Synthesis of Tetranuclear Palladium(II) Complexes and Their Catalytic Activity for Cross-Coupling Reactions

Article abstract of DOI:10.1002/chem.201703926

We have developed a short and simple synthesis of tetranuclear palladium(II) complexes that have been structurally confirmed by X-ray analysis. These complexes were formed in about 30 % overall yield by spontaneous metalation of dimethylaminoarene derivatives and exhibit a high stability. We have studied the utility of the tetranuclear palladium(II) complexes as precatalysts for Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions. Our novel complexes show excellent catalytic activities with high turnover numbers (TON) and high turnover frequencies (TOF) (e.g., for the Suzuki–Miyaura reaction: TON up to 538000 and TOF up to 23400 h^?1 at room temperature).

Full text of DOI:10.1002/chem.201703926

A Journal of
Accepted Article
Title: Synthesis of Tetranuclear Palladium(II) Complexes and their
Catalytic Activity for Cross Coupling Reactions
Authors: Florian Puls, Nils Richter, Olga Kataeva, and Hans-Joachim
Knölker
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Synthesis of Tetranuclear Palladium(II) Complexes and their
Catalytic Activity for Cross Coupling Reactions**
Florian Puls, Nils Richter, Olga Kataeva, and Hans-Joachim Knölker*
Abstract: We have developed a short and simple synthesis of
tetranuclear palladium(II) complexes which have been structurally
confirmed by X-ray analysis. These complexes were formed in about
30% overall yield by spontaneous metalation of dimethylaminoarene
derivatives and exhibit a high stability. We have studied the utility of
the tetranuclear palladium(II) complexes as precatalysts for
Mizoroki–Heck and Suzuki–Miyaura cross coupling reactions. Our
novel complexes show excellent catalytic activities with high turnover
numbers and high turnover frequencies (e.g. for the Suzuki–Miyaura
reaction: TON up to 538000 and TOF up to 23400 h–1 at room
temperature).
to get more structural insights into the 4-diarylpalladium(II)
complexes, we prepared the corresponding
with a
dimethylaminomethylene-chelated complexes
coordinating nitrogen atom.
3
Introduction
In recent years, organopalladium chemistry has experienced a
tremendous development with a variety of applications in
organic synthesis.[1] Palladacycles, especially multinuclear
palladium complexes, have proven to be simple and efficient
catalyst precursors which slowly release the catalytically active
palladium species for cross coupling reactions.[2] Besides the
Hermann–Beller catalyst,[3]
a
large variety of dinuclear
palladacycles has been synthesized.[4] While dinuclear
palladacycles were shown to be useful precatalysts in C–C bond
forming cross coupling reactions (e.g. Mizoroki–Heck, Suzuki–
Miyaura, Stille and Sonogashira–Hagihara),[5] multinuclear
palladacycles containing four palladium centers have been
rarely applied to catalysis.[6] However, multinuclear complexes
offer the advantage of providing several catalytically active metal
centers from a comparatively small organic ligand. Herein, we
describe a simple synthetic route to tetranuclear palladium(II)
complexes and their suitability to serve as precatalysts for
palladium-catalyzed cross coupling reactions with excellent
turnover numbers and turnover frequencies.
The palladium(II)-catalyzed oxidative cyclization is a key
reaction in our project directed towards the total synthesis of
biologically active carbazole alkaloids.[7] Recently, we have
isolated the 4-diarylpalladium(II) complexes 1 and 2 as key
intermediates of the oxidative cyclization and studied their
subsequent reductive elimination process (Figure 1).[8] In order
Figure 1. Structures of the diarylpalladium(II) complexes 1–3 and of the
tetranuclear palladium(II) complexes 4.
Results and Discussion
For the synthesis of the new ligands, we first prepared the
ethylene acetals 7 and 8 by protection of the corresponding
aldehydes 5 and 6 (Scheme 1). Ullmann coupling of 7 and 8
followed by cleavage of the acetal afforded the diaryl ether 9a.[9]
Twofold
hydrochloride and the bromoarene 8 led to the diarylamine
9b.[10] Reductive amination of
using dimethylamine
Buchwald–Hartwig
coupling
of
methylamine
9
hydrochloride and sodium triacetoxyborohydride provided the
diaryl ligands 10a and 10b.
Heating of the diaryl ether 10a in the presence of
stoichiometric amounts of palladium(II) acetate in an inert
solvent provided the tetracoordinated complex 3a (Table 1).
Increasing the reaction temperature led to higher yields of
complex 3a. The best result (33% yield of 3a) was achieved by
reaction in o-xylene at 144 °C for 1 h. Reaction of the
diarylamine 10b with palladium(II) acetate led to complex 3b
surprisingly in only 4% yield using toluene at 110 °C. The
structures of the complexes 3a and 3b have been confirmed by
their spectroscopic data (1H and 13C NMR), the ESI-MS (Figures
S1 and S2) and by comparison with the data of the complexes 1
and 2 reported previously.[8]
[*]
F. Puls, Dr. N. Richter, Prof. Dr. H.-J. Knölker
Department Chemie, Technische Universität Dresden
Bergstraße 66, 01069 Dresden (Germany)
https://tu-dresden.de/mn/chemie/oc/oc2
E-mail: hans-joachim.knoelker@tu-dresden.de
Dr. O. Kataeva
A. M. Butlerov Chemistry Institute, Kazan Federal University
Kremlevskaya Str. 18, Kazan 420008 (Russia)
Part 135 of “Transition Metals in Organic Synthesis”; for part 134,
see: ref. [8c].
[**]
Supporting information for this article can be found under:
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dimethylaminomethylene group and by the aryl carbon atom at
the position para to the oxygen atom. A structural arrangement
in which two pairs of palladium atoms are each bridged by two
acetate ligands leads to a dimerization of the diaryl ether unit in
a tetranuclear complex. The distance between the two palladium
atoms of 2.93 and 2.94 Å indicates a Pd–Pd interaction as the
van-der-Waals radius of palladium is 1.6–1.7 Å.[11]
Table 2. Synthesis of the tetranuclear palladium(II) complexes 4.
