Template-directed synthesis of palladium(II) sulfonate-NHC complexes and catalytic studies in aqueous Mizoroki-Heck reactions

Article abstract of DOI:10.1021/om500140g

Oxidation of easily accessible thiolato-functionalized dinuclear Pd(II) NHC complexes 1-3 by Oxone gave rise to sulfonate-NHC complexes 4-6. This represents the first template-directed approach to NHC complexes bearing sulfonate functions, where the sulfu

Full text of DOI:10.1021/om500140g

Article
pubs.acs.org/Organometallics
Template-Directed Synthesis of Palladium(II) Sulfonate-NHC
Complexes and Catalytic Studies in Aqueous Mizoroki−Heck
Reactions
Dan Yuan, Qiaoqiao Teng, and Han Vinh Huynh*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
S
* Supporting Information
ABSTRACT: Oxidation of easily accessible thiolato-functionalized
dinuclear Pd(II) NHC complexes 1−3 by Oxone gave rise to
sulfonate-NHC complexes 4−6. This represents the first template-
directed approach to NHC complexes bearing sulfonate functions,
where the sulfur atoms undergo a six-electron oxidation, changing
their oxidation states from −II to +IV. The catalytic activities of
water-soluble 4−6 were also tested in aqueous Mizoroki−Heck
reactions.
Synthetic methodologies to introduce an alkyl sulfonate
group into carbene precursors usually involve (i) ring-opening
reaction of sultones with azoles,2 (ii) N-alkylation with
haloalkyl sulfonates,3 and (iii) reaction of N-haloalkylazolium
salts with Na2SO3 (Scheme 1).4 For method (i), only limited
sultones are commercially available, e.g., 1,3-propanesultone
and 1,4-butanesultone. As a result, azolium salts with certain
length of the sulfonate tether (e.g., n = 2) are not accessible
through this method. The choice of commercially available
haloalkyl sulfonates is similarly limited for method (ii). The
third method, on the other hand, suffers from low yields and
the difficulty to separate the product from starting materials.
Ag(I), Au(I), Rh(I), Ru(II), and Pd(II) NHC complexes
bearing a sulfonate function have been synthesized,2−5 and
some of them have been tested in aqueous Suzuki−
Miyaura2f,5a,b and Sonogashira5b reactions, showing good
activities.
INTRODUCTION
■
Sulfur-functionalized N-heterocyclic carbenes (NHCs) have
attracted increasing attention due to their unique properties
and versatile coordination chemistry, which has recently been
reviewed.1 Since the sulfur atom can easily adopt various stable
oxidation states in its compounds ranging from −2 to +6, many
different types of sulfur functions are available, which further
diversifies the already rich NHC chemistry. Furthermore,
complexes with sulfur-NHCs possess a versatile and diverse
coordination chemistry with different bonding modes of the
sulfur function, which is considered a useful prerequisite for a
wide range of applications in catalysis.1 Among various sulfur
functions, there has been growing interest in the monoanionic
alkyl or aryl sulfonate group,2−5 especially in the field of green
chemistry, mainly due to the fact that it can impart water
solubility to complexes, making them suitable precatalysts in
aqueous media.6
As our contribution to sulfur-NHC chemistry, we have
developed a convenient method to synthesize thiolato-bridged
dinuclear complexes of group 10 metals, which involves the in
situ generation of thiols via the hydrolysis of thioester groups, as
exemplified in Scheme 2 for complex 1.7 Reactivity studies of
such dinuclear complexes bearing [M2S2] (M = Ni, Pd, Pt)
Scheme 1. Common Synthetic Routes to Azolium Salts
Bearing Alkyl Sulfonate Groups
Scheme 2. Reported Synthesis of Thiolato-Bridged Pd(II)
Dinuclear Complex 17a
Received: February 7, 2014
Published: March 21, 2014
© 2014 American Chemical Society
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Scheme 3. Synthesis of Thiolato-Bridged Pd(II) Dinuclear Complexes 2 and 3
cores revealed interesting results. For example, tetranuclear
macrocyclic complexes formed after the halido ligands were
removed by either Me3OBF4 or NaSCH(CH3)2.7a,b On the
other hand, trinuclear complexes with capping sulfido or oxido
ligands were obtained in reactions with Na2S and NaOH,
respectively.7c In the aforementioned studies, the oxidation
states of the sulfur atoms remained unchanged. To further
explore the reactivities of these thiolato-functionalized com-
plexes, we investigated sulfur-centered oxidations by treatment
of thiolato-bridged [Pd2S2] dimers with oxidizing reagents with
the aim to yield NHC complexes with sulfur functions of higher
oxidation states. Herein, we report a novel template-directed
approach to sulfonate-functionalized NHC complexes by
oxidation of [Pd2S2] complexes with Oxone and a preliminary
Figure 1. Molecular structure of 3 showing 50% probability ellipsoids;
solvent molecules and hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and bond angles [deg]: Pd1−C1 2.002(8), Pd1−Br1
2.4641(12), Pd1−S1A 2.363(2), Pd1−S1 2.300(2); C1−Pd1−Br1
94.79(6), C1−Pd1−S1 92.2(2), Br1−Pd1−S1A 96.79(6), S1A−Pd1-
S1 76.47(9), C1−Pd1−S1A 168.3(2), Br1−Pd1−S1 172.41 (6).
