Mechanistic Insights in the Exchange of Arylthiolate Groups in Aryl(arylthiolato)palladium Complexes Supported by a Dippe Ligand

Article abstract of DOI:10.1021/acs.organomet.5b00662

Carbon-sulfur activation of phenyl p-tolyl sulfide by a mixture of [Pd(dippe)(μ-H)]₂ (1a) and dinuclear Pd(0), [(μ-dippe)Pd]₂ (1b) (dippe = 1,2-bis(diisopropylphosphino)ethane), to yield four carbon-sulfur activation products, (dippe)Pd(p-tolyl)(SPh) (3a), (dippe)Pd(Ph)(S-p-tolyl) (3b), (dippe)Pd(SPh)(Ph)(3c), and (dippe)Pd(p-tolyl)(S-p-tolyl) (3d), was investigated. The carbon-sulfur complexes 3a-3d were completely characterized by ¹H, ³¹P, and ¹³C NMR spectroscopy, elemental analysis, and X-ray diffraction. Exchange interactions between arylthiolate groups in (dippe)Pd(Ar)(SAr′) (3a-3d) were investigated, leading to understanding the mechanism of interconversions among the complexes.

Full text of DOI:10.1021/acs.organomet.5b00662

Article
pubs.acs.org/Organometallics
Mechanistic Insights in the Exchange of Arylthiolate Groups in
Aryl(arylthiolato)palladium Complexes Supported by a Dippe Ligand
Lloyd Munjanja, William W. Brennessel, and William D. Jones*
Department of Chemistry, University of Rochester, Rochester, New York, United States
S
* Supporting Information
ABSTRACT: Carbon−sulfur activation of phenyl p-tolyl sulfide
by a mixture of [Pd(dippe)(μ-H)]2 (1a) and dinuclear Pd(0),
[(μ-dippe)Pd]2 (1b) (dippe = 1,2-bis(diisopropylphosphino)-
ethane), to yield four carbon−sulfur activation products,
(dippe)Pd(p-tolyl)(SPh) (3a), (dippe)Pd(Ph)(S-p-tolyl) (3b),
(dippe)Pd(SPh)(Ph)(3c), and (dippe)Pd(p-tolyl)(S-p-tolyl)
(3d), was investigated. The carbon−sulfur complexes 3a−3d
were completely characterized by 1H, 31P, and 13C NMR
spectroscopy, elemental analysis, and X-ray diffraction. Exchange interactions between arylthiolate groups in (dippe)Pd(Ar)-
(SAr′) (3a−3d) were investigated, leading to understanding the mechanism of interconversions among the complexes.
(diisopropylphosphino)ethane), to generate palladium(II) C−
S activated products.9 The mixture of complexes 1a and 1b
furnishes the highly reactive 14-electron (dippe)Pd fragment,
which is responsible for the activation of the carbon−sulfur
bonds in thiophenes and thioethers. In previous studies9 we
observed that the nucleophilic (dippe)Pd fragment inserts into
the C−S bond of diphenyl sulfide to yield (dippe)Pd(SPh)(Ph)
(3c) as a light yellow solid in 41% yield (eq 1). Complex 3c was
shown to be thermally stable and remain unchanged as the
temperature was raised to 80 °C for several weeks.
1. INTRODUCTION
Primary interest in carbon−sulfur activation chemistry has been
in the hydrodesulfurization process that removes sulfur from
petroleum.1 Over the last few decades, however, interest has
been growing in the applications of C−S bond activation in
synthetic chemistry, mainly in cross-coupling studies.2 A diverse
array of thioorganics can act as electrophilic coupling partners,
thus showing promise as building blocks in cross-coupling
reactions.3 Thioorganics have high stabilities, are structurally
diverse, and are more available compared to the traditional
electrophilic organohalides and other analogues used in cross-
coupling.4 The ability to tune the polarization of thioorganics
offers chemists the opportunity to increase selectivity when
using thioorganics in cross-coupling.5
Recent unique examples in carbon−sulfur activation for
carbon−carbon bond formation have been reported. Dong et
al. reported a novel palladium-catalyzed system that produces 2-
quinolinones from ketene dithioacetals and arynes via C−S
activation and aryne insertion into the carbon−sulfur bond.6
Using various aryl methyl sulfides and diaryl sulfides, Wang and
co-workers recently showed a palladium-catalyzed system that
can successfully couple azoles or thiazoles with thioethers via
C−H/C−S activation.7 The reversibility of the carbon−sulfur
oxidation-addition with late transition metals such as palladium
forms the fundamental foundation for the success of cross-
coupling reactions, leading to a number of reported palladium-
catalyzed cross-coupling reactions via C−S bond cleavage.8
With the growing field of employing thioorganics in cross-
coupling, there is a need for a fundamental mechanistic
understanding of the behavior of C−S bond activated
intermediates to promote selective synthesis in cross-coupling
reactions.
As an extension to the understanding of the carbon−sulfur
bond activation of thioethers with dippe-supported palladium
we investigated C−S bond activation chemistry with a
nonsymmetric substrate, phenyl p-tolyl sulfide. We are
interested to learn, given a choice, if the (dippe)Pd species
would selectively insert into a specific C−S bond over the
other. Slight polarization introduced into phenyl p-tolyl sulfide
by the methyl substituent introduces two choices for C−S
bonding for oxidative addition by the (dippe)Pd species
(Scheme 1). Oxidative addition could happen either on the C−
S bond a or on the slightly less electron rich C−S bond b, or
activation could happen on both bonds although one should be
slightly favored. Subsequently we hypothesized the formation
of two palladium complexes, (dippe)Pd(p-tolyl)(SPh) (3a) and
(dippe)Pd(Ph)(S-p-tolyl) (3b) in the reaction of a mixture of
We have recently reported rapid C−S activation of
thiophenes by a mixture of [Pd(dippe)(μ-H)]2 (1a) and
dinuclear Pd(0), [(μ-dippe)Pd]2 (1b) (dippe = 1,2-bis-
Received: July 31, 2015
© XXXX American Chemical Society
A
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(3b, 38%), (dippe)Pd(SPh)(Ph) (3c,9 10%), and (dippe)Pd(p-
tolyl)(S-p-tolyl) (3d, 11%), respectively. A singlet resonance at
δ 83.6 was assigned to (dippe)Pd(S-p-tolyl)2 (3e, 13%). A
multiplet resonance at δ 83.2−83.8 was assigned to (dippe)-
Pd(S-p-tolyl)(SPh) (3f, 12%). The complex (dippe)2Pd (13%)
shows a singlet at δ 45.3.11 No (dippe)Pd(SPh)2 was observed
in the 31P{1H} NMR spectrum. When the reaction mixture was
exposed to temperatures between −80 and 80 °C in THF-d8,
the product distribution ratios of 3a−3f remained constant. No
other organic products were observed in that temperature range
either by NMR spectroscopy or by GC/MS analysis except for
unreacted phenyl p-tolyl sulfide.