Scheme 1. Synthesis of compounds 10. Reagents and conditions: a) 1. p-
TsCl (1.1 equiv), NEt3 (1.1 equiv), CH2Cl2, 2 °C to RT, 2.5 h; 2. ethylene glycol
(3.9 equiv), p-TsOH · H2O (3 mol%), toluene, reflux, 18 h; 3. NaOH (4.0 equiv),
H2O/EtOH (3:1), 90 °C, 3 h (71%, 3 steps); b) ethylene glycol (3.8 equiv), p-
TsOH · H2O (1 mol%), toluene, reflux, 18 h (88%); c) 1. 7 (1.2 equiv), CuCl
(1.0 equiv), Cs2CO3 (2.0 equiv), TMHD (0.25 equiv), NMP, 130 °C, 17 h; 2. 6 M
HCl/NMP (7:9), RT, 2 h (9a: 82%, 2 steps); d) 1. Pd(dba)2 (2 mol%), XPhos (6
mol%), NaOt-Bu (3.8 equiv), H2NMe · HCl (1.8 equiv), 1,4-dioxane, sealed
tube, 90 °C, 22.5 h; 2. 6 M HCl/NMP (6:5), RT, 2 h (9b: 61%, 2 steps); e)
HNMe2 · HCl (2.5 equiv), NaBH(OAc)3 (5.9–6.3 equiv), NEt3 (2.2–3.8 equiv),
4 Å MS, DCE, RT, 17–18 h (10a: 79%, 10b: 82%). NMP = N-methyl-2-
Reactant
10a
Reaction conditions
acetone, RT, 5 d
4, Yield [%]
4a, 37
pyrrolidinone, TMHD
dibenzylideneacetone,
triisopropylbiphenyl.
=
2,2,6,6-tetramethylheptane-5,7-dione, dba
=
XPhos 2-dicyclohexylphosphino-2′,4′,6′-
=
10a
ethyl acetate, RT, 7 d
ethyl acetate, RT, 3 – 6 d
acetone, RT, 4 d
4a, 52
Table 1. Synthesis of the 4-diarylpalladium(II) complexes 3.
10b
4b, 0
10b
4b, 65
Reactant
10a
Reaction conditions
acetone, RT, 3 h
3, Yield [%]
3a, traces
3a, 17
10a
acetone, 59 °C, 3 h
toluene, 110 °C, 3 h
o-xylene, 144 °C, 1 h
toluene, 110 °C, 3 h
10a
3a, 24
10a
3a, 33
10b
3b, 4
To increase the yields of the 4-diarylpalladium(II)
complexes 3, we applied an excess of palladium(II) acetate.
However, reaction of the diaryl ether 10a with 2.0 equivalents of
Pd(OAc)2 at room temperature afforded the tetranuclear
palladium(II) complex 4a which could be obtained in up to 52%
yield (Table 2). The ESI-MS of complex 4a shows the [M–OAc]+
peak resulting from loss of one of the bridging acetate ligands
and an isotopic pattern characteristic of four palladium atoms
(Figure 2). The molecular structure of complex 4a was
unequivocally confirmed by an X-ray diffraction analysis of single
crystals (Figure 3). Each of the four palladium atoms is
Figure 2. ESI-MS: [M–OAc]+ peaks of the tetranuclear palladium(II) complex
4a and calculated isotopic distribution.
Reaction of the diarylamine 10b with 2.0 equivalents of
palladium(II) acetate in acetone provided the tetranuclear
palladium(II) complex 4b in 65% yield (Table 2). Variation of the
concentration in different solvents, prolonging of the reaction
time and application of more equivalents of palladium(II) acetate
led to no further improvement of the yield for complex 4b. The
structure of complex 4b was confirmed by ESI-MS (Figure 4)
coordinated
by
the
nitrogen
atom
of
the
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and an X-ray single crystal structure determination (Figure 5).
The Pd–Pd distance of 2.92 Å indicates also for complex 4b a
significant interaction between the two palladium atoms of each
pair.
Starting from m-bromobenzaldehyde (6), the tetranuclear
palladium(II) complexes 4 are available in five steps and high
overall yields (4a: 30% and 4b: 29%). Owing to their structural
features with the presence of four labile μ-acetate ligands, we
hypothesized that the complexes 4 may represent useful pre-
catalysts and investigated their catalytic activity for cross
coupling reactions. The palladium-catalyzed C–C cross coupling
is one of the most powerful tools in synthetic organic chemistry
with numerous applications in academic laboratories and in
industry.[12] First, we have tested the catalytic activity of the
complexes 4a and 4b in the Mizoroki–Heck coupling[13] using p-
bromoacetophenone and styrene in the presence of potassium
carbonate as base in NMP at 100 °C as model system (Table 3).
Both complexes proved to be efficient catalysts for the Mizoroki–
Heck coupling. In the presence of small amounts of water,[13a]
the catalyst loading could be reduced to 0.05 mol% without any
loss of activity (entries 3 and 4). A beneficial feature of our
tetranuclear palladium(II) complexes is the high thermal stability.
Figure 3. Molecular structure of the tetranuclear palladium complex 4a in the
crystal. ORTEP plot showing thermal ellipsoids at the 50% probability level.
Methyl groups at nitrogen atoms and a solvent molecule (ethyl acetate) are
omitted for clarity. Selected bond lengths [Å]: Pd–Pd 2.9427(5) and 2.9291(4),
Pd–C 1.963(3), Pd–N 2.070(3), Pd–O 2.048(3) and 2.138(3).
Formation of palladium black is not observed up to
a
temperature of 100 °C. The thermal stability may be explained
by the cyclometalated structure.[2c,14]
Table 3. Mizoroki–Heck coupling reaction.[a]
Entry
Cat. (mol%)[b]
Yield [%][c]
TON[d]
TOF [h–1 [e]
]
1
2
4a (0.20)
4a (0.05)
4a (0.05)
4b (0.05)
88
32
83
87
451
113
167
407
427
668
3[f]
4[f]
1630
1706
Figure 4. ESI-MS: [M–OAc]+ peaks of the tetranuclear palladium(II) complex
4b and calculated isotopic distribution.
[a] Reaction conditions: p-bromoacetophenone (1.25 mmol), styrene (1.2
equiv), K2CO3 (1.2 equiv), NMP (5 mL), Ar, 4 h, 100 °C. [b] Amount of catalyst
in mol% based on bromoarene. [c] Isolated yield for the pure E-isomer; in
addition, traces of the Z-isomer were detected and the 1,1-regioisomer was
obtained in 1–3% yield. [d] Turnover number based on complex 4. [e] Turnover
frequency in h–1 at 100 °C. [f] Solvent: NMP/H2O (24:1, 5 mL).