catalytic study of the respective complexes in the aqueous
Mizoroki−Heck reaction.
RESULTS AND DISCUSSION
■
Synthesis of Thiolato-Bridged Pd(II) NHC Complexes.
Thiolato-functionalized dinuclear complexes 2 and 3 were
synthesized in analogy to 1 (Scheme 3). Ligand precursors D
and E were prepared from the reactions of KSAc with the
bromopropyl- or bromo-o-xylenyl-substituted benzimidazolium
salts B and C, which were in turn synthesized by reacting 1-
benzylbenzimidazole with 1,3-dibromopropane or o-xylene
dibromide. Subsequent reactions of D and E with Pd(OAc)2
gave rise to complexes 2 and 3, respectively, in yields of 60%
and 76%. As expected, the in situ hydrolysis of the thioester
group occurred in wet DMSO, and the resulting thiolato groups
coordinated to the Pd(II) centers. In comparison, the synthesis
of a thioester-functionalized complex could be achieved in an
earlier study only through a postmodification approach,8 in
which a preformed Pd(II)-NHC complex bearing a bromo-
propyl arm was subjected to an SN2 reaction with KSAc.
Similar to 1, dinuclear complexes 2 and 3 show poor
solubility in DMSO and precipitated from the reaction mixture.
They were isolated as yellow powders after simple filtration and
washing with H2O. The solids are slightly soluble in CH2Cl2,
CHCl3, and DMF. Their formation is corroborated by positive
ESI mass spectrometry, where predominant signals at m/z 856
and 979 are observed for their respective [M − Br]+ fragments.
The aliphatic protons become diastereotopic upon complex
Reactions with Oxidants. The oxidation of the thiolato-
bridges of 1 may lead to the formation of a dinuclear or
mononuclear complex bearing sulfur donors of higher oxidation
states, such as sulfinate, sulfenate, or sulfonate. Oxidation of the
thiolato group of a Ni(II) complex to the sulfinate ligand by
aerial oxygen or H2O2 has been reported.9 Our previous study
also showed that the thioether function of a dibenzimidazolium
salt was easily oxidized to a sulfoxide group by 3 equiv of H2O2
in acetic acid at ambient temperature.10 Complex 1 was
therefore treated with the same reagents, and DMF was added
to ensure a homogeneous reaction. However, the compound
remained intact below 70 °C and started to decompose to
palladium black at higher temperatures. The fact that 1 is
resistant to oxidation by H2O2 may be ascribed to the lower
electron density at the sulfur atoms in the bridging mode,
which calls for a stronger oxidant.7a
Indeed, thiolato to sulfonate oxidation occurs with complex 1
when Oxone (KHSO5) is used, which is a cheap and strong
oxidant. The reaction proceeds smoothly at ambient temper-
ature with 3 equiv of Oxone in a DMF/H2O mixture with
excess of NMe4Br, which provides additional bromido ligands
as well as a suitable countercation (Scheme 4). Other alkali or
ammonium salts proved less suitable for the isolation and
crystallization of the product. In comparison, the reaction did
not even occur in boiling CH2Cl2 due to the insolubility of the
oxidant. With 2 equiv of Oxone a lower yield of the sulfonate
species was obtained, and no sulfinate or sulfenate
intermediates could be detected. In contrast, Oxone was
reported to be a chemoselective reagent for the oxidation of
organic sulfides to sulfoxides and further to sulfones.11
1
formation, which complicates their H NMR spectra. Due to
insufficient solubility, 13C NMR spectra could not be obtained.
