Palladium dichloride adduct of N,N-bis-(diphenylphosphanylmethyl)-2- aminopyridine: Synthesis, structure and catalytic performance in the decarboxylative cross-coupling of 4-picolinic acid with aryl bromide

Article abstract of DOI:10.1039/c4dt00815d

Reaction of PdCl₂ with N,N-bis-(diphenylphosphanylmethyl)-2- aminopyridine (bdppmapy) afforded a mononuclear complex [(bdppmapy)PdCl ₂] (1). Compound 1 was characterized by elemental analysis, IR, ¹H, ¹³C and ³¹P NMR, electrospray ion mass spectra (ESI-MS) and X-ray single crystal crystallography. The Pd(ii) center in 1 is chelated by bdppmapy, showing a cis-square planar geometry. With the assistance of additive Cu₂O, complex 1 exhibited good catalytic activity toward the decarboxylative cross-coupling reactions between 4-picolinic acid and aryl bromides. In the presence of only 2 mol% catalyst, a family of 4-aryl-pyridines could be isolated in up to 83% yield.

Full text of DOI:10.1039/c4dt00815d

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Palladium dichloride adduct of
N,N-bis-(diphenylphosphanylmethyl)-
Cite this: Dalton Trans., 2014, 43,
9786
2-aminopyridine: synthesis, structure and catalytic
performance in the decarboxylative cross-
coupling of 4-picolinic acid with aryl bromide†
Run-Tian He,a Jian-Feng Wang,a Hui-Fang Wang,a Zhi-Gang Ren*a and
Jian-Ping Lang*a,b
Reaction of PdCl2 with N,N-bis-(diphenylphosphanylmethyl)-2-aminopyridine (bdppmapy) afforded a
mononuclear complex [(bdppmapy)PdCl2] (1). Compound 1 was characterized by elemental analysis, IR,
1H, 13C and 31P NMR, electrospray ion mass spectra (ESI-MS) and X-ray single crystal crystallography. The
Pd(II) center in 1 is chelated by bdppmapy, showing a cis-square planar geometry. With the assistance of
additive Cu2O, complex 1 exhibited good catalytic activity toward the decarboxylative cross-coupling
reactions between 4-picolinic acid and aryl bromides. In the presence of only 2 mol% catalyst, a family of
4-aryl-pyridines could be isolated in up to 83% yield.
Received 18th March 2014,
Accepted 29th April 2014
DOI: 10.1039/c4dt00815d
www.rsc.org/dalton
breakthroughs. Among them, the most common substrates
were benzoic acid and substituted aryl acids, whereas a very
Introduction
In recent years, the palladium-catalyzed C–C coupling reac- limited number of examples were reported to use heteroaryl-
tions have attracted considerable attention because of their carboxylic acids such as pyridine-, thiophene- and furan-based
application in the formation of a large range of new organic carboxylic acids.9 For instance, Wu developed a novel synthetic
compounds with straightforward procedures.1 As an expansion route to 2-aryl- and hetero-aryl-pyridines with up to 75% yields
of the traditional C–C coupling reactions, efforts to achieve by the coupling of 2-picolinic acid with aryl- and heteroaryl-
higher yields, better selectivity and more friendly envi- bromides using 5 mol% PdCl2.10 Considering that 4-aryl-pyri-
ronmental effects have led to exploration of new substrates2 dines are quite popular in some natural products, medicines
containing aryl cyanides,2a aryl acids,2b,c aryl esters,2d as well and organic ligands,11 it is of importance to investigate the
as activated C–H simple2e,f or substituted arenes such as decarboxylative coupling reaction between 4-picolinic acid and
anilides,2g,h aryl pyridine,2i,j fluoroarenes2k,l and phenols.2m aryl halides.