It was surprising to observe the presence of complexes 3c
and 3d in the reaction shown in Scheme 2 since oxidative
addition of phenyl p-tolyl sulfide was expected to yield two
isomers, 3a and 3b, as C−S activation products similar to
Yamamoto’s observations.10 In Scheme 2 the ratio of the
isomers (dippe)Pd(p-tolyl)(SPh) (3a) and (dippe)Pd(Ph)(S-p-
tolyl) (3b) is 1:13, respectively, highly favoring the formation
of 3b over 3a. A plausible explanation for this observation
might be that the 14-electron nucleophilic (dippe)Pd favors
oxidative addition of the less electron rich C−S bond b to yield
3b over 3a. To gain mechanistic information for the formation
of 3c and 3d in the reaction of phenyl p-tolyl sulfide with a
mixture of 1a and 1b (Scheme 2), complexes 3a−3f were
independently synthesized since it was challenging to separate
the mixture of complexes shown in Scheme 2 by any physical
methods.
Scheme 1. Proposed Formation of 3a and 3b by Reaction of
(dippe)Pd with Phenyl p-Tolyl Sulfide
1a and 1b and phenyl p-tolyl sulfide (Scheme 1). Previously,
Yamamoto and co-workers reported the formation of only two
related isomers in the reaction of a Ni(cod)(PEt3)2 complex
with phenyl p-tolyl sulfide.10 The two isomers formed were
trans-Ni(PEt3)2(SPh)(p-tolyl) and trans-Ni(PEt3)2(S-p-tolyl)-
(Ph) in the ratio 14:86.
Herein, we report a unique example of rapid C−S cleavage of
the asymmetric phenyl p-tolyl sulfide and anionic ligand
exchange via a bimetallic palladium species to yield four
different C−S activation products.
2. RESULTS AND DISCUSSION
Reaction of Phenyl p-Tolyl Sulfide with Mixture of 1a
and 1b. Addition of 4 equiv of phenyl p-tolyl sulfide to a
mixture of 1a and 1b at room temperature resulted in
instantaneous formation of C−S activation products, 3a−3d,
(dippe)palladium bisthiolates (3e and 3f), and (dippe)2Pd
(Scheme 2). A 31P{1H} NMR spectrum of the reaction mixture
in C6D6 shows six phosphorus-containing species. There are
four pairs of doublet resonances; the first one at δ 69.7 and 67.6
Independent Syntheses and Structures of Palladium
Complexes 3a−3f.12 Complexes 3a−3d were independently
synthesized following a similar synthetic procedure developed
by Hartwig and co-workers for related palladium complexes of
the form L2Pd(Ar)(SAr′) (eq 2).13 The synthesis was modified
with 2JP−P = 19.8 Hz, the second at δ 69.3 and 67.4 with 2JP−P
=
20.2 Hz, the third at δ 69.6 and 67.5 with 2JP−P = 19.9 Hz, and
2
the fourth at δ 69.5 and 67.5 with JP−P = 19.7 Hz assigned to
(dippe)Pd(p-tolyl)(SPh) (3a, 3%), (dippe)Pd(Ph)(S-p-tolyl)
Scheme 2. Reaction of Phenyl p-Tolyl Sulfide with
[(dippe)Pd]
by the use of Et3N with either thiophenol or 4-methylbenzene
thiol instead of using sodium thiolate salts. The complexes were
isolated in pure form in 58−69% yield as yellow solids.
1
The complexes were completely characterized by H, 31P-
{1H}, and 13C NMR spectroscopy, as well as by X-ray
diffraction and elemental analysis.
Complexes 3a and 3b are isomers and are easily distinguish-
able in the 1H NMR spectrum in C6D6 by their methyl
substituents on either the phenyl or thiolate groups. In complex
3a the phenyl-methyl appears as a singlet resonance at δ 2.21;
the thiolate-methyl appears as a singlet resonance at δ 2.13 for
3b. On the other hand, complex 3d shows two singlet
resonances in the aliphatic region, one at δ 2.22 and the second
at 2.14 for the phenyl-methyl and thiolate-methyl, respectively.
Complex 3e was synthesized using a modified literature
procedure based on that of Nicholson and co-workers for
( d p p e ) P d ( S A r ) 2 c o m p l e x e s w h e r e d p p e
=
Ph2PCH2CH2PPh2.14 An equivalent of (dippe)PdCl2 was
reacted at room temperature with 2 equiv of 4-methybenze-
nethiol in methanol to yield 3e. Light yellow, X-ray quality
crystals were grown for each of the complexes 3a−3e15 (Figure
B
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1−4), and their bond angles and bond lengths compared
(Table 1).
Figure 4. ORTEP plot of (dippe)Pd(p-tolyl)(S-p-tolyl), 3d. All H
atoms have been omitted for clarity. Thermal ellipsoids are plotted at
the 50% probability level.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complexes 3a−3d
3a
3b
3c
3d
Figure 1. ORTEP plot of (dippe)Pd(p-tolyl)(SPh), 3a. All H atoms
have been omitted for clarity. Thermal ellipsoids are plotted at the
50% probability level.
