uni6ersity of Hawaii, Honolulu, HI 96822, USA
c Inorganic Chemistry Laboratory, Uni6ersity of Oxford, South Parks Road, Oxford OX1 3QR, UK
Received 4 June 2001; accepted 28 August 2001
Abstract
The novel palladium phosphino-thioether P–S chelate complex, [PdCl2{MeSC6H4-2-(CH2PPh2)}] (3) is a highly efficient catalyst
for the olefinic coupling of aryl bromo and iodo compounds to olefins under aerobic conditions, leading to more than 106
turnovers for the reactions with bromo and iodo benzenes. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Heck reaction; P–S hybrid ligands; Palladium complexes; Catalysis
mechanism involving Pd(IV) intermediates has been
suggested [3,2e,2f,2g].
1. Introduction
Sulfur containing complexes have not been widely
used in catalysis due in part to the assumed tendency of
The formation of vinylic CꢀC bonds from the palla-
dium catalyzed coupling of aryl bromides, iodides, and
sulfur to act as catalyst poison. This is certainly not the
triflates with alkenes (the Heck reaction) is one of the
case and recently sulfur-containing species have found
‘true power tools of contemporary organic synthesis’
application in different catalytic processes such as al-
[1]. However, the reaction has been little utilized indus-
lylic alkylation, hydroformylation, etc. [4]. Notably the
trially due to the instability of the palladium catalysts
and the high cost of the aryl starting materials. In hope
of expanding the scope of the Heck reaction, a number
of recent studies have focused on the development of
complexes [PdX{C6H3(CH2SR)2-2,6}] [5] (X=Cl−,
¸¹¹º
CF3CO2−, R=But, Ph) and [PdCl{C6H4(CH3CHSR)-
2}]2 [6] (R=But, Me) are efficient catalysts for the
Heck couplings of iodobenzenes. In recent years hybrid
ligands have received much attention due to the possi-
bility of enhanced reactivity by cooperative effects.
Thus PꢀN [7] and PꢀO [8] ligands have been tested and
proved useful for some catalytic transformations. Inter-
estingly, however, P–S hybrid ligands have not been
extensively used in catalysis [9]. This is particularly true
in the case of the Heck reaction.
improved catalysts [2]. Among those, authors have
agreed that the structure of the catalyst should be
known in order to better understand the effects of the
different substituents and chelating ligands have be-
come the standard for several thermally stable com-
plexes [2a,2b,2c,2d,2e,2f,2g]. The same species have also
generated a debate, which renders the traditional Pd(0)/
Pd(II) cycle to be unlikely in these cases and a new
We report here that the phosphino-thioether palla-
dium complex 3 is an outstanding catalyst for the Heck
reaction under aerobic conditions with turnover num-
bers of 106 even for those cases where aryl bromides are
used.
* Corresponding author. Tel.: +52-56-224514; fax: +52-56-162
217.
E-mail address: damor@servidor.unam.mx (D. Morales-Morales).
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00659-4

40
D. Morales-Morales et al. / Inorganica Chimica Acta 328 (2002) 39–44
2.3. Preparation of MeSC6H4-2-(CH2PPh2) (2)
2. Experimental
2.1. General
To a solution of BrC6H4-2-(CH2PPh2) (2 g) in 30 ml
of dry THF, cooled to −78 °C, was added 2.25 ml of
n-BuLi (2.5 M in n-hexane). The reaction mixture was
stirred for 6 h. The resulting dark brown solution was
reacted with 1.2 ml of MeS–SMe. The solvent was
removed and the residue recrystallized from methanol.
Yield 1.5 g (82%). NMR spectroscopy showed the
product to be better than 98% pure and it was used in
further steps without further purification. 1H NMR
(400.00 MHz, CDCl3): l=6.73–7.44 (m, 14H, arom.),
3.56 (s, 2H, CH2PPh2), 2.47 (s, 3H, SCH3). 13C NMR
(100.00 MHz, CDCl3): l=124.71–138.33 (m, ArC),
All manipulations were carried out using standard
Schlenk and glovebox techniques under purified argon.