Next, we tested our tetranuclear palladium(II) complexes
4a and 4b as catalysts for the Suzuki–Miyaura coupling[15] of
para-substituted aryl bromides with phenylboronic acid in NMP–
water as solvent system (Table 4).[16] All reactions were
performed at room temperature and afforded the corresponding
biaryl compounds in high yields and under optimized conditions
with excellent TON and TOF values. Using complex 4b as
precatalyst a TON of over half a million and a TOF of 23400 h–1
was observed. In conclusion, without any further additives the
complexes 4 show high catalytic activities for the Suzuki–
Miyaura coupling reaction.
Figure 5. Molecular structure of the tetranuclear palladium complex 4b in the
crystal. ORTEP plot showing thermal ellipsoids at the 30% probability level.
Methyl groups at nitrogen atoms and two solvent molecules (dichloromethane)
are omitted for clarity. Selected bond lengths [Å]: Pd–Pd 2.9218(12), Pd–C
1.961(15), Pd–N 2.075(9), Pd–O 2.042(8) and 2.159(10).
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Table 4. Suzuki–Miyaura coupling reactions.[a]
(0.035–0.070 mm). TLC was performed with TLC plates from Merck (60
F254) using UV light for visualization. Melting points were measured on a
Gallenkamp MPD 350 melting point apparatus. Ultraviolet spectra were
recorded on a PerkinElmer 25 UV/Vis spectrometer. Fluorescence
spectra were measured on a Varian Cary Eclipse spectrophotometer. IR
Cat. (mol%)[b]
t [h]
Yield [%][c]
TON[d]
TOF [h-1][e]
spectra were recorded on
a Thermo Nicolet Avatar 360 FT-IR
R
spectrometer using the ATR method (Attenuated Total Reflectance).
NMR spectra were recorded on Bruker DRX 500 and Avance III 600
spectrometers. Chemical shifts δ are reported in parts per million with the
solvent signal as internal standard. Standard abbreviations were used to
denote the multiplicities of the signals. Mass spectra were recorded on a
Finnigan MAT-95 spectrometer (electron impact, 70 eV) or by GC/MS-
coupling using an Agilent Technologies 6890 N GC System equipped
with a 5973 Mass Selective Detector (electron impact, 70 eV). ESI-MS
spectra were recorded on an Esquire LC with an ion trap detector from
Bruker. Positive and negative ions were detected. Elemental analyses
were measured on an EuroVector EuroEA3000 elemental analyzer. X-
ray crystal structure analyses were performed with a Bruker Kappa Apex
II CCD that was equipped with a 700 series Cryostream low temperature
device from Oxford Cryosystems. SHELXS,[18] SADABS version 2.10,[19]
SHELXL,[20] POV-Ray for Windows version 3.7.0.msvc10.win64, and
ORTEP-3 for Windows[21] were used as software.
OMe
4a (0.21)
4a (0.23)
4a (0.25)
4a (0.012)
4
93
93
97
86
89
52
97
455
114
Me
4
403
101
NO2
4
393
98
COMe
COMe
COMe
COMe
1
9100
9100
11400
3150
23400
4a (390·10–6
4a (97·10–6
4b (180·10–6
)
20
168
23
228·103
530·103
538·103
)
)
[a] Reaction conditions: aryl bromide (1.25 mmol), phenylboronic acid (1.1
equiv), K2CO3 (2.0 equiv), NMP/H2O (1:1, 4 mL), air, RT. [b] Amount of catalyst
in mol% based on bromoarene. [c] Isolated yield. [d] Turnover number based
on complex 4. [e] Turnover frequency in h–1 at RT.
3-(1,3-Dioxolan-2-yl)phenol (7):
1. Freshly distilled triethylamine (4.71 g, 6.49 mL, 46.6 mmol) was added
to a solution of 3-hydroxybenzaldehyde (5, 5.05 g, 41.3 mmol) in CH2Cl2
(25 mL). A suspension of p-toluenesulfonyl chloride (8.37 g, 43.9 mmol)
in CH2Cl2 (25 mL) was slowly added via a syringe at a temperature of
2 °C. After complete addition, the resulting reaction mixture was stirred at
room temperature for 2.5 h. Subsequently, the mixture was diluted with
For the palladium(0)-catalyzed cross coupling, we propose
that by nucleophilic attack of ligand molecules the tetranuclear
palladium(II) complexes 4 fall apart into two halves (Scheme 2).
The resulting dinuclear palladium(II) moieties are capable to
generate catalytically active palladium species at both termini.[17]
ethyl acetate (100 mL), washed using 2
M HCl (50 mL), saturated
aqueous Na2CO3 solution (50 mL), and brine (50 mL), dried over MgSO4,
and the solvent was evaporated under reduced pressure. The crude 3-
formylphenyl p-toluenesulfonate crystallized as an off-white solid and
was used for the second step.
2.
A solution of 3-formylphenyl p-toluenesulfonate, ethane-1,2-diol
(10.02 g, 161.4 mmol) and p-toluenesulfonic acid monohydrate (214 mg,
1.24 mmol) in toluene (50 mL) was heated at reflux for 18 h using a
Dean-Stark apparatus. After cooling to room temperature, the reaction
mixture was diluted with ethyl acetate (50 mL), washed with a saturated
aqueous solution of NaHCO3 (40 mL), water (40 mL), then with brine
(40 mL) and dried with MgSO4. Removal of the solvent under reduced
pressure afforded 3-(1,3-dioxolan-2-yl)phenyl p-toluenesulfonate as a
light pink crystalline solid. The crude product was used for the third step.
3. A solution of 3-(1,3-dioxolan-2-yl)phenyl p-toluenesulfonate in ethanol
(10 mL) was added to a solution of NaOH (6.56 mg, 164 mmol) in a
mixture of water/ethanol (3:1, 120 mL). The reaction mixture was stirred
at 90 °C for 3 h. After cooling to room temperature, the mixture was
diluted with ethyl acetate (60 mL) and washed with water (50 mL). The
aqueous layer was adjusted to pH 7 with 2 M HCl and extracted with ethyl
acetate (50 mL). The combined organic layers were dried using MgSO4.