Distances
a
b
a
b
Pd1−Caryl
Pd1−P1
Pd1−P2
Pd1−S1
2.0453(14)
2.2603(4)
2.3188(4)
2.3486(4)
2.051(4)
2.077(13)
2.245(4)
2.339(3)
2.332(4)
2.064(4)
2.2604(12)
2.3269(11)
2.3455(12)
angles
2.2598(11)
2.3353(11)
2.3455(11)
a
b
a
b
Caryl−Pd1− 89.62(4)
89.29(11)
89.9(4)
89.08(11)
P1
a
b
a
b
Caryl−Pd1− 175.24(4)
174.89(12)
86.76(4)
175.8(4)
175.18(11)
86.48(4)
P2
P1−Pd1−
87.288(13)
86.76(12)
P2
a
b
a
b
Caryl−Pd1− 93.17(4)
82.00(11)
81.9(4)
82.48(11)
S1
P1−Pd1−
172.33(14)
90.35(13)
171.20(4)
102.01(4)
115.04(14)
171.68(12)
101.35(12)
112.8(5)
171.02(4)
102.05(4)
114.15(13)
S1
P2−Pd1−
S1
C1−S1−Pd 110.75(5)
Figure 2. ORTEP plot of (dippe)Pd(Ph)(S-p-tolyl), 3b. All H atoms
have been omitted for clarity. Thermal ellipsoids are plotted at the
50% probability level.
a
b
Caryl = C7. Caryl = C8.
in the solid-state crystal structures (3b−3d) is shown to be
bent toward one of the phosphorus atoms except in 3a, where
the thiolate ligand is closer to the Pd-bound phenyl ring.
Thiolate ligand orientations in the crystalline state indicate that
Pd−S rotation in solution might not be so restricted. All
thiolate complexes 3a−3d exhibit an AB-type splitting system
in the 31P{1H} NMR spectra, showing a pair of doublets for
each phosphorus. The Pd−S−C angle is found to be between
110° and 116° for the complexes. Complexes 3a and 3c, with
the SPh moiety, show slightly smaller Pd−S−C angles, at
110.75(5)° and 112.8(5)°, respectively, while 3b and 3d, with
the S-p-tolyl moiety, have Pd−S−C angles of 115.04(14)° and
114.15(13)°, respectively. All of the complexes are square
planar with RMS deviations from planarity of 0.095, 0.038,
0.019, and 0.041 for 3a, 3b, 3c, and 3d, respectively. The twist
angle between the plane defined by Pd(1), P(1), and P(2) and
the plane defined by S(1) Pd(1), and C(7)/(8) is 8.00(5)°,
3.5(2)°, 3.0(6)°, and 3.6(1)° for 3a, 3b, 3c, and 3d,
respectively. In complexes 3a−3d the Pd1−P2 bond, which is
trans to the Pd-bound aryl moiety, appears longer, 2.339(3)−
2.3188(4) Å, than the Pd1−P1 bond, which is trans to the
sulfur-bound thiolate (2.245(4)−2.2604(12) Å). The Pd−S
bond distances fall in the typical range for square-planar
Figure 3. ORTEP plot of (dippe)Pd(SPh)(Ph), 3c. All H atoms have
been omitted for clarity. Thermal ellipsoids are plotted at the 50%
probability level.
X-ray diffraction shows monomeric palladium species 3a, 3b,
3c, and 3d, which are virtually superimposable with slight
differences in the thiolate ligand orientation. The thiolate ligand
C
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palladium complexes with terminal aryl thiolate ligands.9,13 The
dippe bite angles, P1−Pd−P2, for complexes 3a−3d are in the
range 86.76(4)−87.288(13)°, typical for related (dippe)Pd
complexes.16 The softness of the thiolate ligands in these
complexes matches well with late transition metals, resulting in
the formation of stable Pd−S bonds.13 Complexes 3a−3d are
stable indefinitely in air and in the presence of moisture. Diaryl
species of (dippe)Pd(aryl)2 are unknown, as these would be
expected to reductively eliminate biaryls.
Scheme 3. Equilibrium between Complexes 3a, 3b, 3c, and
3d via Proposed Bimetallic Intermediate B
Mechanistic Consideration for the Formation of 3c
and 3d from 3a and 3b. With the isolated Pd complexes 3a−
3d, we desired to understand the relationship between these
complexes as it applied to Scheme 2. Following the
observations from Scheme 2 of the presence of four C−S
activation products, 3a, 3b, 3c, and 3d, we envisioned that these
complexes could all be in equilibrium via fast thiolate ligand
exchange.
To test if 3c and 3d were generated from reacting 3a and 3b,
experiments were set up with differing concentrations of pure
3a and 3b in C6D6 at room temperature, and the reactions were
Table 3. Addition of Varying Amounts of 3c and 3d in C6D6
at Room Temperature
1
followed by H NMR and 31P{1H} NMR spectroscopy (Table
initial ratio of final equilibrium ratios (NMR)
2). Formation of 3c and 3d was immediately observed in a 1:1
entry [3c], M [3d], M
3c:3d
3a:3b:3c:3d
1
2
3
0.036
0.007
0.064
0.034
0.065
0.007
5:5
1:9
9:1
2.53:2.52:2.49:2.46
0.53:0.53:0.09:8.85
0.52:0.52:8.77:0.19
Table 2. Addition of Varying Amounts of 3a and 3b in C6D6
at Room Temperature
initial ratio of final equilibrium ratios (NMR)
entry [3a], M [3b], M
3a:3b
3a:3b:3c:3d
Efforts to spectroscopically identify intermediate B were
carried out by mixing equimolar amounts of 3a and 3b at low
temperatures in toluene-d8. At −80 °C, only 3a and 3b are
observed; however, as the mixture is warmed to −64 °C,
formation of 3c and 3d commences. Equilibrium was
established for 3a:3b:3c:3d in the ratio 1:1:1:1 as the mixture
was warmed to room temperature. Intermediate B could not be
observed at these low temperatures given that the exchange
between 3a, 3b, 3c, and 3d is very fast.