Recently, the decarboxylative coupling reaction has been
On the other hand, organic ligands can play a crucial role
proved to be effective in refraining from stoichiometric organo- in associating the Pd salts to catalyze the C–C coupling reac-
metallic reagents and to be less polluting as it usually pro- tions as well as decarboxylative cross-coupling reactions to
duces CO2 instead of toxic metal salts.3 In this area, Myers,4 improve the yields and selectivity.4b,5c,6b,7b Up to now, several
Gooßen,5 Liu,6 Su,7 and Larrosa8 have achieved some exciting types of ligands such as N-donor ligands,5a,e,f mono-phosphine
ligands7c and bi-phosphine ligands12 have been introduced
into various decarboxylative reaction systems. A catalytic
system with both 1,10-phenanthroline and bi-phosphine
ligands was reported by Gooßen to simplify the synthetic
methodology of azomethines and ketones via one-pot decar-
boxylative cross-coupling reactions of α-oxo carboxylates,
amines and aryl halides with yields up to 85%.13 However, in
the case of hybrid P–N ligands, although they have shown
excellent performance in some C–C cross-coupling reactions,14
their catalytic activity toward the decarboxylative cross-coup-
ling reactions has been rarely reported to date. In addition,
aCollege of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, People’s Republic of China.
E-mail: jplang@suda.edu.cn; Fax: +86 512 65882865; Tel: +86 512 65882865
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic
of China
†Electronic supplementary information (ESI) available: ESI and crystallographic
data in CIF format, ESI-MS and copies of the 1H NMR, 13C NMR and 31P NMR
spectra for 1 and copies of the 1H NMR and 13C NMR spectra of all final pro-
ducts. CCDC 990305. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt00815d
9786 | Dalton Trans., 2014, 43, 9786–9794
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Cu(I) compounds may exert an important impact on such a Synthesis
decarboxylative reaction because it assists the decarboxylation
[(bdppmapy)PdCl2] (1). To a solution of bdppmapy (491 mg,
1.0 mmol) in CH2Cl2–CH3CN (5 mL/5 mL) was added PdCl2
(178 mg, 1.0 mmol). The mixture was stirred for 5 min. The
resulting pale yellow solution was then covered by acetonitrile
(2 mL) and diethyl ether (20 mL). Yellow rhombic crystals of
1·MeCN were afforded after one hour. The crystals were further
dried in vacuo to give pure 1. Yield: 630 mg (94%). Anal. calcd
for C33H31Cl2N3P2Pd: C, 55.91; H, 4.41; N, 5.93. Found: C,
55.76; H, 4.44; N, 5.44%. IR (KBr disk): 3421 (m), 3049 (s),
2967 (s), 2891 (s), 1593 (w), 1566 (m), 1471 (w), 1435 (w), 1309
(s), 1296 (m), 1224 (w), 1162 (s), 1099 (w), 998 (s), 864 (w), 782
(s), 770 (s), 751 (m), 729 (m), 705 (m), 692 (w), 669 (m), 626 (s),
process via the coordination of Cu(I) and the carboxylic
group.5a Homo-coupling reactions have always occurred, which
caused some trouble in optimizing the reaction conditions.15
Therefore the introduction of a pyridyl group into the phos-
phine ligands might be helpful in preventing the homo-coup-
ling reactions.
We have been engaged in the synthesis and reactivity of
metal complexes derived from multiple phosphine and hybrid
P–N ligands.16 Some hybrid phosphine–pyridyl (P–N) ligands
such as N-diphenylphosphanylmethyl-4-aminopyridine (dppmapy)
and N,N-bis-(diphenylphosphanylmethyl)-2-aminopyridine
(bdppmapy) were used to react with Cu(I) and Ag(I) to produce
a family of Cu(I) and Ag(I) coordination polymers with interest-
ing structures.16c,d,f The bdppmapy ligand has a pyridyl group
in its structure. When PdCl2 reacted with bdppmapy, it simply
formed a mononuclear complex [(bdppmapy)PdCl2] (1). Could
complex 1 show catalytic activity toward decarboxylative cross-
coupling reactions? Could bdppmapy prevent the homo-
coupling reactions? With these two questions in mind, we
performed the decarboxylative cross-coupling reactions of
4-picolinic acid and aryl bromide catalyzed by 1, Cu2O, and
K2CO3. A set of 4-aryl-pyridines were isolated in good yields.
The results showed that bdppmapy was effective in preventing
the homo-coupling reactions and complex 1 was efficient in
catalyzing such a cross-coupling reaction. Herein we report its
synthesis, structural characterization and catalytic perform-
ance in the decarboxylative cross-coupling reactions of 4-picoli-
nic acid and aryl bromide.