After removal of the solvent in vacuo, the crude product was purified by
automated column chromatography on silica gel (isohexane/ethyl acetate,
1‒20%, 90 min; then 20–30%, 30 min) to afford 3-(1,3-dioxolan-2-
yl)phenol (7) (4.90 g, 29.5 mmol, 71% over three steps) as a colorless
crystalline solid. IR (ATR): ν = 3323 (br), 3055, 2976, 2886, 1696, 1604,
1460, 1404, 1348, 1295, 1225, 1180, 1161, 1073, 1015, 989, 959, 943,
914, 867, 791, 724, 696, 606 cm–1; 1H NMR (500 MHz, CDCl3): δ = 4.08
(m, 4 H), 5.17 (s, 1 H), 5.78 (s, 1 H), 6.82 (ddd, J = 8.0, 2.6, 0.9 Hz, 1 H),
6.96 (m, 1 H), 7.04 (d, J = 7.6 Hz, 1 H), 7.24 (d, J = 1.0 Hz, 1 H); 13C
NMR (125 MHz, CDCl3) δ = 65.40 (2 CH2), 103.51 (CH), 113.34 (CH),
116.43 (CH), 118.97 (CH), 129.89 (CH), 139.66 (C), 155.87 (C); MS (EI):
m/z (%) = 166 (43, [M]+), 165 (100), 149 (17), 135 (13), 121 (85), 107
(35), 94 (74), 73 (91), 65 (44), 51 (20), 45 (39), 39 (36); elemental
analysis: calcd (%) for C9H10O3: C 65.05, H 6.07; found: C 65.05, H 6.30.
Scheme 2. Postulated disaggregation of the tetranuclear palladium(II)
complexes 4 in solution. L = ligand (e.g. NMP); X = O, NMe.
Conclusion
In summary, we have developed a convenient synthesis of
tetranuclear palladium(II) complexes which have been
characterized by X-ray analysis. These complexes are air,
moisture, and thermally stable and showed excellent catalytic
activities in Mizoroki–Heck and Suzuki–Miyaura cross coupling
reactions. Further optimization of the catalytic activity by using
novel tetranuclear palladium(II) complexes with the same
structural motif is currently in progress.
Experimental Section
General: All reactions were carried out in oven-dried glassware using
anhydrous solvents under an argon atmosphere, unless stated otherwise.
CH2Cl2, THF, and toluene were dried using a solvent purification system
(MBraun-SPS). Pd(OAc)2 was recrystallized from glacial acetic acid. All
other chemicals were used as received from commercial sources.
Automated flash chromatography was performed on a Büchi Sepacore
system equipped with an UV monitor using silica gel from Acros Organics
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2-(3-Bromophenyl)-1,3-dioxolane
(8):
A
solution
of
3-
λem = 297 nm; IR (ATR): ν = 3071, 2969, 2941, 2854, 2815, 2771, 1509,
1482, 1442, 1361, 1256, 1148, 1030, 988, 878, 840, 775, 692 cm–1; 1H
NMR (500 MHz, CD2Cl2): δ = 2.18 (s, 12 H), 3.37 (s, 4 H), 6.87 (dd, J =
8.0, 2.4 Hz, 2 H), 6.96 (t, J = 1.0 Hz, 2 H), 7.04 (d, J = 7.6 Hz, 2 H), 7.27
(t, J = 7.9 Hz, 2 H); 13C NMR (125 MHz, CD2Cl2) δ = 46.34 (4 CH3), 65.11
(2 CH2), 118.78 (2 CH), 120.68 (2 CH), 125.24 (2 CH), 130.80 (2 CH),
142.53 (2 C), 158.71 (2 C); ESI-MS (+ 10 V): m/z = 285 [M+H]+;
elemental analysis calcd (%) for C18H24N2O: C 76.02, H 8.51, N 9.85;
found: C 75.37, H 8.89, N 9.74.
bromobenzaldehyde (6) (10.1 g, 54.5 mmol), ethane-1,2-diol (12.9 g, 208
mmol) and p-toluenesulfonic acid monohydrate (120 mg, 0.69 mmol) in
toluene (40 mL) was heated at reflux for 18 h using a Dean-Stark
apparatus. After cooling to room temperature, the reaction mixture was
diluted with ethyl acetate (20 mL), washed with a saturated aqueous
solution of NaHCO3 (40 mL), water (40 mL) and brine (40 mL), and dried
with MgSO4. Removal of the solvent under reduced pressure and
purification of the residue by automated chromatography on silica gel
(isohexane/ethyl acetate, 0‒1%, 70 min; then 1–3 %, 70 min) provided
compound 8 (11.1 g, 48.2 mmol, 88%) as colorless liquid. IR (ATR): ν =
3058, 2955, 2885, 1572, 1475, 1432, 1377, 1277, 1263, 1209, 1099,
Palladium(II) complex 3a: The synthesis was performed in a GC vial.
The diaryl ether 10a (102 mg, 0.36 mmol) and Pd(OAc)2 (81 mg, 0.36
mmol) were placed in the vial. After addition of o-xylene (1.5 mL) the vial
was sealed and the mixture was stirred at 144 °C for 1 h. After cooling to
room temperature the crude product was adsorbed on silica gel and
purified by chromatography on silica gel with isohexane/ethyl acetate
(1:1). After removal of the solvent complex 3a (46 mg, 0.12 mmol, 33 %)
was obtained as a light yellow to off-white solid. M.p. 114 °C (dec.); UV
(MeOH): λ = 196, 199, 222, 240, 279, 307 nm; fluorescence (MeOH): λex
= 222 nm, λem = 325 nm; IR (ATR): ν = 3436, 3041, 3000, 2966, 2864,
2830, 2781, 2701, 1457, 1432, 1360, 1301, 1239, 1208, 1173, 1004, 981,
958, 874, 849, 808, 767, 755, 705, 628 cm–1; 1H NMR (600 MHz, CDCl3):
δ = 2.74 (s, 12 H), 3.84 (s, 4 H), 6.81 (d, J = 7.2 Hz, 2 H), 7.00 (d, J = 7.9
Hz, 2 H), 7.07 (t, J = 7.6 Hz, 2 H); 13C NMR (150 MHz, CDCl3) δ = 50.66
(4 CH3), 71.32 (2 CH2), 113.82 (2 CH), 116.08 (2 CH), 125.11 (2 CH),
128.34 (2 C), 149.08 (2 C), 152.12 (2 C); ESI-MS (+ 10 V): m/z = 389
[M+H]+ for the 106Pd isotope (Figure S1); elemental analysis calcd (%) for
C18H22N2OPd: C 55.61, H 5.70, N 7.21; found: C 55.28, H 5.78, N 7.49.