To confirm 3a−3d can participate in thiolate anionic ligand
exchange, the complexes were reacted with sodium thiolate
salts. An excess of sodium thiolate salt NaSPh was added to a
THF-d8 solution of 3b. The simultaneous disappearance of the
pair of doublet resonances assigned to 3b and the appearance
of another pair of doublet resonances for 3c were observed in
the 31P{1H} NMR spectroscopy instantly (eq 3).
1
2
3
0.035
0.007
0.063
0.035
0.063
0.007
5:5
1:9
9:1
2.50:2.50:2.50:2.50
0.29:7.30:1.21:1.20
7.40:0.14:1.23:1.23
ratio independent of the starting concentrations of 3a and 3b
(entries 1−3) at room temperature. The reaction reaches
equilibrium instantly, making it difficult to study the reaction
rate. The product distribution ratios do not change with time at
this temperature. No reductive elimination of phenyl p-tolyl
sulfide, diphenyl sulfide, or di-p-tolylsulfide was observed at
room temperature. Complexes 3e and 3f were not observed in
any of the entries above. Complexes 3c and 3d are formed in a
1:1 ratio, consistent with the stoichiometry of the ligand
exchange reaction.
From these results we propose the involvement of a thiolate-
bridged palladium(II) bimetallic intermediate B, [(dippe)-
Pd]2(μ-SPh)(μ-S-p-tolyl), for the formation of complexes 3c
and 3d (Scheme 3). There have been a few thiolate-bridged
palladium(II) bimetallic complexes reported in the literature,17
but none have been proposed or shown for this type of
equilibrium. Thiolate ligands have been shown to be capable of
diverse coordination from monodentate, μ2-bridging to μ3-
bridging.18 To show that intermediate B might be involved in
the formation of 3a and 3b from the reaction of 3c and 3d, the
concentrations of 3c and 3d were varied and the amounts of
the four complexes measured at equilibrium (Table 3).
Similar to the reactions in Table 2, equilibrium was
established instantly at room temperature. Regardless of the
starting concentrations of 3c and 3d (entries 1−3) in C6D6
solution, 3a and 3b are always formed in a 1:1 ratio. The
equimolar formation of 3a and 3b from 3c and 3d supports an
associative mechanism that forms B (Scheme 3), which
subsequently can rapidly exchange the thiolate ligands to
form 3a and 3b.
Addition of excess Na-S-p-tolyl salt to 3c also showed the
reverse formation of 3b. In a related experiment thiolate ligand
exchange was observed when 3a was reacted with excess Na-S-
p-tolyl to give 3d. Reaction of 3d with excess NaSPh gave back
3a. Since 3a−d and NaSaryl remain un-ionized in C6D6, these
observations support the establishment of a rapid equilibrium
between 3a + 3b and 3c + 3d via proposed intermediate B,
which facilitates the thiolate ligand exchange.
Evidence for transformation of 3a to 3b, 3c, and 3d has been
1
observed in the H and 31P{1H} NMR spectrum of a benzene
solution of complex 3a upon heating to 100 °C. Rather than
observing the expected product from interchange of C−S bond
D
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cleavage, equilibrium is reached in 24 h to give 3a, 3b, 3c, and
3d in the ratio 1:1:1:1. In addition to the formation of these
complexes, reductive elimination products from the respective
complexes are also observed. Heating of 3a over 11 d results in
complete reductive elimination of 3a and 3b to yield phenyl p-
tolyl sulfide, while 3c and 3d eliminate diphenyl sulfide or di-p-
tolyl sulfide, respectively. The ratio of phenyl p-tolyl sulfide,
diphenyl sulfide, and di-p-tolyl sulfide is 2:1:1, respectively. For
comparison, 3b was also heated at 100 °C in a separate
experiment. However, after 11 d of heating 3b, there is still a
significant amount of 3a, 3b, 3c, and 3d in solution in the ratio
1:1:1:1, respectively. Reductive elimination products from the
4. EXPERIMENTAL SECTION
General Procedures. All operations and routine manipulations
were performed under a nitrogen atmosphere, either on a high-vacuum
line using modified Schlenk techniques or in a Vacuum Atmospheres
Corp. Dri-Lab. Benzene-d6 and THF-d8 were purchased from
Cambridge Isotope Laboratories, Inc. Prior to use, benzene-d6 and
THF-d8 were distilled under vacuum from a dark purple solution of
benzophenone ketyl and stored in an ampule with a Teflon valve.
CD2Cl2 was dried with CaH2 and stored over molecular sieves.
Benzene was distilled from dark purple solutions of benzophenone
ketyl and stored over molecular sieves in an ampule with a Teflon
valve. Phenyl p-tolyl sulfide was purchased from Sigma-Aldrich Co.
and used without any further purification. (PPh3)4Pd was purchased
1
from Strem. All H and 13C NMR spectra were collected on either a
1
respective complexes are also observed by H NMR spectros-
Bruker Avance 400 or Avance 500 MHz spectrometer. All chemical
shifts were reported in ppm (δ) relative to tetramethylsilane and
referenced to the chemical shifts of residual solvent resonances
(C6HD5, δ 7.16 or 128.0). While 1H chemical shifts are given to three
decimal places ( 0.4 Hz), these values can vary slightly with
concentration and temperature. 13C shifts are given to two decimal
places ( 1 Hz). 31P{1H} NMR spectra were referenced relative to
external H3PO4. Elemental analysis was performed by the CENTC
Elemental Analysis Facility at the University of Rochester using a
PerkinElmer 2400 Series II elemental analyzer in CHN mode. GC-MS
spectra were recorded on a Shimadzu QP2010 GCMS. Infrared
spectra were recorded in the solid state on a Thermo Scientific Nicolet
4700 FT-IR spectrometer equipped with a Smart Orbit diamond
attenuated total reflectance (ATR) accessory.
copy. From these experiments it was deduced that complex 3b
is thermodynamically more stable than 3a and therefore less
willing to reductively eliminate than complex 3a. The formation
of 3b from heating 3a at 100 °C can be explained by a
reversible process that involves the reductive elimination of
phenyl p-tolyl sulfide, giving the chance for the oxidative
addition of the C−S bond b (Scheme 1) to form 3b. Once 3a
and 3b are in solution, the thiolate ligand exchange equilibrium
is established via proposed intermediate B to form 3c and 3d
(Scheme 3).