The identity of complex 3 was further confirmed by X-ray
diffraction of a single crystal obtained via slow evaporation of a
saturated CHCl3/toluene solution. Both palladium centers are
in a square planar environment coordinated by one terminal
bromido ligand and one o-xylenyl-linked thiolato-NHC in a
κ2C,μ-S chelating and bridging mode (Figure 1). The carbene
planes are twisted by ∼60° out of the [PdCS2Br] coordination
plane. The [Pd2S2] core is significantly bent with a hinge angle
of ca. 109°, which is smaller than that of complex 1 (119°).
The negative ion ESI mass spectrum of the oxidation product
of 1 shows a predominant signal at m/z 581 corresponding to a
monoanionic Pd(II) complex fragment bearing a sulfonate-
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Scheme 4. Synthesis of Sulfonate Complexes 4−6
Figure 2. Isotopic pattern of the fragment from X1/4 in the negative ESI mass spectrum (left) and a simulated pattern (right).
NHC ligand and two bromido ligands (Figure 2). In solution,
formation of solvates or dimerization to X1 is proposed.
Indeed, the solvate 4 can be isolated in a good yield of 86%
when the crude product is treated with CH3CN. Complex 4 is a
yellow powder, which is soluble in DMF, CH3CN, and DMSO,
slightly soluble in H2O, but insoluble in diethyl ether and
hexane. The negative ion ESI mass spectrum of 4 is identical
with the one discussed earlier, i.e., X1. The 1H NMR spectrum
recorded in CD3CN shows a singlet at 6.03 ppm for the
benzylic protons. Two multiplets centered at 5.10 and 3.32
ppm are assigned to the two methylene groups of the tether. All
three signals do not show the diastereotopic patterns typical of
1, implying a pendant sulfonate moiety. A singlet at 3.10 ppm
+
attributed to NMe4 supports the successful incorporation of
the ammonium cation. Moreover, a triplet was observed for the
cation at 56.0 ppm in the 13C NMR spectrum due to the
coupling with the NMR-active 14N nucleus. The carbene
carbon atom resonates at 161.4 ppm, which is shifted upfield in
comparison to that of 1.
Figure 3. Molecular structure of 4 showing 50% probability ellipsoids;
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and
bond angles [deg]: Pd1−C1 1.959(4), Pd1−Br1 2.4295(11), Pd1−N3
2.099(3), Pd1−Br2 2.4210(10); C1−Pd1−Br1 89.18(11), C1−Pd1−
Br2 87.74(11), Br1−Pd1−N3 92.35(10), Br2−Pd1−N3 90.70(10),
C1−Pd1−N3 178.33(15), Br1−Pd1−Br2 175.95(2). PdCNBr2/NHC
dihedral angle 89.1°.
The identity of 4 was also confirmed by X-ray analysis on
single crystals obtained from diffusion of diethyl ether into a
concentrated CH3CN solution. The molecular structure
depicted in Figure 3 shows that the Pd(II) center is
coordinated by one carbene carrying a pendant sulfonate
function and two bromido ligands trans to each other. The
fourth coordination site is completed by a CH3CN molecule
from the solvent used for crystallization. The negative charge of
solubility to that of 4. In their negative ESI mass spectra,
isotopic patterns centered at m/z 595 or 659 for their [M −
NMe4 − CH3CN]− complex fragments support the formation
of the desired sulfonate complexes. A singlet resonating at 6.05
assignable to the benzylic protons and three multiplets located
in the range 4.99−2.47 ppm corresponding to the methylene
+
the complex is balanced by an NMe4 countercation. The
1
Pd(II) center adopts a slightly distorted square planar
geometry, and the dihedral angle between the [PdCNBr2]
coordination plane and the carbene plane measures 89°.
The generality and versatility of this new template-assisted
approach was tested in the oxidations of 2 and 3, which
afforded complexes 5 and 6 as yellow solids (Scheme 2) in
yields of 75% and 74%, respectively. They show similar
groups of the arm were observed in the H NMR spectrum of
5. For complex 6, three singlets were recorded at 6.41, 6.13, and
4.17 ppm due to the SCH2 protons and the two sets of NCH2
protons. The singlets for the NMe4 cations appear at ∼3.1 ppm
of both complexes. In their 13C NMR spectrum, the chemical
shifts of the carbene carbon atoms are found at 160.8 and 162.6
ppm, respectively.
