Palladium-catalyzed acetoxylation of arenes by novel sulfinyl n-heterocyclic carbene ligand complexes

Article abstract of DOI:10.1021/om400981x

A series of novel ligands based on N-heterocyclic carbene and sulfoxide functionalities have been prepared and characterized. Pd(II) complexes have been synthesized by transmetalation from the corresponding NHC-Ag derivatives, and their behavior as catalysts has been studied in arene C-H bond oxidative activation. Studies conducted toward the elucidation of the reaction mechanism of the acetoxylation suggest a C-H activation step at Pd(IV) rather than Pd(II) intermediates.

Full text of DOI:10.1021/om400981x

Article
pubs.acs.org/Organometallics
Palladium-Catalyzed Acetoxylation of Arenes by Novel Sulfinyl
N‑Heterocyclic Carbene Ligand Complexes
́
́
Francisco Tato,* Andres Garcıa-Domınguez, and Diego J. Cardenas*
́
́
́
Departamento de Quımica Organ
́
ica, Facultad de Ciencias, Universidad Auton
́
oma de Madrid, Cantoblanco, 28049 Madrid, Spain
S
* Supporting Information
ABSTRACT: A series of novel ligands based on N-heterocyclic carbene and
sulfoxide functionalities have been prepared and characterized. Pd(II)
complexes have been synthesized by transmetalation from the corresponding
NHC−Ag derivatives, and their behavior as catalysts has been studied in arene
C−H bond oxidative activation. Studies conducted toward the elucidation of
the reaction mechanism of the acetoxylation suggest a C−H activation step at
Pd(IV) rather than Pd(II) intermediates.
INTRODUCTION
■
The activation of C−H bonds by palladium catalysis in high
oxidation states has gained a relevant significance over the last
few years, due mainly to the discovery of novel applications and
the development of mechanistic studies concerning Pd(IV).1,2
Although the C−H activation of substrates with directing
groups has been widely studied,3 the functionalization of
unactivated C−H bonds remains a significant challenge in
organic chemistry and still presents several drawbacks related to
reactivity, selectivity, catalytic activity, and scope, among others.
Traditionally, the palladium catalysts involved in Pd(II)/
Pd(IV) catalytic cycles possess carboxylates or pyridine
derivatives as ancillary ligands.4 However, other types of
ligands such as N-heterocyclic carbenes (NHC) have
demonstrated a great utility in Pd chemistry5 (mainly in
cross-coupling reactions) due to the high stability of the
complexes, resistance to acidic conditions, and a strong σ-donor
coordination mode which can both facilitate oxidative addition
and stabilize high oxidation states. For these reasons, some
examples of NHC−Pd complexes have recently appeared
describing the oxidation of both Csp −H and Csp −H bonds
involving Pd(IV) species.6 On the other hand, sulfoxides are
well-known in coordination chemistry and form stable
complexes with many metals.7 Among those, sulfoxides have
recently shown to be exceptional ligands in Pd catalysis for the
functionalization of allylic C−H bonds, overcoming the
regiochemistry troubles inherent to these oxidations.8 In spite
of this, the sulfinyl group has not been previously studied as a
part of NHC ligands, and only one example has been reported,
to the best of our knowledge.9 In the present work, we report
the synthesis of an adaptable NHC family of ligands containing
a sulfinyl group (Figure 1) which could represent a suitable
choice for Pd-catalyzed C−H bond activation. Thus, we intend
to take advantage of the different properties of both kinds of
ligands. Whereas NHC is able to stabilize Pd(IV) species, we
expect that the sulfinyl group can act as a hemilabile
Figure 1. Proposed sulfinyl-NHC ligand and catalyst.
functionality10 capable of coordinating the reactive metal or
creating vacant coordination sites. In order to evaluate these
novel Pd catalysts, we have selected the C−H acetoxylation
reaction of arenes by hypervalent iodine oxidants.
RESULTS AND DISCUSSION
■
These new ligand precursors were prepared by nucleophilic
substitution of the corresponding N-substituted imidazoles with
bromomethyl sulfoxide.11 The reaction conditions were
optimized by the use of microwave irradiation, which improved
the yields and shortened the reaction times (Scheme 1).
Accordingly, aromatic substituents at the imidazole moiety such
as 2,4,6-trimethylphenyl (Mes) and 2,6-diisopropylphenyl
groups reacted with the racemic sulfinyl bromide 2 to form
the imidazolium salts 3a,b in good yields (76% and 78%,
respectively). On the other hand, the alkyl-substituted
3
2
Scheme 1. Synthesis of the Imidazolium Bromide Ligand
Precursors 3a−d
Received: October 7, 2013
Published: November 26, 2013
© 2013 American Chemical Society
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Table 1. Synthesis of the Ag Complexes 4a−d and Sulfinyl-NHC−Pd Complexes 5a−d
yield, %
a
b
entry
R
(NHC)2−Ag (4)
NHC−Pd (5)
1
2
3
4
Mes (3a)
94 (4a)
93 (4b)
95 (4c)
91 (4d)
91 (5a)
82 (5b)
95 (5c)
72 (5d)
2,6-(iPr)-C6H3 (3b)
Me (3c)
t-Bu (3d)
a
The synthesis of Ag and Pd complexes was carried out in a vial with 3a−d (0.1 mmol) and Ag2O (0.1 mmol) in MeCN (1 mL) at room
temperature and stirring for 5 h with protection from light. Then, the mixture was filtered through a Celite pad and Pd(MeCN)2Cl2 (0.1 mmol) was
added to the filtrate; the mixture was stirred for 8 h at room temperature. Yield of isolated complex synthesized in a one-pot reaction from the
b
imidazolium bromide.
imidazole rings also gave the corresponding imidazolium salts
3c,d in 65% and 79% yields for the Me and t-Bu derivatives,
respectively.