To confirm the reversible process of reductive elimination
and oxidative addition, a benzene solution of 3d and 1 equiv of
diphenyl sulfide was heated at 100 °C. In 30 min, production of
3a, 3b, and 3c was observed in the 31P{1H} NMR spectrum.
The results suggest a reversible reductive elimination and
oxidative addition process of ArSAr′ is occurring at 100 °C.
Consequently, the equilibrium between 3a, 3b, 3c, and 3d
occurs simultaneously via the mechanism depicted in Scheme 3.
No reaction was observed when a 3d and diphenyl sulfide
solution was stirred at room temperature, suggesting that no
reductive elimination occurs at room temperature. Further-
more, the observation that 3a is stable in solution at room
temperature and does not convert to 3b rules out a reversible
equilibrium with an S-bound phenyl p-tolyl sulfide complex. It
is worth noting that Hartwig’s extensive studies of alkyl and aryl
sulfide elimination from (diphos)Pd(Ar) (SR) complexes
showed no evidence for reversible C−S addition.13
Reaction of Mixture of 1a, 1b, and Phenyl p-Tolyl Sulfide.
Under a nitrogen atmosphere, NaHBEt3 (91 μL of a 1 M solution in
toluene, 0.091 mmol) was added slowly to a stirred suspension of
[(dippe)PdCl2] (20 mg, 0.046 mmol) in 0.5 mL of toluene and 4
equiv (37 mg, 0.184 mmol) of phenyl p-tolyl sulfide. The resulting
dark red solution was stirred for 30−45 min and then filtered through
Celite on a fine glass frit. The solvent was evaporated under vacuum
overnight to yield C−S insertion products, which were then washed
with 2 mL of hexanes and dried under vacuum. A 31P{1H} NMR
spectrum in C6D6 revealed seven sets of resonances: four pairs of
2
doublet resonances, the first one at δ 69.7 and 67.6 with JP−P = 19.8
Hz (3a), the second at δ 69.3 and 67.4 with 2JP−P = 20.2 Hz (3b), the
third at δ 69.6 and 67.5 with 2JP−P = 19.9 Hz (3c), and the fourth at δ
69.5 and 67.5 with 2JP−P = 19.7 Hz (3d); a singlet resonance at δ 83.6
(3e); a multiplet resonance at δ 83.2−83.8 (3f); and a singlet at δ 45.3
((dippe)2Pd). The percentages of the products were determined on
the basis of the area ratios of these resonances in the 31P{1H} NMR
spectra.
General Procedure for the Synthesis of (PPh3)2Pd(I)(Ar).
Under a nitrogen atmosphere, benzene was added to a mixture of
(PPh3)4Pd and 1.5 equiv of ArI (where Ar = p-tolyl for 2a′ and Ph for
2b′). The resulting cloudy, light yellowish solution was stirred at room
temperature for 3 h. The precipitate that formed was filtered and
washed with minimal benzene, then dried under vacuum to afford a
cream-white powder.
(PPh3)2Pd(p-tolyl)(I) (2a′). This compound was prepared from
(PPh3)4Pd (1446 mg, 1.251 mmol) and p-tolyl iodide (409 mg, 1.877
mmol) in 82% yield (874 mg). 1H NMR (400 MHz, CDCl3): δ 7.40−
7.35 (m, 12H), 7.21−7.17 (m, 6H), 7.14−7.09 (m, 12H), 6.29 (d, J =
7.6 Hz, 2H), 5.95 (d, J = 7.8 Hz, 2H), 1.79 (s, 3H). 31P NMR (162
MHz, CDCl3): δ 19.22. Anal. Calcd (found): C, 60.83 (60.50); H,
4.39 (4.41).
(PPh3)2Pd(Ph)(I) (2b′). This compound was prepared from
(PPh3)4Pd (2000 mg, 1.731 mmol) and iodobenzene (0.30 mL,
2.596 mmol) in 58% yield (840 mg). 1H NMR (500 MHz, CDCl3): δ
8.08−7.38 (m, 30H), 6.80−5.80 (m, 5H). 31P NMR (202 MHz,
CDCl3): δ 24.38. Anal. Calcd (found): C, 60.41 (60.10); H, 4.23
(4.51).
General Procedure for the Synthesis of (dippe)Pd(I)(Ar).
Under a nitrogen atmosphere, benzene was added to a mixture of
(PPh3)2Pd(I)(Ar) (where Ar = p-tolyl for 2a′ and Ph for 2b′) and 1
3. CONCLUSIONS
In summary, we have successfully shown that the carbon−sulfur
bond activation of phenyl p-tolyl sulfide with dippe-supported
palladium results in the preferential formation of (dippe)Pd-
(Ph)(S-p-tolyl) (3b) over (dippe)Pd(p-tolyl)(SPh) (3a).
However, the mixtures of 3a and 3b undergoes a fast thiolate
ligand exchange via a proposed 18-electron palladium bimetallic
species B to give additional C−S palladium complexes
(dippe)Pd(SPh)(Ph) (3c) and (dippe)Pd(p-tolyl)(S-p-tolyl)
(3d). Heating complex 3a or 3b separately at 100 °C results in
the equilibrium formation of 3a, 3b, 3c, and 3d complexes in
the ratio 1:1:1:1 via a reversible reductive elimination and
oxidation addition process. Continued heating of 3a or 3b at
100 °C for several weeks results in the formation of phenyl p-
tolyl sulfide, diphenyl sulfide, and di-p-tolyl sulfide in the ratio
2:1:1, respectively. We believe a mechanistic understanding of
the C−S bond activation in thioethers could be used to
determine synthetic outcomes of cross-coupling reactions that
employ these substrates.