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Figure 4. Molecular structure of 6 (different views) showing 50% probability ellipsoids; hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and bond angles [deg]: Pd1−C1 1.949(5), Pd1−Br1 2.4280(10), Pd1−N3 2.077(5), Pd1−Br2 2.4409(10); C1−Pd1−Br1 88.23(15),
C1−Pd1−Br2 88.03(15), Br1−Pd1−N3 93.45(15), Br2−Pd1−N3 90.44(15), C1−Pd1−N3 177.28(2), Br1−Pd1−Br2 174.52(2). PdCNBr2/NHC
dihedral angle 85.7(8)°.
a
In analogy to 4, single crystals of complex 6 were obtained by
diffusion of diethyl ether to a CH3CN solution. Figure 4 shows
the expected molecular structure of complex 6. The dihedral
angle between the [PdCNBr2] coordination plane and the
carbene plane measures ∼86°, which is smaller than that of
complex 4. This may be due to the less flexible o-xylenyl linker
in complex 6, which prevents the carbene ring from rotating to
the ideal perpendicular position. The phenyl ring of the linker
was almost perpendicular to the carbene ring with a dihedral
angle of 89°. Notably, complex 6 is not accessible via any of the
general routes (i)−(iii) mentioned above.
Catalytic Studies. Since sulfonate complexes 4−6 are
soluble in H2O, their catalytic activities were tested in aqueous
reactions. Although Mizoroki−Heck reactions catalyzed by
Pd(II) complexes have been extensively investigated,12 not
many studies focused on reactions in aqueous media, which still
remains a challenge.13 Reported studies also suffer from some
limitations. For example, good yields were obtained only with
aryl iodides,13c,f and organic solvents such as DMF had to be
added to water.13b,i,m
Table 1. Mizoroki−Heck Coupling Reactions Catalyzed by
Complexes 4−6
temp
[°C]
yield
b
entry precatalyst
base
solvent
additive [%]
1
2
3
4
5
6
7
4
4
4
4
4
4
4
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NEt3
110
110
110
110
100
110
110
H2O (1 mL)
H2O (3 mL)
H2O (1 mL)
H2O (3 mL)
iPrOH (1 mL)
H2O (1 mL)
H2O (1 mL)
13
0
TBAB
TBAB
9
11
0
25
25
NEt3 (2.5
equiv)
8
4
4
5
6
6
4
5
6
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
110
110
110
110
110
110
110
110
H2O (1 mL)
H2O (3 mL)
H2O (1 mL)
H2O (1 mL)
H2O (1 mL)
TBAB
TBAB
TBAB
TBAB
TBAB
86
26
88
9
10
11
12
13
14
15
97
The catalytic activity of precatalyst 4 was first tested in the
reaction of 4-bromoacetophone and tert-butyl acrylate in 1 mL
of H2O with 1.5 equiv of NaOAc and 1 mol % of 4 at 110 °C
for 24 h. A low yield of 13% was obtained (Table 1, entry 1). In
comparison, a more diluted reaction with 3 mL of H2O gave no
conversion (entry 2). The addition of 1.5 equiv of TBAB did
not lead to higher yields (entries 3 and 4).14 Moreover, no
c
58
H2O (0.5 mL) TBAB
H2O (0.5 mL) TBAB
H2O (0.5 mL) TBAB
86
86
96
a
Reaction conditions: 0.3 mmol of 4-bromoacetophenone; 0.42 mmol
of tert-butyl acrylate; 0.45 mmol of base; TBAB (0.45 mmol when
necessary); 1 mol % of precatalyst. Yields were determined by H
b
1
i
conversion was detected from the reaction in boiling PrOH
c
NMR spectroscopy for an average of two runs. Reaction was heated
(entry 5). To modify the reaction conditions, the base was
changed to 1.5 equiv of NEt3, and the reaction in H2O (1 mL)
gave an improved yield of 25% (entry 6). In comparison, an
increased loading of NEt3 (2.5 equiv) did not lead to an
improvement (entry 7). However, in the presence of 1.5 equiv
of TBAB, a good yield of 86% was obtained in 1 mL of H2O
(entry 8). The reaction in a diluted mixture dropped to 26%
(entry 9). When the reaction was carried out in 1 mL of H2O in
the presence of 1.5 equiv of NEt3, 1.5 equiv of TBAB, and 1
mol % of 5, a slightly better yield of 88% was obtained (entry
10). Complex 6, with the more bulky o-xylene linker, showed
the best activity, affording an excellent yield of 97% (entry 11)
under the same conditions. Shortening of the reaction time to 2
h led to a drop of yield (58%, entry 12). When the solvent
volume was decreased to 0.5 mL, the yield remained essentially
for 2 h instead.
unchanged when catalyzed by 5 (86%, entry 13), while it
slightly dropped when catalyzed by 4 and 6 (entries 14 and 15).