1069, 1027, 965, 941, 881, 782, 721, 695, 672, 630 cm–1; H NMR (500
1
MHz, CDCl3): δ = 4.07 (m, 4 H), 5.78 (s, 1 H), 7.25 (t, J = 7.9 Hz, 1 H),
7.40 (d, J = 7.9 Hz, 1 H), 7.49 (ddd, J = 7.9, 1.9, 0.9 Hz, 1 H), 7.64 (t, J =
1.7 Hz, 1 H); 13C NMR (125 MHz, CDCl3) δ = 65.47 (2 CH2), 102.87 (CH),
122.61 (C), 125.30 (CH), 129.67 (CH), 130.10 (CH), 132.34 (CH), 140.45
(C); MS (EI): m/z (%) = 229 (90), 227 (100, [M]+), 183 (49), 149 (48), 119
(29), 89 (76), 73 (78), 50 (41); elemental analysis calcd (%) for
C9H9BrO2: C 47.19, H 3.96; found: C 47.41, H 4.08.
Bis(3-formylphenyl) ether (9a): CuCl (433 mg, 4.37 mmol) and Cs2CO3
(2.85 g, 8.73 mmol) were suspended in N-methyl pyrrolidinone (NMP,
12 mL) and 2,2,6,6-tetramethylheptane-3,5-dione (201 mg, 1.09 mmol).
The phenol 7 (873 mg, 5.25 mmol) and the bromoarene 8 (1.00 g,
4.37 mmol) were added via syringes. The resulting reaction mixture was
stirred at 130 °C for 17 h. After cooling to room temperature, the mixture
was diluted with methyl tert-butyl ether (MTBE, 80 mL) and filtered. The
MTBE solution was washed with 2 M HCl (2  60 mL), 2 N NaOH (2 
60 mL), water (60 mL), and brine (60 mL). The combined aqueous layers
were extracted with MTBE (20 mL). The combined organic layers were
dried over MgSO4 and the solvent was removed under reduced pressure.
The residue was diluted with MTBE (50 mL) and the organic layer was
washed using 2 N NaOH (50 mL), water (50 mL) and brine (50 mL), and
dried over MgSO4. The mixture was concentrated to about 3 mL and
NMP (13 mL) and 6 M HCl (10 mL) were added. Then, the mixture was
stirred at room temperature for 2 h, diluted with MTBE (50 mL), washed
with 2 M HCl (50 mL), water (50 mL) and brine (50 mL). After drying over
MgSO4, the solvent was evaporated in vacuo and the crude product was
purified by automated chromatography on silica gel (isohexane/ethyl
acetate, 1‒25%, 60 min) to provide compound 9a (813 mg, 3.59 mmol,
82 % over two steps) as an orange to brown, highly viscous oil. UV
(MeOH): λ = 199, 226, 304 nm; fluorescence (MeOH): λex = 199 nm, λem
= 296 nm; IR (ATR): ν = 3068, 2926, 2841, 2731, 1692, 1476, 1445,
1387, 1242, 1164, 1144, 1120, 786, 678, 643 cm–1; 1H NMR (500 MHz,
CDCl3): δ = 7.33 (ddd, J = 8.2, 2.5, 0.9 Hz, 2 H), 7.50 (m, 2 H), 7.56 (t, J
= 7.9 Hz, 2 H), 7.67 (d, J = 7.6 Hz, 2 H), 9.98 (s, 2 H); 13C NMR (125
MHz, CDCl3) δ = 118.71 (2 CH), 125.35 (2 CH), 125.98 (2 CH), 130.93 (2
CH), 138.41 (2 C), 157.53 (2 C), 191.51 (2 CHO); MS (EI): m/z (%) = 226
(100, [M]+), 169 (46), 141 (57), 115 (41), 77 (39), 63 (41), 51 (50);
elemental analysis calcd (%) for C14H10O3: C 74.33, H 4.46; found: C
74.68, H 4.46.
N,N-Bis(3-formylphenyl)methanamine (9b): In a glove box, a glass
tube was filled with Pd(dba)2 (23.6 mg, 41.0 μmol), XPhos (64.9 mg,
0.136 mmol), and NaOtBu (802 mg, 8.35 mmol) and sealed with a
septum. Under argon, methylamine hydrochloride (271 mg, 4.02 mmol)
was added. Using a syringe, a solution of the bromoarene 8 (509 mg,
2.22 mmol) in 1,4-dioxane (10 mL) was added and the tube was sealed
with a screw cap. The reaction mixture was stirred at 90 °C for 22.5 h
and after cooling to room temperature, diluted with ethyl acetate (50 mL)
and filtered through Celite®. The reaction mixture was washed with water
(50 mL) and brine (50 mL), and dried over MgSO4. After removal of the
solvent under reduced pressure, the crude product was purified by
automated chromatography on silica gel (isohexane/ethyl acetate, 5‒
30%, 90 min). NMP (5 mL) and 6 M HCl (6 mL) were added to the
resulting intermediate and the mixture was stirred at room temperature
for 2 h. The mixture was diluted with ethyl acetate (30 mL), washed with
sat. aqueous NaHCO3 (50 mL), water (50 mL) and brine (50 mL), and
dried over MgSO4. After removal of the solvent under reduced pressure,
the crude product was purified by automated chromatography on silica
gel (isohexane/ethyl acetate, 5–30 %, 90 min) to afford compound 9b
(161 mg, 0.673 mmol, 61 % over two steps) as a yellow oil. IR (ATR): ν =
3369, 3058, 2814, 2730, 2057, 1917, 1844, 1771, 1734, 1690, 1654,
1575, 1483, 1455, 1389, 1338, 1262, 1185, 1120, 1091, 993, 893, 778,
1
683, 645 cm–1; H NMR (500 MHz, CDCl3): δ = 3.42 (s, 3 H), 7.30 (m, 2
H), 7.45 (t, J = 7.7 Hz, 2 H), 7.50 (d, J = 7.6 Hz, 2 H), 7.54 (t, J = 1.9 Hz,
2 H), 9.96 (s, 2 H); 13C NMR (125 MHz, CDCl3) δ = 40.53 (CH3), 120.32
(2 CH), 124.19 (2 CH), 126.76 (2 CH), 130.21 (2 CH), 137.92 (2 C),
149.22 (2 C), 192.32 (2 CHO); MS (EI): m/z (%) = 239 (100, [M]+), 238
(22), 222 (12), 192 (19), 182 (39), 167 (77), 105 (16), 77 (38), 51 (27);
elemental analysis calcd (%) for C15H13NO2: C 75.30, H 5.48, N 5.85;
found: C 75.67, H 5.49, N 6.15.