E
DOI: 10.1021/acs.organomet.5b00662
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Article
equiv of dippe ligand. The resulting solution was stirred for 2 h. A
precipitate was formed after addition of pentane. The precipitate was
then filtered and washed with pentane to yield white solids 2a and 2b.
(dippe)Pd(p-tolyl)(I) (2a). This compound was prepared from 2a′
(200 mg, 0.236 mmol) and dippe (62 mg, 0.236 mmol) in 55% yield
(75.8 mg). 1H NMR (400 MHz, THF-d8): δ 7.23 (t, J = 7.5 Hz, 2H),
6.74 (d, J = 6.6 Hz, 2H), 2.60−2.37 (m, 2H), 2.30−2.20 (m, 2H), 2.17
(s, 3H), 1.97 (m, 2H), 1.78 (m, 2H), 1.42−1.37 (m, 6H), 1.25−1.14
(m, 12H), 0.99−0.93 (m, 6H). 13C NMR (126 MHz, CD2Cl2): δ
150.32 (d, J = 128.9 Hz), 138.85, 132.08, 128.12 (d, J = 8.8 Hz), 26.10
(d, J = 17.9 Hz), 25.66 (d, J = 25.7 Hz), 24.86 (t, J = 23.1 Hz), 21.07,
20.62 (d, J = 5.0 Hz), 20.53−20.16 (m), 19.25 (d, J = 2.8 Hz), 18.98,
18.02. 31P NMR (162 MHz, THF-d8): δ 71.26 (d, J = 18.0 Hz), 66.31
(d, J = 18.0 Hz). Anal. Calcd (found): C, 42.98 (43.13); H, 6.70
(6.54).
3H), 2.14 (s, 3H), 2.13−2.06 (m, 2H), 1.84−1.67 (m, 4H), 1.45−0.65
(m, 26H). 13C NMR (126 MHz, CD2Cl2): δ 156.95−155.56 (m),
143.48 (dd, J = 6.4, 2.8 Hz), 138.01, 134.48 (d, J = 2.2 Hz), 134.21,
131.12, 127.90, 127.71 (d, J = 8.2 Hz), 25.39 (d, J = 23.8 Hz), 25.17
(d, J = 16.9 Hz), 23.85 (t, J = 21.9 Hz), 21.06, 21.01, 20.34 (d, J = 5.6
Hz), 20.24−19.88 (m), 19.36 (d, J = 2.9 Hz), 18.76, 18.14. 31P NMR
(162 MHz, C6D6): δ 69.48 (d, J = 19.7 Hz), 67.45 (d, J = 19.6 Hz).
Anal. Calcd (found): C, 57.68 (57.90); H, 7.95 (7.69).
General Description of X-ray Structural Determinations:
Single-Crystal X-ray Crystallography. Crystals were placed onto
the tips of thin glass optical fibers and mounted on a Bruker SMART
CCD platform diffractometer for data collection.20 For each crystal a
preliminary set of cell constants and an orientation matrix were
calculated from reflections harvested from three orthogonal wedges of
reciprocal space. Full data collections were carried out using Mo Kα
radiation (0.710 73 Å, graphite monochromator) with frame times
ranging from 25 to 60 s and at a detector distance of approximately 4
cm. Randomly oriented regions of reciprocal space were surveyed: four
to six major sections of frames were collected with 0.50° steps in ω at
four to six different φ settings and a detector position of −38° in 2θ.
The intensity data were corrected for absorption.21 Final cell constants
were calculated from the xyz centroids of approximately 4000 strong
reflections from the actual data collections after integration.22
Structures were solved using SIR201123 and refined using SHELXL-
2014.24 Space groups were determined based on systematic absences,
intensity statistics, or both. Direct-method solutions were calculated,
which provided most non-hydrogen atoms from the E-map. Full-
matrix least-squares/difference Fourier cycles were performed, which
located the remaining non-hydrogen atoms. All non-hydrogen atoms
were refined with anisotropic displacement parameters. All hydrogen
atoms were placed in ideal positions and refined as riding atoms with
relative isotropic displacement parameters. Full matrix least-squares
refinements on F2 were run to convergence.
(dippe)Pd(Ph)(I)19 (2b). This compound was prepared from 2b′
(400 mg, 0.479 mmol) and dippe (126 mg, 0.479 mmol) in 71% yield
1
(195 mg). H NMR (400 MHz, THF-d8): δ 7.37 (t, J = 7.6 Hz, 2H),
6.87 (t, J = 6.4 Hz, 2H), 6.70 (t, J = 7.2 Hz, 1H), 2.53−2.47 (m, 2H),
2.28−2.22 (m, 2H), 2.03−1.92 (m, 2H), 1.82−1.70 (m, 2H), 1.45−
1.35 (m, 6H), 1.27−1.13 (m, 12H), 1.01−0.89 (m, 6H). 13C NMR
(126 MHz, CD2Cl2): δ 156.74−154.18 (m), 139.23, 127.11 (d, J = 8.6
Hz), 123.11, 26.11 (d, J = 17.9 Hz), 25.59 (d, J = 25.6 Hz), 24.85 (t, J
= 23.1 Hz), 20.62 (d, J = 4.6 Hz), 20.38 (dd, J = 18.7, 11.7 Hz), 19.14
(d, J = 2.2 Hz), 18.98, 17.99. 31P NMR (162 MHz, THF-d8): δ 74.01
(d, J = 18.2 Hz), 69.37 (d, J = 18.2 Hz). Anal. Calcd (found): C, 41.94
(41.34); H, 6.51 (6.12)
General Procedure for the Synthesis of (dippe)Pd(SR)(Ar).
Under a nitrogen atmosphere, benzene was added to a mixture of
(dippe)Pd(I)(Ar) (where Ar = p-tolyl for 2a and Ph for 2b), 2.5 equiv
of Et3N, and 2.5 equiv of thiol (thiophenol to make 3a and 4-
methylbenzene thiol to make 3b). The resulting clear yellowish
solution mixture was stirred for 15 min at room temperature and left
to stand at −30 °C for 5−10 min, after which it was filtered through
Celite on a fine frit. To the yellow filtrate was added pentane, and then
the solution was left to stand at −30 °C for another 5−10 min, before
filtration again through Celite. Solvent was removed under high
vacuum to yield yellow solids.