Notably, no sign of decomposition was observed in all the runs.
To evaluate the thermal stability under the optimized
conditions further, complex 6 was heated without substrates
ceteris paribus for a prolonged time (24 h), and again no
decomposition was observed. In comparison, 1 mol %
Pd(OAc)2 also gave a good yield of ∼80% under the same
conditions. However, almost immediate formation of palladium
black was observed in this case.
Having identified the suitable reaction condition for the
aqueous Heck reaction, the scope of the reaction was then
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explored with complexes 4 and 6. In the coupling of the
activated substrate of 4-bromobenzaldehyde, both catalysts give
excellent yields. Again, complex 6 outperforms 4 (>99% vs
92%, Table 2, entries 1 and 2) and even gives quantitative yield
a
Table 2. Mizoroki−Heck Coupling Reactions Catalyzed by
Complexes 4 and 6
Figure 5. Concentration/time profile for the coupling reaction of 4-
bromobenzaldehyde with tert-butyl acrylate catalyzed by complex 6.
b
entry cat.
cat. loading
aryl halide
time yield [%]
are described. In comparison to other known catalytic Pd
systems, complexes 4−6 are well active in aqueous Mizoroki−
Heck reactions of aryl bromides, with complex 6 being the best
performer. However, the more difficult coupling of aryl
chlorides in aqueous media remains a challenge to be tackled
in the future. Research in our lab is currently on going to
extend this new synthetic strategy to other transition metals
1
4
1 mol %
1 mol %
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromotoluene
24 h
24 h
2 h
92
>99
>99
99
70
>99
48
>99
17
56
6
2
6
3
6
1 mol %
4
6
0.5 mol %
0.1 mol %
0.1 mol %
0.05 mol %
0.05 mol %
0.01 mol %
0.01 mol %
1 mol %
2 h
5
6
2 h
6
6
24 h
2 h
−
and to further investigate the catalytic activities of NHC−SO3
complexes in aqueous reactions.
7
6
8
6
24 h
2 h
9
6
EXPERIMENTAL SECTION
10
11
12
13
14
15
16
17
6
24 h
24 h
24 h
24 h
24 h
24 h
24 h
4 h
■
4
General Considerations. Unless otherwise noted, all operations
were performed without taking precautions to exclude air and
moisture, and all solvents and chemicals were used as received. Salt
B has been synthesized according to a reported procedure.15 The
6
1 mol %
4-bromotoluene
25
5
4
1 mol %
4-bromoanisole
6
1 mol %
4-bromoanisole
6
1
chemical shifts (δ) for the H and 13C NMR spectra were internally
4/6
4/6
6
1 mol %
4-chloroacetophone
4-chlorobenzaldehyde
4-chlorobenzaldehyde
0
referenced to the residual solvent signals relative to tetramethylsilane.
Salt C. α,α′-Dibromo-o-xylene (5.3 g, 20 mmol) and benzylbenzi-
midazole (208 mg, 1 mmol) were dissolved in toluene (50 mL) and
stirred at 50 °C overnight, resulting in a suspension. The white solid
was collected by centrifuging and washed with toluene (3 × 10 mL)
and diethyl ether (3 × 10 mL). The solid was redissolved in CH2Cl2
and filtered. Slow evaporation of the filtrate gave the product as a
1 mol %
0
c
1 mol %
0
a
Reaction conditions: 0.3 mmol of 4-bromoacetophenone; 0.42 mmol
of tert-butyl acrylate; 0.45 mmol of base; H2O (1 mL); TBAB (0.45
mmol). Yields were determined by H NMR spectroscopy for an
b
1
c
average of two runs. Microwave reaction at 160 °C.
1
white crystalline solid (179 mg, 0.38 mmol, 38%). H NMR (500
MHz, CDCl3): δ 11.44 (s, 1 H, NCHN), 7.58 (d, 1 H, Ar−H), 7.54−
7.42 (m, 5 H, Ar−H), 7.35−7.28 (m, 6 H, Ar−H), 7.18 (d, 1 H, Ar−
H), 6.11 (s, 2 H, NCH2Ph), 5.82 (s, 2 H, NCH2Ph), 4.70 (s, 2 H,
CH2Br). 13C{1H} NMR (125.77 MHz, CDCl3): δ 144.0 (NCHN),
136.9, 133.1, 132.33, 132.25, 131.9, 131.7, 130.4, 129.9, 129.83,
129.76, 129.0, 128.9, 127.83, 127.82, 114.6, 114.5 (Ar−C), 52.4, 49.6
(NCH2Ph), 32.0 (CH2Br). MS (ESI): m/z 391 [M − Br]+.