Solvents were degassed and dried using standard proce-
dures. The following were purchased and used without
further purification; PdCl2, 2-bromobenzylbromide,
dimethyldisulphide, LiAlH4, n-BuLi and CDCl3
(Aldrich). The complex [PdCl2(COD)] [10] was synthe-
1
sized by the literature methods. The H NMR spectra
were recorded on a Varian Unity Inova 400 spectrome-
ter. Chemical shifts are reported in ppm down field of
TMS using the solvent as internal standard (CDCl3, l
7.26). 13C and 31P NMR spectra were recorded with
complete proton decoupling and are reported in ppm
downfield of TMS with solvent as internal standard
(CDCl3, l 77.0) and external 85% H3PO4, respectively.
GC analyzes were carried out in a HP 5890A flame
ionization detector (FID) and HP 5890 SERIES II with
a 5971A mass selective detector gas chromatographs,
and an HP-1 capillary column (25.0 m) from Hewlett–
Packard.
1
34.02 (d, JPC=16.1 Hz, CH2PPh2), 16.42 (s, SCH3).
31P NMR (161.90 MHz, CDCl3): l= −13.65. Anal.
Calc. for C20H19PS (m/z=322.40): C, 74.51; H, 5.94.
Found: C, 74.5; H, 6.10%.
2.4. Preparation of [PdCl2{MeSC6H4-2-(CH2PPh2)}]
(3)
An acetone (50 ml) solution of MeSC6H4-2-
(CH2PPh2) (500 mg, 1.46 mmol), and [PdCl2(COD)]
[10] (417 mg, 1.46 mmol) was heated under reflux for 2
h. The solvent was evaporated under vacuum and the
crude product was extracted with acetone. Following
recrystalization from CH2Cl2–MeOH, purified product
was obtained as a bright yellow microcrystalline pow-
2.2. Preparation of BrC6H4-2-(CH2PPh2) (1)
Approximately 150 ml of liquid ammonia were con-
densed into a 500 ml three-necked round bottom flask
cooled to −78 °C (dry ice-acetone) and equipped with
a dry ice condenser and N2 inlet. Small pieces of
sodium (2.6 g) were added to give a deep blue solution.
After stirring for 30 min, triphenylphosphine (15 g) was
slowly added under vigorous stirring via the open neck
of the flask. After 2 h stirring, the cool bath was
removed, allowing the ammonia reflux gently turning
the color of the solution to a dark orange. The cooling
bath was replaced and dry NH4Br (5.6 g) was slowly
added. The stirring was continued for one extra hour to
yield a pale orange solution. A degassed solution of
BrC6H4-2-(CH2Br) (14.4 g) in 60 ml of dry, degassed
THF was added dropwise discharging the orange col-
oration. The liquid ammonia was then allowed to evap-
orate. The resulting white solid was washed with 200 ml
of distilled water (vigorously), and then with acetone.
Yield 16 g (78%). NMR spectroscopy showed the
product to be better than 98% pure and it was used in
further steps without further purification. 1H NMR
(400.00 MHz, CDCl3): l=7.6–6.50 (m, 14H, arom.),
3.5 (s, 2H, CH2PPh2). 13C NMR (100.00 MHz, CDCl3):
der. Yield 630 mg (90%). 1H NMR (400.00 MHz,
CDCl3) l=6.75–7.76 (m, 14H, arom.), 3.59 (d, JPH
2
=
10.4 Hz, 2H, CH2PPh2), 3.17 (s, 3H, SCH3). 13C NMR
(100.00 MHz, CDCl3) l=116.62–133.52 (m, ArC),
33.26 (d, JPC=24.14 Hz, CH2PPh2), 18.70 (s, SCH3).
31P NMR (161.90 MHz, CDCl3) l=64.2 (s, 1P). Anal.
Calc. for C20H19Cl2PPdS (m/z=499.73): C, 48.07; H,
3.83.C. Found: C 48.03; H 3.78%.
2.5. General procedure for the catalytic reactions
A DMF solution (5 ml) of 50.0 mmol of halogen
benzene, 60.0 mmol of alkene, and the prescribed
amount of 3 as catalyst was introduced into a Schlenk
tube in the open air. The tube was charged with a
magnetic stir bar and an equimolar amount of base,
sealed, and fully immersed in a 120 °C silicon oil bath.
After the prescribed reaction time (72 h), the mixture
was cooled to room temperature (r.t.) and the organic
phase was analyzed by gas chromatography (GC/FID,
1
l=127.13–138.81 (m, ArC), 36.78 (d, JPC=18.3 Hz,
GC–MS).