In structures with multiple molecules in the asymmetric unit, only
one is shown in the figures and referred to in the discussions because
either the second independent molecule was disordered or the two
independent molecules were metrically identical. Full details for each
structure can be found in the SI.
(dippe)Pd(p-tolyl)(SPh) (3a). This compound was prepared from 2a
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00662.
(298 mg, 0.507 mmol), thiophenol (130 μL, 1.27 mmol), and Et3N
■
1
(177 μL, 1.27 mmol) in 61% yield (175 mg). H NMR (400 MHz,
S
C6D6): δ 7.62 (d, J = 7.6 Hz, 2H), 7.42 (t, J = 7.4 Hz, 2H), 7.00−6.81
(m, 5H), 2.40 (m, 2H), 2.21 (s, 3H), 2.08 (m, 2H), 1.79 (s, 2H),
1.43−0.65 (m, 26H). 13C NMR (126 MHz, C6D6): δ 157.18 (dd, J =
123.2, 8.3 Hz), 147.28 (dd, J = 8.2, 3.5 Hz), 138.43, 136.14 (d, J = 1.8
Hz), 134.95, 127.41, 127.16, 122.41, 25.26 (d, J = 5.0 Hz), 25.10,
23.80 (t, J = 21.6 Hz), 21.48, 20.42−20.22 (m), 20.15 (d, J = 5.2 Hz),
19.35 (d, J = 3.4 Hz), 18.80, 17.99. 31P NMR (162 MHz, C6D6): δ
69.66 (d, J = 19.7 Hz), 67.55 (d, J = 19.8 Hz). Anal. Calcd (found): C,
56.99 (56.39); H, 7.79 (8.13).
A listing of crystallographic information including data
collection parameters, bond lengths and angles, and
anisotropic thermal parameters for complexes 3a−3e, 2a,
and 2b (PDF)
Crystallographic data for complexes 3a−3e, 2a, and 2b
(CCDC deposition nos. 1416386−1416392) (CIF)
(dippe)Pd(Ph)(S-p-tolyl) (3b). This compound was prepared from
2b (238 mg, 0.4149 mmol), 4-methylbenzene thiol (129 mg, 1.04
mmol), and Et3N (145 μL, 1.04 mmol) in 69% yield (164 mg).
Purification of the complex was achieved by layering a benzene
solution of 3b with diethyl ether to yield a yellow solid, which was
AUTHOR INFORMATION
Corresponding Author
*E-mail: william.jones@rochester.edu.
Notes
■
1
dried under high vacuum. H NMR (400 MHz, C6D6): δ 7.57−7.46
(m, 4H), 7.05−6.95 (m, 2H), 6.90 (t, J = 7.1 Hz, 1H), 6.76 (d, J = 7.7
Hz, 2H), 2.13 (s, 3H), 2.11−2.05 (m, 2H), 1.87−1.63 (m, 2H), 1.45−
0.60 (m, 28H). 13C NMR (126 MHz, C6D6): δ 162.96 (dd, J = 122.8,
7.9 Hz), 142.96 (dd, J = 8.1, 3.6 Hz), 138.76, 136.00, 131.26, 130.41
(d, J = 14.7 Hz), 127.18 (d, J = 7.9 Hz), 122.46, 25.22 (d, J = 3.8 Hz),
25.06 (d, J = 10.0 Hz), 23.77 (t, J = 21.5 Hz), 21.52, 20.45−20.26 (m),
20.17 (d, J = 5.1 Hz), 19.23 (d, J = 3.3 Hz), 18.82, 17.95. 31P NMR
(162 MHz, C6D6): δ 69.25 (d, J = 20.1 Hz), 67.44 (d, J = 20.2 Hz).
Anal. Calcd (found): C, 56.99 (55.89); H, 7.79 (8.21).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Science Foundation (NSF Grant
1360985) for their support of this work.
■
REFERENCES
(dippe)Pd(p-tolyl)(S-p-tolyl) (3d). This compound was prepared
■
(1) (a) Topsoe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating
Catalysis; Springer-Verlag: Berlin, 1996. (b) Sanchez-Delgado, R. A.
Organometallic Modelling of the Hydrodesulfurization and Hydro-
nitrogenation Reactions; Kluwer Academic Publishers: Dordrecht, 2002.
from 2a (136 mg, 0.232 mmol), 4-methylbenzene thiol (72 mg, 0.579
1
mmol), and Et3N (81 μL, 0.579 mmol) in 58% yield (78 mg). H
NMR (400 MHz, C6D6): δ 7.51 (d, J = 7.8 Hz, 2H), 7.42 (t, J = 7.2
Hz, 2H), 6.86 (d, J = 8.4 Hz, 2H), 6.75 (d, J = 7.8 Hz, 2H), 2.22 (s,
F
DOI: 10.1021/acs.organomet.5b00662
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(2) (a) Wang, L.; He, W.; Yu, Z. Chem. Soc. Rev. 2013, 42, 599−621.
(b) Modha, S. G.; Mehta, V. P.; Van der Eycken, E. V. Chem. Soc. Rev.
2013, 42, 5042−5055. (c) Jafarpour, F.; Barzegar, M.; Olia, A.; Hazrati,
H. Adv. Synth. Catal. 2013, 355, 3407−3412. (d) Wang, M.; Li, D.-K.;
Zhou, W.; Wang, L. Tetrahedron 2012, 68, 1926−1930. (e) Dubbaka,
S. R.; Vogel, P. Angew. Chem., Int. Ed. 2005, 44, 7674−7684.
(3) (a) Thottumkara, A. P.; Kurokawa, T.; Du Bois, J. Chem. Sci.
2013, 4, 2686−2689. (b) Pan, F.; Wang, H.; Shen, P.-X.; Zhao, J.; Shi,
Z.-J. Chem. Sci. 2013, 4, 1573−1577. (c) Henke, A.; Srogl, J. Chem.