NMR spectra. Complete shifting of the dynamic equilibrium to
the solvent-coordinated complex 5 may be the reason for this
behavior (Figure 2). Coordination of solvent was confirmed by
X-ray diffraction of a single crystal obtained by slow
evaporation of a MeCN solution of 5a.12
Once the synthesis of ligands 3a−d was complete, we
directed our efforts to the preparation of novel potentially
active metal complexes. Sulfinyl-NHC−Pd complexes were
synthesized according to the Ag(I)−NHC transfer protocol.
Thus, imidazolium precursors 3a−d were treated with Ag2O in
MeCN at room temperature and subjected to subsequent
transmetalation to a Pd source. This route involves the
formation of [(NHC)2Ag]·[AgBr2] complexes 4a−d, as
shown by mass spectrometry. In a further step, these silver
carbene complexes were reacted with Pd(MeCN)2Cl2, affording
the Pd complexes 5a−d in good yields (95−72%) as yellow
crystalline products starting from the corresponding imidazo-
lium bromides 3a−d (Table 1).
The molecular structure of complex 5a exhibits the usual
square-planar geometry for a Pd(II) complex with nearly no
1
distortion. As expected from H NMR experiments, a MeCN
molecule completes the coordination sphere in a position trans
to the NHC, while the sulfinyl function is orientated in an axial
position with regard to Pd. This makes the torsion angle
between the NHC unit and the coordination main plane reach
121.9°. In fact, the Pd···S distance is 3.34(2) Å, which is slightly
lower than the sum of the van der Waals radii and suggests a
weak interaction between sulfur and Pd (Figure 3, left).
Unexpectedly, heating complex 5d in a AcOH/Ac2O (9/1)
mixture allowed us to isolate 5d′, which exhibits a dimeric
square-planar geometry (Figure 3, right). It is important to note
that the sulfoxide remains uncoordinated even in this case,
suggesting that the coordinating ability of this ligand is not
strong enough to open up the dimer.12
We were interested in the reactivity of these novel complexes
in the activation of aromatic C−H bonds and how the
combined properties conferred by the presence of both
sulfoxide and NHC ligands could influence the reactivity. The
initial report by Crabtree on the acetoxylation of arenes
revealed an interesting process involving oxidation of initially
formed Ph−Pd derivatives, although the reaction was not
regioselective.13 From then on, studies on arene acetoxylation
have mainly centered on substituted systems, in which the
presence of coordinating groups determines the regioselectiv-
ity.3 Thus, ortho acetoxylation of arylpyridines and other
substituted arenes has enabled elegant mechanistic studies
(mainly by the group of Sanford) based on the isolation of
intermediate complexes and the separate study of the different
reaction steps: C−H activation, oxidation of Pd(II) pallada-
cycles, and reductive elimination. Alternatively, acetoxylation of
activated substrates such as indoles14 and quinaxolines15
proceeds with high regioselectivity due to the special features
of the C−H bonds. Recently, more reactive catalytic systems
based on pyridine ligands have been reported for the
Although the reaction was performed in one pot, the
intermediate NHC−Ag complexes were isolable and relatively
1
stable. The H NMR of complexes 5a−d in noncoordinating
solvents (CD2Cl2, CDCl3, toluene-d8) only showed broad
signals, probably due to dynamic equilibration between
monomeric and dimeric species (Figure 2). However,
coordinating solvents such as MeCN-d3, CD3OD, and THF-
1
d8 allowed the observation of well-defined signals in the H
Figure 2. Putative species involved in the dynamic equilibrium of
complex 5 derivatives.
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Figure 3. ORTEP view of complexes 5a (left) and 5d′ (right). Ellipsoids are represented at the 50% probability level. Hydrogen atoms and acetic
acid (5d′) have been omitted for clarity. The Pd···S interaction in complex 5a has been represented as a dashed line.
nondirected acetoxylation of arenes.4c Previously, the simple
Table 2. Evaluation of Novel Pd Complexes 5a−d in
Acetoxylation of Toluene
Pd(OAc)2 has been used as the catalyst for these processes.
In our work, we intended to study the catalytic activity of
NHC-sulfoxide derivatives. The high electron richness of the
NHC ligand seems to promote a different pathway, which starts
with oxidation prior to C−H activation, in contrast to the
reported mechanisms, as will be shown below.
Complexes 5a−d were evaluated in the Pd-catalyzed
acetoxylation of arenes, involving C−H bond oxidative
activation. Toluene was used as the model substrate, and
PIDA was the oxidant of choice. The results are shown in Table
2.