1
533 (m) cm−1. H NMR (400 MHz, DMSO-d6, ppm): δ 7.98 (t,
J = 8.8 Hz, 8H), 7.81 (d, J = 4.8 Hz, 1H), 7.57 (t, J = 7.2 Hz, 4H),
7.50 (t, J = 7.2 Hz, 8H), 7.20 (t, J = 7.2 Hz, 1H), 6.50 (m, 2H),
5.04 (s, 4H). 13C NMR (400 MHz, DMSO-d6, ppm): δ 155.9,
146.4, 137.3, 134.3, 131.4, 128.2, 114.0, 106.8, 54.8. 31P NMR
(400 MHz, DMSO-d6, ppm): δ 9.37 (m).
X-ray structure determination
The measurements were made on a Agilent Xcalibur CCD X-ray
diffractometer using graphite monochromated Mo-Kα (λ =
0.71073 Å) radiation. A single crystal of 1·MeCN was obtained
directly from the above preparation and mounted in a capillary
and measured at room temperature. The program CrysAlisPro
(Agilent Technologies, Ver. 1.171.35.21, 2012) was used for the
refinement of cell parameters and the reduction of collected
data, while absorption corrections (multi-scan) were applied.
The reflection data were also corrected for Lorentz and polari-
zation effects.
The crystal structure of 1·MeCN was solved by direct
methods and refined on F2 by full-matrix least-squares
methods with the SHELX-97 program package.18 All non-H
atoms were refined anisotropically. All H atoms were placed in
geometrically idealized positions (C–H = 0.97 Å for methyl
groups, C–H = 0.98 Å for methylene groups and C–H = 0.94 Å
for pyrazolyl groups) and constrained to ride on their parent
atoms with Uiso(H) = 1.2Ueq(C). A summary of key crystallo-
graphic information is given in Table 1.
Experimental section
General procedures
The ligand bdppmapy was prepared according to a literature
method.17 Other chemicals were obtained from commercial
sources and used as received. All reactions and manipula-
tions were carried out under an oxygen-free nitrogen atmos-
phere with standard Schlenk techniques. Solvents were dried
and degassed before use. The NMR spectra were recorded at
room temperature on a Varian UNITY-400 (400 MHz) spectro-
meter with CDCl3 or DMSO-d6 as a solvent. Chemical shifts
are quoted relative to tetramethylsilane (TMS) and solvent
Typical procedure for the decarboxylative cross-coupling
reactions
signals. Elemental analyses for C, H, and N were performed A solution containing PdCl2 (178 mg) and bdppmapy (491 mg)
on a Carlo-Erbo CHNO-S microanalyzer. The IR spectra (KBr in N,N-dimethylacetamide (DMA) (10 mL) was prepared as the
disc) were recorded on a Nicolet MagNa-IR550 FT-IR spectro- catalyst. The decarboxylative cross-coupling reactions were
meter (4000–400 cm−1). Electrospray ion mass spectra carried out in a 25 mL tube equipped with a stirring bar. The
(ESI-MS) were recorded on an Agilent 1200/6200 mass catalyst solution (0.08 mL, 2%) was loaded into the tube with
spectrometer. GC measurements were recorded on an Agilent 4-picolinic acid (2, 0.4 mmol), aryl bromides (5, 0.6 mmol),
7820A Gas Chromatograph with an Agilent HP-5 chromato- Cu2O (0.6 mmol), K2CO3 (1.2 mmol) and 1,2-dimethoxyethane
graphic column and N2 as the mobile phase. The LC-MS were (DME) (1.4 mL)–DMA (0.32 mL)–DMSO (0.2 mL). The tube was
recorded in
a Rapid Resolution HT-3 chromatographic sealed and stirred at 130 °C for 24 h. After cooling to room
column on an Agilent 1260 Infinity Liquid Chromatograph temperature, the resulting mixture was diluted with 10 mL
with a 6120 Quadrupole Mass Spectrometer and MeCN as the ethyl ether and then filtered. The filtrate was washed with satu-
mobile phase.
rated NaCl solution (30 mL). The organic phase was dried over
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4-(2-Methylphenyl)-pyridine (6d).23 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 1-bromo-2-methyl-
benzene (0.6 mmol) was performed. After extraction and
concentration, the reaction mixture was purified by column
chromatography on silica gel (30% diethyl ether–petroleum
ether) to afford 6d as a colorless oil (43 mg, 63% yield).