Bis{(3-[(dimethylamino)methyl]phenyl} ether (10a): Dimethylamine
hydrochloride (227 mg, 2.78 mmol), sodium triacetoxyborohydride (1.42
g, 6.71 mmol) and 100 mg of molecular sieves (4 Å) were placed in a 25-
mL reaction flask. Then, 1,2-dichloroethane (4 mL) and distilled
triethylamine (0.35 mL, 2.5 mmol) were added via syringes. Compound
9a (257 mg, 1.14 mmol) in 4 mL of 1,2-dichloroethane was added via a
syringe pump over a period of 30 minutes at room temperature. After
complete addition, the reaction mixture was stirred at room temperature
for 17 h. The reaction mixture was filtered, washed with sat. aqueous
Na2CO3 (50 mL), water (50 mL), and brine (50 mL). The organic layer
was dried over MgSO4. After evaporation of the solvent, compound 10a
(257 mg, 0.904 mmol, 79 %) was obtained as highly viscous yellow oil.
UV (MeOH): λ = 203, 274, 281 nm; fluorescence (MeOH): λex = 203 nm,
N,N-Bis{3-[(dimethylamino)methyl]phenyl}methanamine
Dimethylamine hydrochloride (138 mg, 1.69 mmol),
(10b):
sodium
triacetoxyborohydride (894 g, 4.22 mmol) and 200 mg of molecular
sieves (4 Å) were placed in 25-mL reaction flask. Then, 1,2-
dichloroethane (15 mL) and distilled triethylamine (300 μL, 2.15 mmol)
a
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10.1002/chem.201703926
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were added via syringes.
A
solution of compound 9b (161 mg,
93.885(8)°, V = 5153.9(14) Å3, Z = 4, ρcalcd = 1.694 g/cm3, μ = 1.436 mm‒
1, λ = 0.71073 Å, T = 198(2) K, θ range: 1.16‒30.50°, reflections
collected: 368413, independent: 15709 (Rint = 0.0940), 627 parameters.
The structure was solved by direct methods and refined by full-matrix
least-squares on F2; final R indices [I > 2σ(I)]: R1 = 0.0365, wR2 = 0.0876;
maximal residual electron density: 1.278 e Å–3. CCDC-1563750 contains
the supplementary crystallographic data for this structure. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
0.673 mmol) in 1,2-dichloroethane (5 mL) was added at room
temperature over a period of 60 minutes via a syringe pump. After
complete addition, the reaction mixture was stirred at room temperature
for another 18 hours. The reaction mixture was filtered and washed with
sat. aqueous Na2CO3 (50 mL), water (50 mL), and brine (50 mL). The
organic layer was dried over MgSO4. After evaporation of the solvent,
compound 10b (164 mg, 0.551 mmol, 82 %) was obtained as a brownish
highly viscous oil. IR (ATR): ν = 3035, 2939, 2853, 2767, 1597, 1487,
1456, 1398, 1360, 1340, 1263, 1172, 1145, 1122, 1095, 1029, 994, 951,
840, 769, 695 cm–1; 1H NMR (500 MHz, CD2Cl2): δ = 2.23 (s, 12 H), 3.32
(s, 3 H), 3.36 (s, 4 H), 6.90 (m, 4 H), 6.98 (s, 2 H), 7.20 (t, J = 7.7 Hz, 2
H); 13C NMR (125 MHz, CD2Cl2) δ = 40.54 (CH3), 45.52 (4 CH3), 64.61 (2
CH2), 119.34 (2 CH), 121.17 (2 CH), 122.19 (2 CH), 129.08 (2 CH),
140.01 (2 C), 149.19 (2 C); MS (EI): m/z (%) = 297 (46, [M]+), 254 (44),
211 (49), 210 (21), 209 (42), 208 (100), 195 (53), 194 (53), 58 (61);
elemental analysis calcd (%) for C19H27N3: C 76.72, H 9.15, N 14.13;
found: C 76.06, H 9.29, N 13.92.
Tetranuclear palladium(II) complex 4b: The synthesis was performed
in a cylindrical vial (8 mL) with a non-air-tight plastic cap. The diaryl
amine 10b (50.0 mg, 0.168 mmol) and palladium acetate (75.7 mg,
0.337 mmol) were stirred in acetone (3 mL) at room temperature for 4
days. The crude product was purified by flash chromatography on silica
gel (ethyl acetate/CH2Cl2 (4:1) + 5 vol.-% ethanol) to provide complex 4b
(68.9 mg, 0.055 mmol, 65%) as colorless crystals. M.p. 198–199 °C
(dec.); UV (CH2Cl2): λ = 229, 306 nm; fluorescence (CH2Cl2): λex
=
229 nm, λem = 353 nm; IR (ATR): ν = 3469, 2981, 2880, 2835, 1569,
1508, 1473, 1457, 1407, 1342, 1235, 1179, 1106, 1021, 988, 940, 865,
843, 805, 723, 679, 660, 622 cm–1; 1H NMR (500 MHz, CDCl3): δ = 2.06
(s, 12 H), 2.16 (s, 12 H), 2.72 (d, J = 13.2 Hz, 4 H), 2.80 (s, 12 H), 3.20 (d,
J = 13.6 Hz, 4 H), 3.25 (s, 6 H), 6.43 (d, J = 2.2 Hz, 4 H), 6.67 (dd, J =
8.2, 2.5 Hz, 4 H), 7.02 (d, J = 8.2 Hz, 4 H); 13C NMR (125 MHz, CDCl3) δ
= 24.70 (4 CH3), 40.62 (2 CH3), 50.72 (4 CH3), 53.08 (4 CH3), 71.16 (4
CH2), 113.79 (4 CH), 116.14 (4 CH), 133.30 (4 CH), 134.37 (4 C), 146.80
(4 C), 147.07 (4 C), 181.22 (4 C=O); ESI-MS (+ 10 V): m/z = 1193 [M‒
OAc]+ for the 106Pd isotope; elemental analysis calcd (%) for
C46H62N6O8Pd4: C 44.10, H 4.99, N 6.71; found: C 44.44, H 5.33, N 6.61.
Palladium(II) complex 3b: The synthesis was performed in a GC vial.
The diarylamine 10b (89.3 mg, 0.300 mmol) and Pd(OAc)2 (68.7 mg,
0.306 mmol) were placed in the vial. After addition of toluene (1.5 mL)
the vial was sealed and the mixture was stirred at 110 °C for 3 h. After
cooling to room temperature the crude product was adsorbed on silica
gel and purified by chromatography on silica gel (isohexane/ethyl acetate,
1:1). After removal of the solvent, the obtained solid was washed with
small amounts of isohexane and drying in vacuo provided complex 3b
(4.3 mg, 11 μmol, 4 %) as a brown solid. M.p. 183 °C (dec.). UV (MeOH):
λ = 239, 311 nm; fluorescence (MeOH): λex = 239 nm, λem = 371 nm; IR
(ATR): ν = 3031, 2959, 2874, 1573, 1553, 1502, 1446, 1359, 1336, 1287,
1242, 1208, 1176, 1146, 1099, 1030, 999, 966, 862, 846, 805, 763, 712,
Single crystals of 4b, suitable for X-ray diffraction analysis, were obtained
by the slow diffusion of isohexane into the solution of 4b in CH2Cl2.