a
b
entry
cat. (mol%)
solvent
yield, %
1
2
3
4
5
6
7
8
9
none
AcOH
AcOH
AcOH
AcOH
AcOH
c
5a (1)
5b (1)
5c (1)
5d (1)
5a (1)
5b (1)
5c (1)
5d (1)
5d (1)
5d (0.5)
5d (0.25)
5d (1)
5d (0.25)
38
34
42
45
63
56
67
69
79
76
74
78
73
Initially, the use of AcOH as solvent afforded low yields of
acetoxylation products for catalysts 5a−d. Slightly higher yields
were obtained for the non-N-aromatic catalysts 5c,d (Table 2,
entries 2−5). Nevertheless, enhanced yields of acetoxy
derivatives, up to 69% with 5d, could be obtained when a
AcOH/Ac2O (9/1) mixture was employed as solvent (Table 2,
entries 6−9). Trace amounts of benzaldehyde were detected in
all assays, which came from the oxidation of the more activated
benzylic position. When the reaction was performed in the
absence of Pd catalyst, we found that the aldehyde was formed
in 35% yield (Table 2, entry 1). On the other hand, PIDA was
observed to decompose in the reaction medium; however, the
decomposition rate decreased with dilution and therefore the
yield of acetoxy derivatives increased up to 79% when the
concentration was lowered (Table 2, entry 10). Catalytic
loading of 5d could be reduced to 0.25 mol % without a
significant loss of yield (Table 2, entries 11 and 12). As
expected, in all cases the process is not regioselective and ca. 1/
1/1 mixtures of acetoxylation at ortho/meta/para positions
were collected for each reaction. Additionally, PIFA was tested
in this reaction, affording similar results in both yield and
regioselectivity (Table 2, entry 13). Moreover, this reaction can
be carried out under microwave irradiation and, although no
improvement concerning the yield was detected, the reaction
time was reduced to 1 h (Table 2, entry 14). We intended to
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
AcOH/Ac2O (9/1)
d
10
d
11
d
12
de
,
13
14
f
a
Unless otherwise noted, a sealed tube was charged with toluene (10
equiv), PIDA (0.167 mmol, 1 equiv), Pd complex (5a−d), and solvent
(0.4 M). Then, the reaction mixture was heated to 95 °C for 16 h.
Yield based on GC using chlorobenzene as internal standard. 35%
yield of benzaldehyde based on GC. Reaction conducted at 0.2 M in
AcOH/Ac2O (9/1). PIFA was used as oxidant. Reaction was carried
out under microwave irradiation for 1 h.
b
c
d
e
f
obtain mechanistic information concerning the reaction
mechanism, since not many examples of strong donating
ligands involved in Pd-catalyzed oxidation reactions have been
reported. Exceptionally, the use of NHC ligands in Pd-catalyzed
C−H activation has been previously described by Sanford et al.
for the chlorination of arenes possessing a directing group.6d
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The oxidation to Pd(IV) with PhICl2 occurs at the preformed
cyclometalated benzoquinoline palladacycle with subsequent
reductive elimination. On the other hand, (NHC)Pd-catalyzed
alkane C−H activation has been proposed at Pd(II) followed
by an oxidation step.6e−h However, we suspected that the
presence of strong σ-donor NHC ligands could play an
additional role in the mechanism. We hypothesized that the
electron-rich Pd(II) catalyst could result in oxidation to Pd(IV)
prior to C−H activation (Figure 4, path A). Although reductive
elimination at Pd(IV) should occur rapidly, C−H activation at
Pd(IV) has been also described.16
other hand, the reaction between the Pd complex and PIDA
was studied by H NMR and GC. While the oxidant slightly
1
decomposed in the presence of a AcOH/Ac2O (9/1) mixture
after heating at 95 °C, as had been previously described,18 the
presence of stoichiometric Pd complex 5d totally consumed the
1
PIDA in 2.5 h. H NMR spectra of this reaction mixture only
showed PhI (coming from the oxidant) and broad signals which
could indicate Pd complexes. Additionally, catalyst 5d and
toluene were heated at 95 °C in AcOH/Ac2O (9/1) for 3 h
without oxidant, and the residue was analyzed by NMR and
GC. The fact that complex 5d did not afford aryl homocoupling
products, along with the absence of deuterium exchange,
suggests that the C−H activation step does not occur at Pd(II),
in contrast to what has been proposed for Pd(OAc)2. (6)
Moreover, the reaction of Pd complex 5d, PIDA or PIFA, and
toluene afforded 64% and 88% yields of chlorotoluene (with
respect to complex 5d), respectively. The presence of this
product suggests the existence of a transient Pd(IV)
intermediate which afforded chlorotoluene by C−Cl reductive
elimination (Scheme 4).
All these data suggest that the reaction mechanism could
involve a Pd(II)/Pd(IV) cycle in which oxidation of Pd(II)
with PIDA takes place prior to C−H activation. Additionally,
C−H activation is relatively slow, in accord with the observed
KIE, but fast in comparison to the acetate scrambling with
solvent. The intimate mechanism of the C−H activation cannot
be elucidated from these data. It could take place in a single
step, with or without acetate assistance, or follow the formation
of an intermediate reminiscent of the Wheland intermediate for
SEAr reactions. In any case, formation of the C−O bond by
reductive elimination must be fast enough to explain the
absence of H/D exchange.
Furthermore, other substrates were subjected to this reaction
conditions using catalyst 5d in other to recover additional
information about the mechanism. For this purpose, different
substitutions on the aromatic ring were evaluated, taking into
account the electronic and the steric effects.
First of all, it is important to note that, in most of the
reactions of Scheme 5, traces of the corresponding chloroarenes
were detected by GC/MS, supporting the formation of Pd(IV)
intermediates, as mentioned above. Substitution of the methyl
group of toluene by propyl provoked a decrease of the
acetoxylation rate at the ortho and meta positions, probably due
to steric and electronic influences, respectively. For the
naphathalene derivative a ca. 1/1 mixture was obtained.
However, electron-rich methoxy-substituted substrates showed
a clear preference for the usual SEAr substitution pattern, and a
single regisomer was obtained from 1,2-dimethoxybenzene. A
competitive substrate such as p-iodoanisole revealed the
prevalence of the influence of strongly activating groups,
yielding mostly acetoxylation at the ortho position. An electron-
withdrawing group such as a methyl ester also gave the reaction
mainly in the meta position, as expected, but in lower yield
(28%). Finally, a substrate carrying an ortho-directing azo group
exclusively afforded the activation at the ortho position. All
these data suggest an electrophilic aromatic substitution for the
Pd-catalyzed C−H activation.