Table 1 Crystal data and structure refinement parameters for 1·MeCN
Formula
Formula weight
Crystal system
Space group
C31H28Cl2N2P2Pd
708.05
Monoclinic
P21/c
a/Å
b/Å
c/Å
9.5873(19)
27.125(5)
13.096(3)
110.38(3)
3192.6(11)
1.475
4-(3-Methylphenyl)-pyridine (6e).24 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 1-bromo-3-methylben-
zene (0.6 mmol) was performed. After extraction and concen-
tration, the reaction mixture was purified by column
chromatography on silica gel (30% diethyl ether–petroleum
ether) to afford 6e as a white solid (29 mg, 42% yield).
β/°
V/Å3
Dc/g cm−3
Z
4
μ (Mo-Kα)/mm−1
F(000)
0.876
1440
Total reflections
Unique reflections
No. of observations
No. of parameters
Rint
13 814
5625
4647
371
0.0310
0.0337
0.0682
1.036
4-(4-Methylphenyl)-pyridine (6f).21 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 1-bormo-4-methylben-
zene (0.6 mmol) was performed. After extraction and concen-
tration, the reaction mixture was purified by column
chromatography on silica gel (15% diethyl ether–petroleum
ether) to afford 6f as a white solid (51 mg, 75% yield).
Ra
wRb
GOFc
4-(2,4,6-Trimethyl-phenyl)-pyridine (6g).25 The typical pro-
cedure using 4-pyridinyl acid (0.4 mmol) and 1-bromo-2,4,6-tri-
methylbenzene (0.6 mmol) was performed. After extraction
and concentration, the reaction mixture was purified by
column chromatography on silica gel (25% diethyl ether–pet-
roleum ether) to afford 6g as a white solid (32 mg, 40% yield).
4-(3,5-Dimethylphenyl)-pyridine (6h).26 The typical pro-
cedure using 4-pyridinyl acid (0.4 mmol) and 1-bormo-3,5-
dimethylbenzene (0.6 mmol) was performed. After extraction
and concentration, the reaction mixture was purified by
column chromatography on silica gel (25% diethyl ether–pet-
roleum ether) to afford 6h as a white solid (32 mg, 44% yield).
4-(4-Fluoro-phenyl)-pyridine (6i).27 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 1-bromo-4-fluoroben-
zene (0.6 mmol) was performed. After extraction and concen-
tration, the reaction mixture was purified by column
chromatography on silica gel (10% diethyl ether–petroleum
ether) to afford 6i as a white solid (34 mg, 48% yield).
a R = ||Fo| − |Fc|/|Fo|. b wR = {w(Fo2 − Fc2)2/w(Fo2)2}1/2
.
c GOF = {w((Fo
−
2
Fc2)2)/(n − p)}1/2, where n = number of reflections and p = total number
of parameters refined.
Mg2SO4 and concentrated in vacuo. The residue was then puri-
fied by chromatography on silica gel to provide the corres-
ponding product.
4-(2,4-Dimethoxyphenyl)-pyridine (4a).19 The typical pro-
cedure using 4-pyridinyl acid (0.4 mmol) and 2,4-dimethoxy
bromobenzene (0.6 mmol) was performed. After extraction
and concentration, the reaction mixture was purified by
column chromatography on silica gel (5% diethyl ether–pet-
roleum ether) to afford 4a as a white solid (67 mg, 78% yield).
2-(2,4-Dimethoxyphenyl)-pyridine (4b).20 The typical pro-
cedure using 2-pyridinyl acid (0.4 mmol) and 2,4-dimethoxy
bromobenzene (0.6 mmol) was performed. After extraction
and concentration, the reaction mixture was purified by
column chromatography on silica gel (50% diethyl ether–pet-
roleum ether) to afford 4b as a white solid (21 mg, 25% yield).
4-(4-Methoxylphenyl)-pyridine (6a).21 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 4-methoxy bromoben-
zene (0.6 mmol) was preformed. After extraction and concen-
tration, the reaction mixture was purified by column
chromatography on silica gel (20% diethyl ether–petroleum
ether) to afford 6a as a white solid (61 mg, 83% yield).
4-(Pyridin-4-yl)-benzonitrile (6j).28 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 4-bromobenzonitrile
(0.6 mmol) was performed. After extraction and concentration,
the reaction mixture was purified by column chromatography
on silica gel (25% diethyl ether–petroleum ether) to afford 6j
as a white solid (42 mg, 58% yield).