A Hewlett–Packard 5980A gas chro-
CH2PPh2). 31P NMR (161.90 MHz, CDCl3): l=
−12.43. Anal. Calc. for C19H16BrP (m/z=355.21): C,
64.24; H, 4.54. Found: C, 64.32; H, 4.47%.
matograph with flame ionization detector (FID), and
an HP-1 capillary column (25.0 m) was used for quanti-
tative GC analysis.

D. Morales-Morales et al. / Inorganica Chimica Acta 328 (2002) 39–44
41
Table 1
Crystal data and structure refinement for complex PdCl2{MeSC6H4-
2-(CH2PPh2)} (3)
Empirical formula
Formula weight
Temperature (K)
C20H19Cl2PPdS
499.68
293(2)
,
Wavelength (A)
0.71073
monoclinic
P21/n
Crystal system
Space group
Unit cell dimensions
,
a (A)
8.744(2)
14.841(5)
16.0310(10)
90
102.960(2)
90
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
Fig. 1. A ZORTEP representation of the structure of PdCl2{MeSC6H4-
2-(CH2PPh2)} (3). Showing the atom labeling scheme. Hydrogen
atoms have been omitted for clarity. The thermal ellipsoids have been
drawn at 50% probability.
,
2027.3(8)
4
Z
Reflections (lattice)
q range (lattice) (°)
250
2.34BqB25.09
1.637
1.361
1000
0.7×0.5×0.5
1.89–25.00
Dcalc (g cm−3
)
v (mm−1
F(000)
)
2.6. Single crystal X-ray structure determination of
[PdCl2{MeSC6H4-2-(CH2PPh2)}] (3)
Crystal size (mm)
q range for data
collection (°)
Intensity data were collected at the EPSRC service,
Cardiff on an Delft-Instruments FAST-TV Area detec-
tor diffractometer with graphite-monochromated Mo
Index ranges
−65h59, −165k514, −185l518
5691
2799 [Rint=0.1072]
SHELXL-97 (Sheldrick, 1997)
SHELXS-86 (Sheldrick, 1991)
Calc. w=1/[|2(Fo2)+(0.0563P)2]
where P=(Fo2+2Fc2)/3
full-matrix least-squares on F2
Reflections collected
Independent reflections
Program used
Solution method (direct)
Weighting scheme
,
Ka radiation (u=0.71069 A). Cell constants were ob-
tained from least-squares refinement of the setting an-
gles of 250 reflections having q=2.34–25.09°. The
structure was solved via direct methods [11] and refined
by Fo2 by full-matrix least-squares. All non-hydrogen
atoms were anisotropic. The hydrogen atoms were in-
cluded in idealized positions with Uiso free to refine.
The weighting scheme gave satisfactory agreement ana-
lyzes. Final R indices [1940I\2|(I)] R1 (on F)=
0.0527, wR2=0.1244. The largest difference peak and
Refinement method
Data/restraints/parameters 2799/0/226
Goodness-of-fit on F2
R indices [1940I\2|(I)]
R indices (all data)
0.927 a
R1=0.0527, wR2=0.1244 b
R1=0.0718, wR2=0.1293 b
1.251 and −0.575
Largest difference peak
−3
,
and hole (e A
Max. shift/esd
)
hole were 1.251 and −0.575 e A−3, the highest peak in
B0.001
,
the final difference Fourier map was in the vicinity of
metal atoms, the final map had no significant features.
The maximum shift/esd was B0.001. Sources of scat-
tering factors were as in reference [12]. The details of
the structure determination are given in Table 1, se-
a S=[w((Fo)2−(Fc)2)2/(n−p)]1/2 where n, number of reflections
and p, total number of parameters.
b R1=ꢀFo−Fcꢀ/ꢀFoꢀ, wR2=[w((Fo)2−(Fc)2)2/w(Fo)2]1/2
.
,
lected bond lengths (A) and angles (°) in Table 2. The
numbering of the atoms is shown in Fig. 1 (ZORTEP
[13]).