Figure 4. Plausible catalytic cycles for C−H activation an acetoxylation
of toluene.
In order to get an insight into the reaction mechanism and
the actual behavior of these electron-rich NHC−Pd(II)
catalysts, different experiments were performed, which gave
the following information. (1) The catalyst stability was studied
under the reaction conditions. Thus, 5d was stirred for 3 h in
AcOH/Ac2O (9/1) at 95 °C and no decomposition was
1
observed by H NMR. (2) The sulfinyl-NHC−Pd catalysts
afforded the highest conversion of the acetoxylation in 5 h; after
this point, solutions became greenish, indicating the presence of
Pd(0), and finally Pd black starts to appear in the crude
reaction mixture, indicating the complete consumption of the
oxidant. (3) The intermolecular kinetic isotopic effect obtained
by comparing the acetoxylation reaction rate of toluene and
toluene-d8 reveals kH/kD = 3.82, which suggests that the C−H
bond cleavage is involved in the rate-determining step (Scheme
2).17 (4) Additionally, when the catalytic reaction was carried
Scheme 2. Intermolecular Kinetic Isotopic Effect
out in CD3CO2D as solvent, the acetoxytoluene derivatives did
not show any deuterium incorporation in either the acetyl
group or in the arene (Scheme 3, eq 1). Therefore, the whole
acetoxy group incorporated into the product comes from PIDA,
and no exchange with the solvent is taking place at the Pd
complex. On the other hand, when the reaction was carried out
with toluene-d8 as substrate in AcOH, only the acetoxylated d7-
arene was produced, which implies the occurrence of an
irreversible C−H activation step (Scheme 3, eq 2). (5) On the
CONCLUSIONS
■
In conclusion, we have developed a novel series of sulfinyl-
NHC−Pd catalysts with different substitutions at the imidazolyl
nitrogen. Their activity in the C−H activation of arenes has
been studied, affording good yields for the acetoxylation of
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a
Scheme 3. H/D Exchange Experiments
a
Yields were calculated by GC using chlorobenzene as internal standard.
Scheme 4. Reductive Elimination at Pd(IV) To Afford Chlorotoluene
a
through activated alumina on a commercial solvent purification system
if necessary. Thin-layer chromatography was carried out using TLC-
aluminum sheets with 0.2 mm of silica gel and visualized by UV or by
staining with phosphomolybdic acid or KMnO4 solution. Chromato-
graphic purifications were carried out using flash grade silica gel (40−
60 μm). 1H (300 MHz) and 13C (75 MHz) NMR spectra were
recorded at 23 °C. Proton and carbon chemical shifts were referenced
internally to residual solvent resonances, and coupling constants are
expressed in Hz. High-resolution mass spectra (HRMS) were recorded
using electrospray ionization (ESI) or fast atom bombardment (FAB)
(70 eV) and a BR-5 ms column (15 m × 0.25 mm i.d., 0.25 μm thick).
Compounds 1a,b,d and 2 were prepared according to the procedure
described in the literature.19 Other chemicals such as 1c, PIDA, PIFA,
PdCl2(MeCN)2, arenes, and chlorobenzene as well as deuterated
solvents were purchased from standard suppliers and used as received.
General Procedure for the Synthesis of Sulfinylimidazolium
Bromides 3a−d. In a microwave vial were added 1-[(bromomethyl)-
sulfinyl]-4-methylbenzene (2; 140 mg, 0.6 mmol, 1.0 equiv), N-
substituted imidazoles 1a−d (0.6 mmol, 1 equiv), and MeCN (2 mL).
The vial was capped, and the reaction mixture was irradiated under
microwave conditions (110 °C, 100 W). After consumption or no
evolution of starting materials (TLC eluted in CH2Cl2/EtOAc 50/50),
the solvent was removed under vacuum and the residue was washed by
sonication in EtOAc until the complete purification of the products.
Finally, sulfinyl imidazolium bromides 3a−d were dried under vacuum.
Additionally, imidazolium bromides can be purified by a chromatog-
raphy column using DCM/MeOH 95/5 → 90/10 as eluent.
Scheme 5. Sulfinyl-NHC−Pd-Catalyzed C−H Acetoxylation
a
Unless otherwise noted, the reaction was carried out as described in
Table 2 at a concentration of 0.2 M and the regioisomeric ratio was
determined by GC. Regioisomeric ratio with respect to OMe group.
Reaction was carried out with 1 equiv of azobenzene and 2 equiv of
b
c
PIDA in MeCN at 80 °C for 72 h.
arenes. A low catalysis loading (0.25 mol %) allows this
transformation, and according to the collected data, the
reaction follows a different mechanism in comparison with
similar oxidations performed in the absence of strongly
donating ligands. Thus, previous oxidation of the catalyst
followed by C−H activation at Pd(IV) species seems to be
involved in this case.
1-Mesityl-3-[(p-tolylsulfinyl)methyl]-1H-imidazol-3-ium Bro-
mide (3a). 1-Mesitylimidazole (1a; 112 mg, 0.6 mmol) was used as
the starting material, and the reaction mixture was irradiated for 6 h,
affording 3a (191 mg) in 76% yield as a hygroscopic off-white solid.