4-(4-Trifluoromethylphenyl)-pyridine (6k).29 The typical pro-
cedure using 4-pyridinyl acid (0.4 mmol) and 1-bromo-4-(tri-
fluoromethyl)benzene (0.6 mmol) was performed. After
extraction and concentration, the reaction mixture was purified
by column chromatography on silica gel (20% diethyl ether–petro-
leum ether) to afford 6k as a white solid (39 mg, 44% yield).
4-Biphenyl-4-yl-pyridine (6l).30 The typical procedure using
4-pyridinyl acid (0.4 mmol) and 4-bromobiphenyl (0.6 mmol)
was performed. After cooling down, a white solid was precipi-
tated in the bottom of the tube. The solid was filtrated and
washed with water and then dissolved in diethyl ether (20 mL).
After concentration, the reaction mixture was purified by
column chromatography on silica gel (pure diethyl ether) to
afford 6l as a white solid (57 mg, 62% yield).
4-(3-Methoxylphenyl)-pyridine (6b).21 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 3-methoxy bromoben-
zene (0.6 mmol) was performed. After extraction and concen-
tration, the reaction mixture was purified by column
chromatography on silica gel (20% diethyl ether–petroleum
ether) to afford 6b as a white solid (40 mg, 54% yield).
4-(2-Methoxylphenyl)-pyridine (6c).22 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 2-methoxy bromoben-
zene (0.6 mmol) was performed. After extraction and concen-
tration, the reaction mixture was purified by column
chromatography on silica gel (20% diethyl ether–petroleum
ether) to afford 6c as a colorless oil (53 mg, 71% yield).
9788 | Dalton Trans., 2014, 43, 9786–9794
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4-(Naphthalen-1-yl)-pyridine (6m).31 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 1-bromonaphthalene
(0.6 mmol) was performed. After extraction and concentration,
the reaction mixture was purified by column chromatography
on silica gel (30% diethyl ether–petroleum ether) to afford 6m
as a white solid (56 mg, 68% yield).
4-(Naphthalen-2-yl)-pyridine (6n).32 The typical procedure
using 4-pyridinyl acid (0.4 mmol) and 2-bromonaphthalene
(0.6 mmol) was performed. After extraction and concentration,
the reaction mixture was purified by column chromatography
on silica gel (45% diethyl ether–petroleum ether) to afford 6n
as a white solid (44 mg, 54% yield).
Results and discussion
Synthetic and spectral aspects
Treatment of PdCl2 with equimolar bdppmapy in CH2Cl2–
MeCN followed by a standard workup produced yellow crystals
of 1 in 94% yield (Scheme 1). Compound 1 is relatively stable
Fig. 1 View of the molecular structure of 1 with a labelling scheme and
50% thermal ellipsoids. All H atoms and the MeCN solvent molecule
towards air and moisture, and soluble in common solvents were omitted for clarity. Selected bond lengths (Å) and angles (°): Pd(1)–
Cl(1) 2.3583(8), Pd(1)–Cl(2) 2.3455(10), Pd(1)–P(1) 2.2512(8), Pd(1)–P(2)
2.2516(10), Cl(1)–Pd(1)–Cl(2) 89.72(3), Cl(1)–Pd(1)–Cl(2) 89.72(3), Cl(1)–
Pd(1)–P(2) 88.22(3), Cl(2)–Pd(1)–P(1) 88.46(3), P(1)–Pd(1)–P(2) 93.57(3).
such as CH2Cl2, CH3CN, N,N-dimethylformamide (DMF) and
DME. The elemental analysis was consistent with its formula.
In the IR spectra, the stretching vibrations at 1435 and
998 cm−1 are assignable to the coordinated P–C groups33 of 1
cene).35 The bite angle P(1)–Pd(1)–P(2) (93.57(3)°) of 1 is in
between that of [(dppp)PdCl2] (90.58(3)°) and that of [(dppf)-
PdCl2] (99.10(4)°). The pyridyl group of the bdppmapy mole-
cule remains intact.
As we know, in the many catalytic systems, the catalysts are
usually added as the mixture of PdCl2 and phosphine ligands,
whereas the exact structures are often left unexplored. Thus at
and the stretching vibrations of phenyl groups and the pyri-
dine group of ligands are located at 1592, 1566 and 1471 cm−1
.