in 2 h (entry 3). Quantitative yield after 2 h was also obtained
when the loading of catalyst 6 was lowered to 0.5 mol % (entry
4). Further reduction of the catalyst loading to 0.1, 0.05, and
0.01 mol % resulted in lower yields ranging from 70% to 17%
(entries 5, 7, and 9). These reactions required longer reaction
time to give high yields (entries 6, 8, and 10). Both catalysts
gave low conversions in the reactions with deactivated
substrates such as 4-bromotoluene and 4-bromoanisole (entries
11−14). No reaction occurred with the more challenging aryl
chlorides (entries 15 and 16) even under microwave irradiation
(entry 17), which shows the limitations of this catalyst system
in aqueous catalysis.
A concentration/time diagram was recorded on the coupling
of the most active substrate, 4-bromobenzaldehyde, employing
1 mol % catalyst 6 (Figure 5). The diagram indicates that the
coupling occurred almost immediately after the catalyst was
added, and no induction period was observed in such time
intervals. The reaction was fastest in the first 20 min, reaching a
yield of 62% and finally completing at 120 min.
Salt D. A mixture of salt B (892 mg, 2.17 mmol) and KSCOCH3
(371 mg, 3.25 mmol) in CH3CN (20 mL) was stirred at 70 °C
overnight. The resulting suspension was filtered, and the solvent of the
filtrate was removed in vacuo. The resulting solid was purified by
column chromatography (SiO2, CH2Cl2/MeOH, 5:1) to give the
1
product as an off-white solid (616 mg, 1.52 mmol, 70%). H NMR
(500 MHz, CDCl3): δ 11.55 (s, 1 H, NCHN), 7.70 (d, 1 H, Ar−H),
7.56−7.48 (m, 5 H, Ar−H), 7.35−7.31 (m, 3 H, Ar−H), 5.85 (s, 2 H,
NCH2Ph), 4.71 (t, 3J(H,H) = 7.0 Hz, 2 H, NCH2), 3.00 (t, 3J(H,H) =
7.0 Hz, 2 H, CH2S), 2.40 (m, 2 H, CH2), 2.27 (s, 3 H, CH3). 13C{1H}
NMR (125.77 MHz, CDCl3): δ 195.9 (CO), 144.3 (NCHN), 133.3,
132.2, 131.8, 130.0, 129.9, 129.0, 127.8, 114.5, 113.7 (Ar−C), 52.2
(NCH2Ph), 46.9 (NCH2), 31.3 (CH2S), 30.0 (CH2), 26.4 (CH3).
One signal for an aromatic carbon atom was not observed due to
accidental overlap. MS (ESI): m/z 325 [M − Br]+.
Salt E. A mixture of salt C (63 mg, 0.13 mmol) and KSCOCH3 (18
mg, 0.16 mmol) in CH3CN (20 mL) was stirred overnight. The
solvent of the mixture was removed in vacuo before CH2Cl2 (30 mL)
was added. The resulting suspension was filtered and dried under
reduced pressure to afford the product as an off-white oil (50 mg, 0.11
mmol, 83%). 1H NMR (300 MHz, CDCl3): δ 11.15 (s, 1 H, NCHN),
7.64−7.61 (m, 1 H, Ar−H), 7.50−7.39 (m, 5 H, Ar−H), 7.32−7.11
CONCLUSION
■
In conclusion, we reported the first versatile template-directed
approach to water-soluble NHC−SO3− complexes 4−6 by easy
oxidation of dinuclear thiolato-NHC Pd(II) complexes 1−3
with Oxone. The solid-state molecular structures of 3, 4, and 6
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(m, 7 H, Ar−H), 5.92 (s, 2 H, NCH2Ph), 5.85 (s, 2 H, NCH2Ph), 4.18
(s, 2 H, CH2S), 2.13 (s, 3 H, CH3). 13C{1H} NMR (75.47 MHz,
CDCl3): δ 195.1 (CO), 143.6 (NCHN), 136.3, 133.2, 132.0, 131.81,
131.79, 131.0, 130.1, 129.7, 129.6, 129.4, 129.1, 129.0, 127.8, 127.7,
114.5, 114.3 (Ar−C), 52.1, 49.5 (NCH2Ph), 31.2 (CH2S), 30.8
(CH3). One signal for an aromatic carbon atom is not observed due to
accidental overlap. MS (ESI): m/z 387 [M − Br]+.