1
Mp 161−163 °C; H NMR (300 MHz, CDCl3) δ 9.73 (s, 1H), 7.83
(t, J = 1.5 Hz, 1H), 7.66 (AA′BB′ system, 2H), 7.34 (AA′BB′ system,
2H), 7.03−6.92 (m, 2H), 7.01 (t, J = 1.5 Hz, 1H), 6.78 (d, J = 13.1 Hz,
1H), 6.02 (d, J = 13.1 Hz, 1H), 2.37 (s, 3H), 2.32 (s, 3H), 2.06 (br s,
3H), 1.79 (br s, 3H) ppm; 13C NMR (75 MHz, CDCl3) δ 142.7,
141.5, 137.7, 134.5, 134.3, 134.0, 130.4 (2C), 130.3, 129.9, 129.8,
124.9 (2C), 124.1, 122.2, 66.8, 21.4, 21.1, 17.6, 17.4 ppm; HRMS
(FAB+) m/z [M − Br]+ calcd for C20H23N2OS 339.1530, found
339.1531.
EXPERIMENTAL DECTION
■
General Considerations. All of the reactions were run in a sealed
tube with a Teflon-lined cap under an air atmosphere. Solvents for
synthesis were of reagent grade or better. Additionally, THF,
dichloromethane, toluene, and acetonitrile were dried by passage
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1-(2,6-Diisopropylphenyl)-3-[(p-tolylsulfinyl)methyl]-1H-imi-
dazol-3-ium Bromide (3b). 1-(2,6-Diisopropylphenyl)imidazole
(1b; 137 mg, 0.6 mmol) was used as the starting material, and the
reaction mixture was irradiated for 6 h, affording 3b (216 mg) in 78%
yield as a hygroscopic off-white solid: mp 157−159 °C; 1H NMR (300
MHz, CDCl3) δ 9.67 (s, 1H), 8.00 (s, 1H), 7.70 (AA′BB′ system, 2H),
7.53 (t, J = 7.8 Hz, 1H), 7.36 (AA′BB′ system, 2H), 7.26 (t, J = 7.8 Hz,
2H), 7.01 (t, J = 1.8 Hz, 1H), 6.89 (d, J = 13.9 Hz, 1H), 6.12 (d, J =
13.9 Hz, 1H), 2.39 (s, 3H), 2.27 (q, J = 6.7 Hz, 1H), 2.04 (q, J = 6.7
Hz, 1H), 1.29 (d, J = 6.8 Hz, 3H), 1.14 (d, J = 6.8 Hz, 3H), 1.12 (d, J
= 6.8 Hz, 3H), 1.09 (d, J = 6.8 Hz, 3H) ppm; 13C NMR (75 MHz,
CDCl3) δ 145.5, 145.2, 142.7, 137.8, 134.4, 132.2, 130.5 (2C), 129.8,
125.0, (2C), 124.9, 124.8, 124.3, 123.2, 66.7, 28.7, 28.6, 24.6, 24.4,
24.4, 24.2, 21.6 ppm; HRMS (FAB+) m/z [M − Br]+ calcd for
C23H29N2OS 381.2001, found 381.1998.
21.6 ppm (Ccarbene not observed); HRMS (ESI+) m/z [M − AgBr2]+
calcd for C46H56N4S2O2Ag [M − AgBr2]+ 869.2892, found 869.2895.
Bis{1-methyl-3-[(p-tolylsulfinyl)methyl]imidazol-2-ylidene}-
silver(I) Dibromoargentate (4c). 1-Methyl-3-[(p-tolylsulfinyl)-
methyl]-1H-imidazolium bromide (3c; 32 mg, 0.1 mmol) was used
as the starting material, and the reaction mixture afforded the silver
1
complex 4c (42 mg) in 95% yield as a white solid. H NMR (300
MHz, acetonitrile-d3) δ 7.39 (AA′BB′ system, 2H), 7.33 (AA′BB′
system, 2H), 7.10 (d, J = 1.8 Hz, 1H), 7.05 (d, J = 1.8 Hz, 1H), 5.38
(d, J = 13.3 Hz, 1H), 5.15 (d, J = 13.3 Hz, 1H), 3.74 (s, 3H), 2.38 (s,
3H) ppm; 13C NMR (76 MHz, acetonitrile-d3) δ 184.1, 143.7, 138.0,
131.3 (2C), 125.5 (2C), 123.7, 123.5, 72.2, 39.4, 21.6 ppm; HRMS
(ESI+) m/z [M − AgBr2]+ calcd for C24H28N4S2O2Ag 575.0699,
found 575.0698; MS (ESI−) calcd for AgBr2 [M] 266.74.
Bis{1-tert-butyl-3-[(p-tolylsulfinyl)methyl]imidazol-2-
ylidene}silver(I) Dibromoargentate (4d). 1-tert-Butyl-3-[(p-
tolylsulfinyl)methyl]-1H-imidazolium bromide (3d; 36 mg, 0.1
mmol) was used as the starting material, and the reaction mixture
afforded the silver complex 4d (46 mg) in 91% yield as a yellowish
solid. 1H NMR (300 MHz, acetonitrile-d3) δ 7.31 (br s, 4H), 7.28 (d, J
= 2.0 Hz, 1H), 7.08 (d, J = 2.0 Hz, 1H), 5.41 (d, J = 13.4 Hz, 1H), 5.28
(d, J = 13.4 Hz, 1H), 2.36 (s, 3H), 1.60 (s, 9H) ppm; 13C NMR (76
MHz, acetonitrile-d3) δ 181.3, 143.5, 137.6, 131.4 (2C), 125.3 (2C),
122.2, 120.3, 73.0, 58.9, 31.9 (3C), 21.5 ppm; HRMS (ESI+) m/z [M
− AgBr2]+ calcd for C30H40N4S2O2Ag 661.1636, found 661.1628.