1
The H NMR spectra of 1 (Fig. S1a, ESI†) in DMSO-d6 at room
temperature showed signals of diphenylphosphine groups
including three triplets at 7.98, 7.57 and 7.51 ppm. The pyri-
dine group could be recognized by the signals including a
doublet at 7.81 ppm, a triplet at 7.20 ppm and a multiplet at
6.49 ppm. The signals of methylene groups were located at
5.04 ppm as a singlet. The 13C NMR spectra of 1 (Fig. S1b,
ESI†) showed three signals at 128–135 ppm for the diphenyl-
phosphine groups, five resonances at 106.8, 114.0, 137.3, 146.4
and 155.9 ppm for the pyridine group and one signal at
54.8 ppm for the methylene groups. In the 31P NMR spectra of
1 (Fig. S1c, ESI†), one signal at 9.37 ppm was clearly observed.
Compound 1·MeCN crystallizes in the monoclinic space
group P21/c, and its asymmetric unit has one discrete
[(bdppmapy)PdCl2] molecule and one MeCN solvent molecule.
The Pd(II) atom is coordinated by the two P atoms from a
bdppmapy molecule and two chlorides, thereby forming a cis
square-planar coordination geometry (Fig. 1). The mean Pd–Cl
bond length (2.3519(10) Å vs. 2.2514(10) Å) is close to those
found in [(dppp)PdCl2] (2.355(2) Å vs. 2.247(3) Å, dppp = 1,3-
bis(diphenylphosphino)-propane)34 and [(dppf)PdCl2] (2.348(2)
Å vs. 2.291(3) Å, dppf = 1,1′-bis(diphenylphosphino)-ferro-
the beginning, we employed
a solution of PdCl2 and
bdppmapy (molar ratio 1 : 1) in DMA as the catalyst. Parallel
experiments proved that similar catalytic activities could be
achieved by using this solution or a solution of the powder of
complex 1 in DMA. In order to confirm the existence of the
same species in both DMA solutions, we measured their posi-
tive-ion ESI mass spectra (Fig. S2, ESI†). In both cases, strong
signals at m/z = 631.05 for [(bdppmapy)PdCl]+ and m/z =
718.11 for [(bdppmapy)PdCl + DMA]+ (Fig. S3, ESI†) were
observed, indicating that the two cations might be the major
species in the catalyst solutions. Thus complex 1 may not be
necessarily isolated before catalysis reactions and a mixture of
PdCl2 and bdppmapy (molar ratio = 1 : 1) could be used
directly in the catalysis reactions.
Decarboxylative cross-coupling reaction
Considering the electronic effects, a model decarboxylative
cross-coupling reaction between 2,4-dimethoxy bromobenzene
(3a) and 4-picolinic acid (2) was performed to produce 1-(4-
pyridyl)-2,4-dimethoxybenzene (4a). Eight phosphine ligands
including tricyclohexylphosphine (PCy3), triphenylphosphine
(PPh3), bis(diphenylphosphino)methane (dppm), 1,2-bis-
(diphenylphosphino)ethane (dppe), dppp, 2-dicyclohexylphos-
phino-2′,4′,6′-triisopropylbiphenyl (Xphos), 2,2′-bis(diphenyl-
Scheme 1 Synthesis of 1.
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Table 2 Screening of Pd sources, ligands and additives for the cross-
coupling of 4-picolinic acid and 2,4-dimethoxy bromobenzenea
Entry
Pd source
Ligand
Additive
Yieldb (%)
1
2
3
4
5
6
7
8
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
Pd(OAc)2
Pd(TFA)2
Pd(acac)2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PCy3
PPh3
dppm
dppe
dppp
XPhos
BINAP
bdppmapy
bdppmapy
bdppmapy
bdppmapy
bdppmapy
bdppmapy
bdppmapy
bdppmapy
bdppmapy
bdppmapy
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Ag2O
Ag2CO3
CuI
20
24
35
38
48
15
32
78
Trace
44
12
Trace
Trace
36
20
Trace
Trace
Scheme 2 The proposed reaction mechanism.