Ar−H), 6.86 (d, 1 H, Ar−H), 6.41 (s, 2 H, NCH2Ph), 6.13 (s, 2 H,
NCH2Ph), 4.17 (s, 2 H,CH2SO3), 3.06 (s, 12 H, NMe4), 1.96 (s,
CH3CN, correct integration is not possible due to ligand exchange
with the solvent). 13C{1H} NMR (125.77 MHz, CD3CN): δ 162.6
(NCN), 136.1, 135.4, 135.2, 134.9, 133.5, 133.2, 129.6, 129.03, 128.96,
128.2, 128.0, 127.8, 124.5, 124.4 (Ar−C), 118.1 (CN), 113.1, 112.4
1
(Ar−C), 56.1 (t, J(C,N) = 3.7 Hz, NMe4), 55.5, 53.8 (NCH2Ph),
51.0 (CH2SO3), 1.32 (m, CH3CN, assignment is tentative due to
overlap with solvent signals). Two signals accidently overlap at 128.8
ppm. Anal. Calcd for C28H34Br2N4O3PdS: C, 43.51; H, 4.43; N, 7.25.
Found: C, 43.23; H, 4.49; N, 7.24. MS (ESI): m/z 74 [NMe4]+, −659
[M − NMe4 − CH3CN]−.
Aqueous Mizoroki−Heck Catalysis. In a typical run, a reaction
tube was charged with a mixture of aryl halide (0.3 mmol), base (0.45
mmol), tert-butyl acrylate (0.42 mmol), catalyst (0.003 mmol), TBAB
(0.45 mmol when necessary), and solvent. The reaction was stirred at
110 °C for the desired time. After the mixture was cooled to ambient
temperature, dichloromethane (2 mL) was added. The organic layer
was then washed with water (6 × 8 mL) and dried over Na2SO4. The
solvent was allowed to evaporate, and the residue was analyzed by 1H
NMR spectroscopy.
X-ray Diffraction Studies. X-ray data for 3, 4, and 6 were
collected with a Bruker AXS SMART APEX diffractometer, using Mo
Kα radiation with the SMART suite of programs.16 Data were
processed and corrected for Lorentz and polarization effects with
SAINT17 and for absorption effect with SADABS.18 Structural solution
and refinement were carried out with the SHELXTL suite of
programs.19 The structure was solved by direct methods to locate
the heavy atoms, followed by difference maps for the light, non-
hydrogen atoms. All non-hydrogen atoms were generally given
anisotropic displacement parameters in the final model. All H atoms
were put at calculated positions. A summary of the most important
crystallographic data is given in Table S1 in the Supporting
Information.
Dinuclear Complexes 2 and 3. A mixture of salt D/E (405/467
mg, 1.0 mmol) and Pd(OAc)2 (224 mg, 1.0 mmol) in DMSO (5 mL)
was stirred at 80 °C overnight. The resulting suspension was filtered,
and the yellow solid was washed with H2O (3 × 20 mL) and dried to
give the products. Complex 2: 277 mg, 0.3 mmol, 60%. 1H NMR (300
MHz, CDCl3): δ 7.50−7.19 (m, 18 H, Ar−H), 6.44 (d, 2J(H,H) = 15.6
2
Hz, 2 H, NCHHPh), 5.37 (d, J(H,H) = 15.6 Hz, 2 H, NCHHPh),
4.83−4.77 (m, 2 H, NCHH), 3.10−2.91 (m, 4 H, CH2), 2.26 (br s, 2
H, CH2). Signals for another two CH2 groups were not resolved. Anal.
Calcd for C34H34Br2N4Pd2S2: C, 43.66; H, 3.66; N, 5.99. Found: C,
43.61; H, 4.07; N, 5.52. MS (ESI): m/z 856 [M − Br]+. Complex 3:
1
403 mg, 0.38 mmol, 76%. H NMR (500 MHz, CDCl3): δ 7.59 (d, 2
H, Ar−H), 7.43−7.39 (m, 4 H, Ar−H), 7.35−7.32 (m, 8 H, Ar−H),
7.22−7.09 (m, 12 H, Ar−H and NCHHPh), 7.07−7.06 (m, 2 H, Ar−
H), 6.96−6.88 (m, 2 H, NCHH), 5.32 (d, 2 H, NCHHPh), 4.91−4.81
2
(m, 2 H, NCHH), 4.39 (d, J(H,H) = 15.1 Hz, 1 H, NCHHS), 4.22
(d, 2J(H,H) = 15.1 Hz, 1 H, NCHHS), 2.63 (d, 2J(H,H) = 15.1 Hz, 2
H, NCHHS). Anal. Calcd for C44H38Br2N4Pd2S2·H2O: C, 49.04; H,
3.74; N, 5.20. Found: C, 48.72; H, 3.48; N, 5.05%. MS (ESI): m/z 979
[M − Br]+. The 13C NMR spectra of complexes 2 and 3 could not be
obtained due to poor solubility.