General Procedure for the Synthesis of Sulfinyl-NHC−Pd
Complexes 5a−d. In a vial were added 1-substituted 3-[(p-
tolylsulfinyl)methyl]-1H-imidazol-3-ium bromides 3a−d (0.30 mmol,
1.0 equiv), Ag2O (70 mg, 0.30 mmol, 1.0 equiv), and MeCN (2 mL).
The vial was protected from the light, and the reaction mixture was
stirred at room temperature for 5 h. Then, the reaction mixture was
filtered through a Celite pad, the pad was rinsed with MeCN (1 mL),
and PdCl2(MeCN)2 (78 mg, 0.30 mmol, 1 equiv) was added to the
filtrate. The reaction mixture was stirred for 16 h at room temperature
and filtered through a Celite pad. The filtrate was evaporated under
reduced pressure, and the residue was dissolved in CH2Cl2. The
suspension was filtered again through a Celite pad, and the solvent was
evaporated under vacuum. Finally, the residue was washed with Et2O
and dried under vacuum, affording the pure complexes. If necessary,
further purification was performed by recrystallization in CH2Cl2/
Et2O.
1-Methyl-3-[(p-tolylsulfinyl)methyl]-1H-imidazol-3-ium Bro-
mide (3c). 1-Methylimidazole (1c; 49 mg, 0.6 mmol) was used as the
starting material, and the reaction mixture was irradiated for 10 h,
affording 3c (123 mg) in 65% yield as a hygroscopic off-white solid:
1
mp 160−162 °C; H NMR (300 MHz, CDCl3) δ 9.68 (s, 1H), 7.56
(AA′BB′ system, 2H), 7.49 (t, J = 1.4 Hz, 1H), 7.45 (t, J = 1.4 Hz,
1H), 7.31 (AA′BB′ system, 2H), 6.14 (d, J = 13.4 Hz, 1H), 5.84 (d, J =
13.4 Hz, 1H), 4.00 (s, 3H), 2.38 (s, 3H) ppm; 13C NMR (75 MHz,
CDCl3) δ 143.1, 137.6, 135.1, 130.5 (2C), 124.7 (2C), 123.6, 122.8,
68.1, 37.1, 21.6 ppm; HRMS (FAB+) m/z [M − Br]+ calcd for
C12H15N2OS 235.0901, found 235.0905.
1-tert-Butyl-3-[(p-tolylsulfinyl)methyl]-1H-imidazol-3-ium
Bromide (3d). 1-tert-Butylimidazole (1d; 75 mg, 0.6 mmol) was used
as the starting material, and the reaction mixture was irradiated for 10
h, affording 3d (169 mg) in 79% yield as an off-white solid: mp 164−
1
166 °C; H NMR (300 MHz, CDCl3) δ 9.91 (s, 1H), 7.54 (AA′BB′
system, 2H), 7.53−7.50 (m, 1H), 7.41 (t, J = 1.9 Hz, 1H), 7.29
(AA′BB′ system, 2H), 6.09 (d, J = 13.3 Hz, 1H), 5.84 (d, J = 13.3 Hz,
1H), 2.34 (s, 3H), 1.52 (s, 9H) ppm; 13C NMR (76 MHz, CDCl3) δ
142.7, 135.6, 134.8, 130.3 (2C), 124.7 (2C), 123.6, 118.6, 67.6, 60.7,
29.9 (3C), 21.4 ppm; HRMS (FAB+) m/z [M − Br]+ calcd for
C15H21N2OS [M − Br]+ 277.1375, found 277.1379.
General Procedure for the Synthesis of Sulfinyl-NHC−Ag
Complexes 4a−d. In a vial were added 1-substituted 3-[(p-
tolylsulfinyl)methyl]-1H-imidazol-3-ium bromides 3 (0.1 mmol, 1.0
equiv), Ag2O (23 mg, 0.1 mmol, 1.0 equiv), and MeCN (1 mL). The
vial was protected from the light, and the reaction mixture was stirred
at room temperature for 5 h. Then, the reaction mixture was filtered
through a Celite pad and the filtrate was evaporated under reduced
pressure, affording the Ag complexes 4a−d, which were used in the
next step without further purification.
Chloro{1-mesityl-3-[(p-tolylsulfinyl)methyl]imidazol-2-
ylidene}palladium(II) (5a). 1-Mesityl-3-[(p-tolylsulfinyl)methyl]-
1H-imidazol-3-ium bromide (3a; 126 mg, 0.30 mmol) was used as
the starting material, and the reaction afforded the Pd complex 5a (141
1
mg) in 91% yield as a crystalline yellow solid: mp 218 °C dec; H
NMR (300 MHz, acetonitrile-d3) δ 7.74 (AA′BB′ system, 2H), 7.44
(AA′BB′ system, 2H), 7.11−7.03 (m, 3H), 6.98 (d, J = 1.9 Hz, 1H),
5.62 (d, J = 7.7 Hz, 1H), 2.44 (s, 3H), 2.38 (s, 3H), 2.17 (s, 3H), 2.13
(s, 3H) ppm; 13C NMR (76 MHz, acetonitrile-d3) δ 150.0, 144.0,
140.6, 139.2, 137.2, 137.1, 135.6, 131.2 (2C), 130.0, 130.0, 125.9,
125.7 (2C), 124.6, 73.7, 21.6, 21.2, 19.1 (2C) ppm; HRMS (ESI+) m/
z [M − Cl − MeCN]+ calcd for C20H22ClN2OSPd 481.0168, found
481.0194. Anal. Calcd for C22H26Cl2N3OPdS: C, 47.37; H, 4.70; N,
7.53; S, 5.75. Found: C, 47.48; H, 4.49; N, 7.42; S, 6.05.