Table 2
,
Selected bond distances (A) and angles (°) for PdCl2{MeSC6H4-2-
(CH2PPh2)} (3)
3. Results and discussion
,
Bond distances (°)
Bond angles (A)
PdꢀP
PdꢀS
2.232(2)
PꢀPdꢀS
90.00(8)
85.81(8)
175.64(8)
175.94(7)
92.70(8)
91.42(8)
110.8(3)
106.1(3)
110.3(4)
3.1. Synthesis of [PdCl2{MeSC6H4-2-(CH2PPh2)}] (3)
2.296(2)
2.321(2)
2.345(2)
1.806(8)
1.810(7)
1.832(8)
1.759(8)
1.797(9)
PꢀPdꢀCl(2)
SꢀPdꢀCl(2)
PꢀPdꢀCl(1)
SꢀPdꢀCl(1)
Cl(2)ꢀPdꢀCl(1)
C(1)ꢀPꢀPd
C(16)ꢀSꢀPd
CꢀSꢀPd
PdꢀCl(2)
PdꢀCl(1)
PꢀC(31)
PꢀC(21)
PꢀC(1)
SꢀC(16)
SꢀC
The synthesis of complex 3 is outlined in Scheme 1.
The ligand precursor diphenylphosphinobenzyl-2-
methylthioether was conveniently prepared in two steps
from the reaction of diphenylphosphine, 2-bromoben-
zylbromide and dimethyldisulphide. Reaction of the
phosphino-thioether compound with [PdCl2(COD)] [10]

42
D. Morales-Morales et al. / Inorganica Chimica Acta 328 (2002) 39–44
in acetone under reflux for 2 h yielded the palladium
complex [PdCl2{MeSC6H4-2-(CH2PPh2)}] (3).
in particular PꢀPdꢀCl(2) 85.81(8)° and SꢀPdꢀCl(1)
92.70(8)°. The closing of the PꢀPdꢀCl(2) is consistent
with the steric effect exerted by the CH2 adjacent to the
phosphorus, whereas the opening of the SꢀPdꢀCl(1) is
consistent with the steric hindrance due to the methyl
of the thioether group. A ZORTEP [13] view of the
crystal structure is shown in Fig. 1. The selected bond
distances and bond angles are listed in Table 2.
The purified compound was isolated in 90% yield as
a yellow powder upon recrystallization from CH2Cl2–
MeOH. 1H and 31P NMR spectroscopic results are
consistent with the formulation proposed. FAB mass
spectrometry and elemental analysis results also agree
1
with the proposed formulation. The H NMR (CDCl3)
spectrum shows a set of signals between 7.8 and 6.8
ppm, (14H, arom.) which is assigned to the protons of
the aromatic rings. A doublet at 3.6 ppm (2H) and a
sharp singlet at 3.17 ppm (3H) are assigned to the
protons on the CH2PPh2 and SCH3 groups, respec-
tively. Integration of these signals is in agreement with
the proposed formulation. The 31P NMR (CDCl3) spec-
trum exhibits a single peak at 64.2 ppm as expected for
the proposed structure where a unique phosphorus
nucleus is present in the complex. The FAB mass
spectrum shows a peak at 465 m/z, which corresponds
to the fragment [M+–Cl].
3.3. Catalytic acti6ity of 3 for the Heck coupling of
aryl halides with olefins
The high thermal and air stability of the phosphino-
thioether complex 3 both in the solid state and in
solution permitted the catalytic reactions to be carried
out completely aerobically.
Preliminary experiments showed that the perfor-
mance of the catalyst was enhanced by the use of
inorganic bases such as Na2CO3 or NaOAc, whereas in
the case of organic bases (amines) the yields were
considerably reduced and the regioselectivity mini-
mized. This may be due in part to steric effects exerted
by competitive coordination of the amine to the Pd
center.
3.2. The molecular structure of
PdCl2{MeSC6H4-2-(CH2PPh2)} (3)
Crystals of complex (3) were obtained from a double
layer system of CH2Cl2–MeOH as orange–yellow
square shaped crystals. The complex is monomeric,
with a slightly distorted square planar coordination
geometry comprising the S and P atoms of the biden-
tate-{MeSC6H4-2-(CH2PPh2)}-ligand and the two chlo-
The catalytic activity of 3 for the Heck reaction was
probed through studies of the reactions of styrene with
iodo and bromobenzenes. As seen in Table 3, quantita-
tive conversion to the corresponding olefins was
achieved under the conditions used. The observed cata-
lytic behavior, in regard to both yield and turnover
numbers (TON), is similar or higher to those obtained
by Milstein [14] and Shaw [2d] using the amino pallada-
cycles or phosphino imino complexes, respectively.