Sulfonate Complex 4. Complex 1 (45 mg, 0.05 mmol), Oxone
(92 mg, 0.15 mmol), and NMe4Br (39 mg, 0.25 mmol) in DMF (5
mL) and H2O (2 mL) were stirred at ambient temperature overnight.
After the solvent was removed, the resulting solid was triturated with
CH3CN (3 × 5 mL) and the suspension filtered. The solvent of the
filtrate was removed, and the residue was washed with H2O (3 × 3
mL) and redissolved in CH3CN (5 mL). Removal of the solvent gave
the product as a yellow powder. A second batch was obtained by
removing the solvent of the aqueous solution, washing the residue with
a little H2O (3 × 1 mL), and recrystallization in CH3CN (5 mL) (60
mg, 0.086 mmol, 86%). 1H NMR (500 MHz, CD3CN): δ 7.62 (d, 1 H,
Ar−H), 7.54 (d, 2 H, Ar−H), 7.53−7.30 (m, 4 H, Ar−H), 7.17−7.14
(m, 2 H, Ar−H), 6.04 (s, 2 H, NCH2Ph), 5.12−5.09 (m, 2 H, NCH2),
3.34−3.31 (m, 2 H, CH2SO3), 3.10 (s, 12 H, NMe4), 1.96 (s, CH3CN,
correct integration is not possible due to ligand exchange with the
solvent). 13C{1H} NMR (125.77 MHz, CD3CN): δ 161.4 (NCN),
135.9, 135.1, 134.6, 129.4, 128.89, 128.87, 124.5, 124.2 (Ar−C), 118.3
ASSOCIATED CONTENT
* Supporting Information
Crystallographic data for 3, 4, and 6 (CCDC 977882−977884)
as CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: chmhhv@nus.edu.sg.
■
1
(CN), 112.2, 111.9 (Ar−C), 56.0 (t, J(C,N) = 4.1 Hz, NMe4), 53.6
Notes
(NCH2Ph), 50.8 (NCH2), 46.4 (CH2SO3), 1.32 (m, CH3CN,
assignment is tentative due to overlap with solvent signals). Anal.
Calcd for C22H30Br2N4O3PdS: C, 37.92; H, 4.34; N, 8.04. Found: C,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
+
37.52; H, 4.38; N, 8.12. MS (ESI): m/z 74 NMe4 , −581 [M − NMe4
■
− CH3CN]−.
The authors thank the National University of Singapore and
the Singapore Ministry of Education (MOE) for financial
support (WBS R-143-000-483-112). Technical support from
staff at the CMMAC of our department is appreciated. In
particular, we thank Ms. Geok Kheng Tan and Prof. Lip Lin
Koh for determining the X-ray molecular structures.
Sulfonate Complexes 5 and 6. Complexes 5 and 6 were
synthesized in analogy to 4 from 2/3 (187/212 mg, 0.2 mmol), Oxone
(369 mg, 0.6 mmol), and NMe4Br (154 mg, 1.0 mmol). Complex 5:
1
214 mg, 0.3 mmol, 75%. H NMR (300 MHz, CD3CN): δ 7.80 (d, 1
H, Ar−H), 7.53−7.52 (m, 2 H, Ar−H), 7.33−7.29 (m, 4 H, Ar−H),
7.15−7.13 (m, 2 H, Ar−H), 6.05 (s, 2 H, CH2Ph), 4.99 (ps t, 2 H,
NCH2), 3.10 (s, 12 H, NMe4), 2.78 (ps t, 2 H,CH2SO3), 2.54−2.47
(m, 2 H, CH2), 1.96 (s, CH3CN, correct integration is not possible
due to ligand exchange with the solvent). 13C{1H} NMR (75.47 MHz,
CD3CN): δ 160.8 (NCN), 136.0, 135.4, 134.5, 129.3, 128.8, 124.4,
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