Chloro{1-(2,6-diisopropylphenyl)-3-[(p-tolylsulfinyl)methyl]-
imidazol-2-ylidene}palladium(II) (5b). 1-(2,6-Diisopropylphenyl)-
3-[(p-tolylsulfinyl)methyl]-1H-imidazol-3-ium bromide (3b; 138 mg,
0.30 mmol) was used as the starting material, and the reaction afforded
the Pd complex 5b (137 mg) in 82% yield as a crystalline yellow solid:
mp 175 °C dec; 1H NMR (300 MHz, acetonitrile-d3) δ 7.74 (AA′BB′
system, 2H), 7.54 (t, J = 8.2 Hz, 1H), 7.43 (AA′BB′ system, 2H), 7.04
(dd, J = 2.1 and 9.7 Hz, 1H), 5.81 (d, J = 13.3 Hz, 1H), 5.58 (d, J =
13.3 Hz, 1H), 2.80 (q, J = 6.7 Hz, 1H), 2.54 (q, J = 6.7 Hz, 1H), 2.44
(s, 3H), 1.38 (d, J = 6.7 Hz, 3H), 1.35 (d, J = 6.7 Hz, 3H), 0.99 (d, J =
6.7 Hz, 3H), 0.97 (d, J = 6.7 Hz, 3H) ppm; 13C NMR (76 MHz,
acetonitrile-d3) δ 147.8, 147.7, 144.1, 139.0, 139.0, 135.2, 131.6, 131.2
(2C), 127.5, 126.0 (2C), 126.0, 125.1, 124.0, 73.3, 29.4, 29.3, 26.3,
26.2, 23.5, 23.3, 21.6 ppm; HRMS (ESI+) m/z [M − Cl − MeCN]+
calcd for C23H28ClN2OPdS 523.0638, found 523.0628.
Bis{1-mesityl-3-[(p-tolylsulfinyl)methyl]imidazol-2-ylidene}-
silver(I) Dibromoargentate (4a). 1-Mesityl-3-[(p-tolylsulfinyl)-
methyl]-1H-imidazolium bromide (3a; 42 mg, 0.1 mmol) was used
as the starting material, and the reaction mixture afforded the silver
complex 4a (53 mg) in a 94% yield as a yellowish solid. 1H NMR (300
MHz, acetonitrile-d3) δ 7.44 (AA′BB′ system, 2H), 7.32 (AA′BB′
system, 2H), 7.18 (d, J = 1.9 Hz, 1H), 7.00−6.93 (m, 2H), 6.98 (d, J =
1.9 Hz, 1H), 5.55 (d, J = 13.7 Hz, 1H), 5.44 (d, J = 13.7 Hz, 1H), 2.36
(s, 3H), 2.34 (s, 3H), 1.85 (s, 3H), 1.66 (s, 3H) ppm; 13C NMR (76
MHz, acetonitrile-d3) δ 184.7, 143.4, 140.2, 137.4, 136.4, 135.9, 135.6,
131.3 (2C), 130.0, 129.9, 125.6 (2C), 123.8, 123.6, 71.0, 21.6, 21.2,
17.9, 17.9 ppm; HRMS (ESI+) m/z [M − AgBr2]+ calcd for
C40H44N4S2O2Ag [M − AgBr2]+ 785.1951, found 785.1959.
Bis{1-(2,6-diisopropylphenyl)-3-[(p-tolylsulfinyl)methyl]-
imidazol-2-ylidene}silver(I) Dibromoargentate (4b). 1-(2,6-
Diisopropylphenyl)-3-[(p-tolylsulfinyl)methyl]-1H-imidazolium bro-
mide (3b; 46 mg, 0.1 mmol) was used as the starting material, and
the reaction mixture afforded the silver complex 4b (57 mg) in 93%
yield as a yellowish solid. 1H NMR (300 MHz, acetonitrile-d3) δ 7.52−
7.34 (m, 5 H), 7.32−6.98 (m, 4H), 5.60 (bs, 1H), 5.35 (d, J = 13.4 Hz,
1H), 2.39 (s, 3H), 2.25 (q, J = 6.8 Hz, 1H), 2.21−2.10 (m, 1H), 1.24−
0.89 (m, 6H), 1.10 (s, 3H), 1.08 (s, 3H) ppm; 13C NMR (76 MHz,
acetonitrile-d3) δ 146.9, 146.5, 143.5, 137.4, 135.7, 131.3 (2C), 125.4
(3C), 125.0, 125.0, 124.8, 123.8, 70.7, 28.9 (2C), 24.8, 24.6, 24.4, 24.3,
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Organometallics
Article
Chloro{1-methyl-3-[(p-tolylsulfinyl)methyl]imidazol-2-
ylidene}palladium(II) (5c). 1-Methyl-3-[(p-tolylsulfinyl)methyl]-1H-
imidazol-3-ium bromide (3c; 95 mg, 0.30 mmol) was used as the
starting material, and the reaction afforded the Pd complex 5c (117
(c) Dudkina, Y. B.; Mikhaylov, D. Y.; Gryaznova, T. V.; Tufatullin, A.