Crystallographic data for 4b: C46H62N6O8Pd4 · 2 CH2Cl2 (4b + 2 CH2Cl2),
M = 1422.46 g mol‒1, crystal size: 0.490  0.410  0.390 mm3, cubic,
space group Pn3¯n, a = b = c = 39.4255(10) Å, V = 61282(5) Å3, Z = 24,
ρcalcd = 0.925 g/cm3, μ = 0.827 mm‒1, λ = 0.71073 Å, T = 150(2) K, θ
range: 0.730‒27.139°, reflections collected: 2002317, independent:
11351 (Rint = 0.1307), 329 parameters. The structure was solved by
direct methods and refined by full-matrix least-squares on F2; final R
indices [I > 2σ(I)]: R1 = 0.1076, wR2 = 0.3014; maximal residual electron
density: 1.414 e Å–3. The crystal contains solvate dichloromethane
molecules; two of them per molecule could be located and refined. The
crystal has also solvent accessible voids of about 47% of the unit cell
volume, filled with disordered solvent molecules which could not be
located (Figure S3). CCDC-1563836 contains the supplementary
crystallographic data for this structure. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
614 cm–1 1H NMR (500 MHz, CDCl3): δ = 2.72 (s, 12 H), 3.56 (s, 3 H),
;
3.87 (s, 4 H), 6.67 (d, J = 6.9 Hz, 2 H), 6.81 (d, J = 8.2 Hz, 2 H), 7.07 (dd,
J = 8.3, 7.3 Hz, 2 H); 13C NMR (125 MHz, CDCl3) δ = 38.37 (CH3), 50.92
(4 CH3), 71.69 (2 CH2), 111.79 (2 CH), 113.88 (2 CH), 124.37 (2 CH),
132.95 (2 C), 144.13 (2 C), 147.77 (2 C); ESI-MS (+ 10 V): m/z = 402
[M+H]+ (Figure S2); ESI-MS (+ 75 V): m/z = 419 [M+NH4]+ for the 106Pd
isotope; elemental analysis calcd (%) for C19H25N3Pd: C 56.79, H 6.27, N
10.46; found: C 56.82, H 6.29, N 10.32.
Tetranuclear palladium(II) complex 4a: The synthesis was performed
in a cylindrical vial (8 mL) with a non-air-tight plastic cap. The diaryl ether
10a (125 mg, 0.440 mmol) and palladium acetate (198 mg, 0.882 mmol)
were stirred in ethyl acetate (4 mL) at room temperature for 7 days. The
crude product was purified by flash chromatography on silica gel (ethyl
acetate/CH2Cl2 (4:1) + 5 vol.-% ethanol) to provide complex 4a (139 mg,
0.113 mmol, 52%) as colorless crystals. M.p. 202–204 °C (dec.); UV
(CH2Cl2): λ = 261 nm; fluorescence (CH2Cl2): λex = 261 nm, λem
=
308 nm; IR (ATR): ν = 3452, 2985, 2915, 2840, 1706, 1633, 1563, 1448,
1404, 1344, 1284, 1249, 1224, 1157, 1111, 1039, 991, 948, 879, 861,
Procedure for the Mizoroki–Heck reaction: 4-Bromoacetophenone
(1.25 mmol), dry potassium carbonate (1.2 equiv) and the complex 4b
(0.05 mol% based on the bromoarene) were placed in a 10-mL round-
bottom flask. A mixture of NMP (4.8 mL) and water (0.2 mL) was added.
After repeated degassing under vacuum and purging with argon, styrene
(1.2 equiv) was injected via a septum. The reaction mixture was
vigorously stirred at 100 °C for 4 h. Subsequently, the cooled reaction
mixture was diluted with ethyl acetate (20 mL) and washed twice with 2 M
HCl (10 mL), sat. aqueous Na2CO3 (10 mL), and brine (10 mL). The
organic layer was evaporated and the crude product was purified by
automated column chromatography on silica gel (isohexane/ethyl acetate,
1–13 %, 45 min) to give (E)-4-acetylstilbene as white solid.
1
835, 816, 682, 624 cm–1; H NMR (500 MHz, CDCl3): δ = 2.06 (s, 12 H),
2.19 (s, 12 H), 2.74 (d, J = 13.6 Hz, 4 H), 2.81 (s, 12 H), 2.84 (d, J = 13.6
Hz, 4 H), 6.21 (d, J = 2.5 Hz, 4 H), 6.77 (dd, J = 8.4, 2.7 Hz, 4 H), 7.09 (d,
J = 8.2 Hz, 4 H); 13C NMR (125 MHz, CDCl3) δ = 24.71 (4 CH3), 50.55 (4
CH3), 53.21 (4 CH3), 71.04 (4 CH2), 109.92 (4 CH), 115.72 (4 CH),
133.72 (4 CH), 135.82 (4 C), 147.32 (4 C), 155.32 (4 C), 181.86 (4 C=O);
ESI-MS (+ 10 V): m/z = 1166 [M‒OAc]+ for the 106Pd isotope; elemental
analysis calcd (%) for C44H56N4O10Pd4: C 43.08, H 4.60, N 4.57; found: C
43.07, H 4.65, N 4.64.
Single crystals of 4a, suitable for X-ray diffraction analysis, were obtained
by slow diffusion of isohexane into a solution of 4a in ethyl acetate.
Crystallographic data for 4a: C44H56N4O10Pd4 · C4H8O2 (4a + EtOAc), M =
1314.63 g mol‒1, crystal size: 0.22  0.16  0.08 mm3, monoclinic, space
group P21/n, a = 11.402(2), b = 12.822(2), c = 35.336(5) Å, β =
(E)-4-Acetylstilbene: 1H NMR (500 MHz, CDCl3): δ = 2.61 (s, 3 H), 7.01
(d, J = 16.4 Hz, 1 H), 7.14 (d, J = 16.4 Hz, 1 H), 7.31 (m, 1 H), 7.39 (m, 2
H), 7.55 (m, 2 H), 7.59 (m, 2 H), 7.96 (m, 2 H); 13C NMR (125 MHz,
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10.1002/chem.201703926
Chemistry - A European Journal
COMMUNICATION
CDCl3) δ = 26.75 (CH3), 126.64 (2 CH), 126.96 (2 CH), 127.57 (CH),
128.46 (CH), 128.94 (2 CH), 129.02 (2 CH), 131.60 (CH), 136.07 (C),
136.82 (C), 142.14 (C) 197.65 (C=O); MS (EI): m/z (%) = 222 (61, [M]+),
207 (100), 179 (26), 178 (78), 152 (19).