However, the phosphino-thioether complex not only
produces exclusively trans-stilbenes in quantitative
yields, but surprisingly turnover numbers of 106 were
observed for bromobenzenes. To our knowledge these
are among the highest TON reported to date for the
coupling reactions of styrene with bromobenzenes.
The reactivities of a variety of aryl halides were
examined under the optimized conditions and uni-
formly showed \99% selective for the trans configured
product. The system is also one of the few reported
systems to show high reactivity with electron-rich and
sterically hindered aryl halides [15]. The practical utility
of this catalytic system was probed by carrying out
preparative scale couplings (Table 3). We uniformly
obtained the corresponding E-stilbenes in \95% iso-
lated yields. To our knowledge, these yields are among
the highest that have been achieved to date for the
Heck coupling of these aryl halides with styrene.
The precise mechanism of the catalytic reaction re-
mains to be elucidated. 31P analyzes of the reaction
mixtures show that complex 3 persists essentially intact
at the end of the reaction, with only halide exchange
having occurred. There has been considerable debate in
,
rine atoms. The PdꢀCl distance trans to P is 2.345(2) A
,
whereas that trans to S is 2.321(2) A. The difference in
bond lengths illustrates the greater structural trans infl-
uence of the phosphine compared to the thioether
donor. The PdꢀP and PꢀS distances of 2.232(2) and
,
2.296(2) A, respectively, are within the expected values.
The distortion of the square planar structure is
reflected in the deviations from 90° of the angles at Pd,
Scheme 1.

D. Morales-Morales et al. / Inorganica Chimica Acta 328 (2002) 39–44
43
Table 3
Heck couplings of halobenzenes with [PdCl2{MeSC6H4-2-(CH2PPh2)}] (3) as catalyst
Entry
X, R
Base
TON
Yield (%) a
Isolated yield (%) b c
E/Z/T
1
2
3
4
5
6
7
8
9
10
11
12
13
I, H
I, H
I, H
I, H
Br, H
Br, H
Br, H
Br, H
Br, 4-Ome
Br, 4-COMe
Br, 4-COH
Br, 4-CH3
Br, 2-Cl
Net3
1 214 219
1 223 964
1 235 832
1 249 325
981 969
1 179 363
1 159 998
1 249 325
1 249 325
1 249 325
1 249 325
1 249 325
1 249 325
\98
\99
\99
\99
79
87.0/10.0/2.0
87.0/11.0/1.4
94.0/4.7/1.1
DIPEA
NaOAc
Na2CO3
NEt3
DIPEA
NaOAc
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
97
65.5/8.1/5.4
88.3/5.5/1.2
98.1/1.4/0.2
95
\99
\99
\99
\99
\99
\99
\99
95
97
94
96
97
96
Reaction conditions 50 mmol of halobenzene, 60 mmol of styrene, 60 mmol of base, 4.0×10−5 mmol of catalyst and 5 ml of DMF, 72 h at
120 °C.
a Yields obtained by GC are based on haloarene.
b Only the trans configured product was detected.
c Isolated yields are the average of two runs.
the literature about the oxidation states of the species
involved in the cycle with Pd(IV)/Pd(II) and Pd(II)/
Pd(0) both being suggested at various times [3]. As is
the case in other examples of Heck catalyst employing
hemilabile chelate ligands [16] we favor the Pd(0)/Pd(II)
mechanism although participation of Pd(IV)/Pd(II) spe-
cies cannot be ruled out.
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
In summary the phosphino-thioether palladium P–S
complex, 3 is a highly efficient catalyst for the olefinic
coupling of aryl bromides and iodides. Most signifi-
cantly, turnover numbers of 106 can be obtained with
bromobenzenes. It is also noteworthy that the attractive
features of this catalytic system: high yields and air and
water stability, approach those required for commercial
viability and opens future perspectives for its use in
other palladium-catalyzed reactions which are currently
under research.
We would like to thank Wesley Yoshida for valuable
help on the running of the NMR experiments and
DGEP-UNAM for financial support for R.R.
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