Commun. 2011, 47, 4282−4284. (d) Zhang, Z.; Lindale, M. G.;
Liebeskind, L. S. J. Am. Chem. Soc. 2011, 133, 6403−6410.
(e) Liebeskind, L. S.; Yang, H.; Li, H. Angew. Chem., Int. Ed. 2009,
48, 1417−1421. (f) Villalobos, J. M.; Srogl, J.; Liebeskind, L. S. J. Am.
Chem. Soc. 2007, 129, 15734−15735. (g) Dubbaka, S. R.; Vogel, P.
Chem. - Eur. J. 2005, 11, 2633−2641. (h) Liebeskind, L. S.; Srogl, J. J.
Am. Chem. Soc. 2000, 122, 11260−11261.
(21) Sheldrick, G. M. SADABS, version 2012/1; University of
Gottingen: Gottingen, Germany, 2012.
̈
̈
(22) SAINT, version 8.34A; Bruker AXS: Madison, WI, 2013.
(23) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.;
Cascarano, G. L.; Giacovazzo, C.; Mallamo, M.; Mazzone, A.; Polidori,
G.; Spagna, R. SIR2011: a new package for crystal structure
determination and refinement, version 1.0; Istituto di Cristallografia:
Bari, Italy, 2012.
(24) Sheldrick, G. M. SHELXL-2014/3; University of Gottingen:
̈
Gottingen, Germany, 2014.
̈
(4) (a) Negishi, E.-i. Handbook of Organopalladium Chemistry for
Organic Synthesis; Wiley-Interscience: New York, 2002. (b) De
Meijere, A. In Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.,
Vols. 1−2; Diederich, F., Ed.; Wiley-VCH: Weinheim, 2004. (c) Jana,
R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111, 1417−1492.
(d) Surry, D. S.; Buchwald, S. L. Chem. Sci. 2011, 2, 27−50.
(5) Liebeskind, L. S.; Srogl, J. Org. Lett. 2002, 4, 979−981.
(6) Dong, Y.; Liu, B.; Chen, P.; Liu, Q.; Wang, M. Angew. Chem., Int.
Ed. 2014, 53, 3442−3446.
(7) Zhu, F.; Wang, Z.-X. Org. Lett. 2015, 17, 1601−1604.
(8) (a) Zhao, F.; Tan, Q.; Xiao, F.; Zhang, S.; Deng, G.-J. Org. Lett.
2013, 15, 1520−1523. (b) Rao, B.; Zhang, W.; Hu, L.; Luo, M. Green
Chem. 2012, 14, 3436−3440. (c) Zhou, C.; Liu, Q.; Li, Y.; Zhang, R.;
Fu, X.; Duan, C. J. Org. Chem. 2012, 77, 10468−10474. (d) Zhou, C.;
Li, Y.; Lu, Y.; Zhang, R.; Jin, K.; Fu, X.; Duan, C. Chin. J. Chem. 2013,
31, 1269−1273. (e) Wenkert, E.; Ferreira, T. W.; Michelotti, E. L. J.
Chem. Soc., Chem. Commun. 1979, 637−638.
(9) Munjanja, L.; Brennessel, W. W.; Jones, W. D. Organometallics
2015, 34, 1716−1724.
(10) Osakada, K.; Maeda, M.; Nakamura, Y.; Yamamoto, T.;
Yamamoto, A. J. Chem. Soc., Chem. Commun. 1986, 442−443.
(11) Fryzuk, M. D.; Clentsmith, G. K. B.; Rettig, S. J. Organometallics
1996, 15, 2083−2088.
(12) Addition of 1 equiv of phenyl p-tolyl sulfide to (dippe)Pd(SPh)2
confirmed the formation of complex 3f as a multiplet resonance at δ
83.2−83.8 in the 31P{1H} NMR spectrum in C6D6.
(13) Mann, G.; Baranano, D.; Hartwig, J. F.; Rheingold, A. L.; Guzei,
I. A. J. Am. Chem. Soc. 1998, 120, 9205−9219.
(14) Decker, C.; Henderson, W.; Nicholson, B. K. J. Coord. Chem.
2010, 63, 2965−2975.
(15) Crystal data files for complexes 3e, 2a, and 2b are included in
the Supporting Information. Only 3a−3d crystal structures are
discussed in this article.
(16) (a) Atesin, A. T.; Oster, S. S.; Skugrud, K.; Jones, W. D. Inorg.
Chim. Acta 2006, 359, 2798−2805. (b) Schager, F.; Bonrath, W.;
Porschke, K- R.; Kessler, M.; Kruger, C.; Seevogel, K. Organometallics
̈
1997, 16, 4276−4286. (c) Portnoy, M.; Milstein, D. Organometallics
1993, 12, 1655−1664.
(17) (a) Zanella, R.; Ros, R.; Graziani, M. Inorg. Chem. 1973, 12,
2736−2738. (b) Yamamoto, T.; Akimoto, M.; Saito, O.; Yamamoto, A.
Organometallics 1986, 5, 1559−1567. (c) Osakada, K.; Chiba;
Nakamura, Y.; Yamamoto, T.; Yamamoto, A. J. Chem. Soc., Chem.
Commun. 1986, 1589−1591. (d) Osakada, K.; Chiba, T.; Nakamura,
Y.; Yamamoto, T.; Yamamoto, A. Organometallics 1989, 8, 2602−
2605. (e) Osakada, K.; Ozawa, Y.; Yamamoto, A. J. Organomet. Chem.
1990, 399, 341−348. (f) Miyauchi, Y.; Watanabe, S.; Kuniyasu, H.;
Kurosawa, H. Organometallics 1995, 14, 5450−5453. (g) Yuan, D.;
Huynh, V. H. Organometallics 2010, 29, 6020−6027.
(18) Krebs, B.; Henkel, G. Angew. Chem., Int. Ed. Engl. 1991, 30,
769−788.
(19) Reid, S. M.; Mague, J. T.; Fink, M. J. J. Organomet. Chem. 2000,
616, 10−18.
(20) APEX2, version 2013.10-0; Bruker AXS: Madison, WI, 2013.
G
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