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Priermeier, M. Beller, H. Fischer, Angew. Chem. Int. Ed. 1995, 34,
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[2]
[3]
[4]
General procedure for the Suzuki–Miyaura reaction: The bromoarene
(1.25 mmol), phenylboronic acid (1.1 equiv), potassium carbonate (1.2
equiv), and the complex 4a or 4b were placed in a 10-mL round-bottom
flask. A mixture of NMP (2 mL) and water (2 mL) was added. The
reaction mixture was vigorously stirred at room temperature under air for
4 or 23 h. Subsequently, the reaction mixture was diluted with ethyl
acetate (30 mL), washed with 2 M HCl twice (10 mL), sat. aqueous
Na2CO3 (10 mL), and with brine (10 mL). The organic layer was
evaporated and the crude product was purified by automated column
chromatography on silica gel to afford the product.
4-Methoxy-1,1′-biphenyl: Complex 4a (3.3 mg, 0.0027 mmol, 0.21
mol%). Reaction time: 4 h. Column chromatography: isohexane/ethyl
acetate, 2–10 %, 30 min. Colorless solid (93 % yield). 1H NMR (500 MHz,
CDCl3): δ = 3.86 (s, 3 H), 6.99 (m, 2 H), 7.31 (m, 1 H), 7.42 (t, J = 7.7 Hz,
2 H), 7.55 (m, 4 H); 13C NMR (125 MHz, CDCl3) δ = 55.47 (CH3), 114.32
(2 CH), 126.79 (CH), 126.87 (2 CH), 128.28 (2 CH), 128.85 (2 CH),
133.90 (C), 140.95 (C), 159.26 (C); MS (EI): m/z (%) = 184 (100, [M]+),
169 (49), 141 (51), 115 (42).
[5]
4-Methyl-1,1′-biphenyl: Complex 4a (3.5 mg, 0.0029 mmol, 0.23 mol%).
Reaction time: 4 h. Column chromatography: isohexane/ethyl acetate, 2–
10 %, 30 min. Colorless solid (93 % yield). 1H NMR (500 MHz, CDCl3): δ
= 2.41 (s, 3 H), 7.26 (d, J = 7.9 Hz, 2 H), 7.33 (m, 1 H), 7.43 (t, J = 7.7 Hz,
2 H), 7.50 (d, J = 7.9 Hz, 2 H), 7.59 (m, 2 H); 13C NMR (125 MHz, CDCl3)
δ = 21.25 (CH3), 127.11 (2 CH), 127.14 (2 CH), 128.85 (3 CH), 129.61 (2
CH), 137.16 (C), 138.49 (C), 141.29 (C); MS (EI): m/z (%) = 168 (100,
[M]+), 167 (68).
[6]
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A. Jäger, A. W. Schmidt, H.-J. Knölker, Org. Biomol. Chem. 2014, 12,
3866–3876; d) R. Hesse, M. P. Krahl, A. Jäger, O. Kataeva, A. W.
Schmidt, H.-J. Knölker, Eur. J. Org. Chem. 2014, 4014–4028; e) K. K.
Julich-Gruner, O. Kataeva, A. W. Schmidt, H.-J. Knölker, Chem. Eur. J.
2014, 20, 8536–8540; f) R. Hesse, O. Kataeva, A. W. Schmidt, H.-J.
Knölker, Chem. Eur. J. 2014, 20, 9504–9509; g) C. Gassner, R. Hesse,
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5673; i) C. Schuster, M. Rönnefahrt, K. K. Julich-Gruner, A. Jäger, A. W.
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J. 2016, 22, 16897–16911.
4-Nitro-1,1′-biphenyl: Complex 4a (3.7 mg, 0.0030 mmol, 0.25 mol%).
Reaction time: 4 h. Column chromatography: isohexane/ethyl acetate,
15–18 %, 20 min. Yellow solid (97 % yield). 1H NMR (500 MHz, CDCl3): δ
= 7.45 (m, 1 H), 7.51 (m, 2 H), 7.63 (m, 2 H), 7.74 (m, 2 H), 8.30 (m, 2
H); 13C NMR (125 MHz, CDCl3) δ = 124.24 (2 CH), 127.51 (2 CH),
127.93 (2 CH), 129.05 (CH), 129.29 (2 CH), 138.90 (C), 147.21 (C),
147.76 (C); MS (EI): m/z (%) = 199 (80, [M]+), 169 (31), 153 (27), 152
(100), 151 (29), 141 (31).
4-Acetyl-1,1′-biphenyl: Complex 4b (2.8 μg, 0.0022 μmol, 0.00018
mol%). Reaction time: 23 h. Column chromatography: isohexane/ethyl
acetate, 1–17 %, 30 min. Colorless solid (97 % yield). 1H NMR (500 MHz,
CDCl3): δ = 2.65 (s, 3 H), 7.41 (tt, J = 7.4 Hz, 1.3 Hz, 1 H), 7.48 (t, J = 7.4
Hz, 2 H), 7.63 (m, 2 H), 7.69 (m, 2 H), 8.04 (m, 2 H); 13C NMR (125 MHz,
CDCl3) δ = 26.83 (CH3), 127.38 (2 CH), 127.43 (2 CH), 128.38 (CH),
129.06 (2 CH), 129.10 (2 CH), 135.99 (C), 140.02 (C), 145.94 (C),
197.93 (C=O); MS (EI): m/z (%) = 196 (44, [M]+), 181 (100), 153 (38),
152 (62).
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10.1002/chem.201703926
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COMMUNICATION
COMMUNICATION
High catalytic activity by multiple
palladium centers: Reaction of
substituted diaryl ethers and
F. Puls, N. Richter, O. Kataeva, H.-J.
Knölker*
Page No. – Page No.
diarylamines with two equivalents of
palladium(II) acetate afforded
tetranuclear palladium(II) complexes.
These complexes showed excellent
catalytic activity for Mizoroki–Heck
and Suzuki–Miyaura cross coupling
reactions with high TON and TOF
values.
Synthesis of Tetranuclear
Palladium(II) Complexes and their
Catalytic Activity for Cross Coupling
Reactions
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