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Laforteza, B. N.; Yu, J.-Q. Org. Lett. 2013, 15, 3058−3061. (b) Cheng,
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1
mg) in 95% yield as a crystalline yellow solid: mp 205 °C dec; H
NMR (300 MHz, acetonitrile-d3) δ 7.70 (AA′BB′ system, 2H), 7.44
(AA′BB′ system, 2H), 7.07 (d, J = 2.0 Hz, 1H), 6.98 (d, J = 2.0 Hz,
1H), 5.63 (d, J = 12.8 Hz, 1H), 5.24 (d, J = 12.8 Hz, 1H), 4.04 (s, 3H),
2.45 (s, 3H) ppm; 13C NMR (76 MHz, acetonitrile-d3) δ 148.7, 143.9,
139.3, 131.2 (2C), 125.6 (2C), 125.0, 124.4, 73.7, 38.8, 21.5 ppm;
HRMS (ESI+) m/z [M − Cl − MeCN]+ calcd for C12H14ClN2OPdS
374.9546, found 374.9545.
Chloro{1-tert-butyl-3-[(p-tolylsulfinyl)methyl]imidazol-2-
ylidene}palladium(II) (5d). 1-tert-Butyl-3-[(p-tolylsulfinyl)methyl]-
1H-imidazol-3-ium bromide (3d; 107 mg, 0.30 mmol) was used as the
starting material, and the reaction afforded the Pd complex 5d (98 mg)
1
in 72% yield as a crystalline yellow solid: mp 191−192 °C; H NMR
(300 MHz, acetonitrile-d3) δ 7.76 (AA′BB′ system, 2H), 7.47 (AA′BB′
system, 2H), 7.29 (d, J = 2.0 Hz, 1H), 7.08 (d, J = 2.0 Hz, 1H), 6.07
(d, J = 12.7 Hz, 1H), 5.31 (d, J = 12.7 Hz, 1H), 2.45 (s, 3H), 2.0.1 (s,
9H) ppm; 13C NMR (76 MHz, acetonitrile-d3) δ 146.4, 143.8, 139.6,
131.3 (2C), 125.4 (2C), 124.0, 122.5, 76.1, 60.9, 32.1 (3C), 21.5 ppm;
HRMS (ESI+) m/z [M
+ Na −
MeCN]+ calcd for
C15H20Cl2N2OPdSNa 476.9591, found 476.9581. Anal. Calcd for
C17H24Cl2N3OPdS: C, 41.18; H, 4.88; N, 8.48; S, 6.47. Found: C,
41.43; H, 4.56; N, 8.31; S, 6.65.
General Procedure for the Acetoxylation Reaction of
Arenes. Arene (2.0 mmol, 10 equiv), PIDA (65 mg, 0.2 mmol, 1.0
equiv), and Pd complex 5d (1 mg, 1 mol %) were place in a sealed
tube. Then, 1 mL of an AcOH/Ac2O (9/1) mixture was added and the
reaction mixture was stirred for 16 h at 95 °C. Unless otherwise noted,
the solvent was evaporated under vacuum and the residue was purified
by a chromatography column in silica gel.
(4) For recent examples, see: (a) Boursalian, G. B.; Ngai, M.-Y.;
Hojczyk, K. N.; Ritter, T. J. Am. Chem. Soc. 2013, 135, 13278−13281.
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Villalobos, J. M.; Sanford, M. S. Angew. Chem., Int. Ed. 2010, 49, 5884−
5886.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental
procedures, intermolecular kinetic isotopic effects, NMR and
MS spectra for all new compounds, and X-ray data for
compounds 5a and 5d′. This material is available free of charge
via the Internet at http://pubs.acs.org.
(5) For recent examples, see: (a) Valente, C.; Calimsiz, S.; Hoi, K.
H.; Mallik, D.; Sayah, M.; Organ, M. G. Angew. Chem., Int. Ed. 2012,
51, 3314−3332. (b) Fortman, G. C.; Nolan, S. P. Chem. Soc. Rev. 2011,
́
40, 5151−5169. (c) Dıez-Gonzal
́
ez, S.; Marion, N.; Nolan, S. P. Chem.
Rev. 2009, 109, 3612−3676.
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.J.C.: diego.cardenas@uam.es.
■
(6) (a) Munz, D.; Meyer, D.; Strassner, T. Organometallics 2013, 32,
3469−3480. (b) McCall, A. S.; Kraft, S. Organometallics 2012, 31,
3527−3538. (c) McCall, A. S.; Wang, H.; Desper, J. M.; Kraft, S. J. Am.
Chem. Soc. 2011, 133, 1832−1848. (d) Arnold, P. L.; Sanford, M. S.;
Pearson, S. M. J. Am. Chem. Soc. 2009, 131, 13912−13913. (e) Hou,
Notes
The authors declare no competing financial interest.
X.-F.; Wang, Y.-N.; Gottker-Schnetmann, I. Organometallics 2011, 30,
̈
ACKNOWLEDGMENTS
This work was supported by the Ministerio de Ciencia e
6053−6056. (f) Meyer, D.; Taige, M. A.; Zeller, A.; Hohlfeld, K.;
Ahrens, S.; Strassner, T. Organometallics 2009, 28, 2142−2149.
(g) Strassner, T.; Muehlhofer, M.; Zeller, A.; Herdtweck, E.;
Herrmann, W. A. J. Organomet. Chem. 2004, 689, 1418−1424.
(h) Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem., Int.
Ed. 2002, 41, 1745−1747. (i) Deprez, N. R.; Kalyani, D.; Krause, A.;
Sanford, M. S. J. Am. Chem. Soc. 2006, 128, 4972−4973.
■
Innovacio
́
n (project CTQ2010-15927) and Comunidad
Auton
́
oma de Madrid (project AVANCAT). We thank the
MICINN for a Juan de la Cierva contract to F.T. and the UAM
for a graduate fellowship to A.G.-D.
(7) Selected examples: (a) Diao, T.; White, P.; Guzei, I.; Stahl, S. S.
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dx.doi.org/10.1021/om400981x | Organometallics 2013, 32, 7487−7494