9
10
11
12
13
14
15
16
17
closer. Thus the cross-coupling reaction could be achieved. In
contrast, when other phosphine ligands were introduced into
such a system (entries 1–5), more homo-coupling products
such as 1,1′-biphenyl and 2,2′,4,4′-tetramethoxy-biphenyl were
isolated (yield 20–50%).
CuBr
CuCl
CuCO3
Larrosa2k and Liu6a once reported that the Density Func-
tional Theory (DFT) calculation could be used to study the
mechanism of coupling reactions. Thus we employed a simple
DFT method36 to calculate the energy level between the [Cu] or
[Pd] intermediate and the supposed Cu–Pd heterometallic
complex (ESI†). As shown in Fig. S4 (ESI†), the binding of the
pyridyl group in bdppmapy to the [Cu] intermediate may lead
to a decrease of 20.8 kcal mol−1 in Gibbs free energy. The
4-pyridyl group in the [Cu] intermediate may be rotated to
form the supposed Cu–Pd heterometallic complex with a sub-
sequent increase of 12.3 kcal mol−1 in Gibbs free energy,
which is quite low and may be easily achieved by the heating
of the reaction system. The potential energy surface obtained
herein suggested that the closer approaching of the [Cu] inter-
mediate and the [Pd] intermediate would be energy profitable
for the transmetallation step, which may finally lead to the
acceleration of the decarboxylative cross-coupling reaction.
The Pd salts and the Cu(I)/Ag(I) additives were also opti-
mized to achieve better yields. Four Pd(II) salts including
PdCl2, Pd(OAc)2, Pd(TFA)2 (TFA = trifluoroacetate) and Pd-
(acac)2 (acac = 2,4-acetylacetonate) were investigated (Table 2,
entries 6–9). It seems that all O-containing anions are less
efficient in this catalytic reaction. The reason is that, relative to
the Pd–Cl bond, the Pd–O bond is stronger and is not easily
broken to form the oxidative addition [Pd] intermediate, which
is one of the key steps of the cross-coupling cycle. Thus PdCl2
exhibited the best catalytic performance among the four Pd(II)
salts. In addition, Cu(I) or Ag(I) salts are essential during the
decarboxylation cycle as they assist the carboxylic acid to elimi-
nate the carboxyl group.5c These salts could be tuned to affect
both homo- and cross-coupling reactions. In this work, seven
a Reaction conditions: 2 (0.4 mmol), 3a (0.6 mmol), Pd salts (5 mol%),
ligand (10 mol% for entries 1–2 and 6 mol% for entries 3–17), K2CO3
(1.2 mmol), DME (1.4 mL), DMA (0.4 mL) and DMSO (0.2 mL),
additive (0.6 mmol), 130 °C, 24 h, a nitrogen atmosphere, 150 mg 3 Å
MS. b Isolated yields based on 2.
phosphino)-1,1′-binaphthalene (BINAP) and bdppmapy were
selected to form the key Pd intermediate during the transme-
tallation and reductive elimination steps of the decarboxylative
coupling reaction. The results (Table 2, entries 1–8) revealed
that the bi-phosphine ligands (dppm, dppe, BINAP and dppp)
were more efficient than the monophosphine ligands (PCy3,
XPhos and PPh3) in this system, whereas the P–N hybrid
ligand bdppmapy achieved the best yield of 78% (entry 8).
Such a difference between phosphine ligands may depend on
the reaction mechanism. Due to the widely established decar-
boxylative cross-coupling mechanism5a which may contain a
decarboxylation cycle and a cross-coupling cycle, the trans-
metallation step connecting the two cycles may require the
[Cu] intermediate to approach the [Pd] intermediate. For the
decarboxylation cycle, its reaction rate is well known to be
affected by the choice of solvents.7b,8g,i As a homo-coupling
reaction (Ullmann type reaction) is likely to occur on the [Pd]
intermediate,15 it has been proved to be a competitive reaction
in such a cross-coupling reaction. The acceleration of decar-
boxylative coupling should be more apparent. In the case of
bdppmapy, as shown in Scheme 2, it is supposed that the
uncoordinated pyridyl group of bdppmapy in the [Pd] inter-
mediate may coordinate at the [Cu] intermediate to form a Cu–
Pd heterometallic complex. The transmetallation step is more
easy to accomplish as the Cu and the Pd centers are drawn
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Table 3 Screening of the solvent, base and temperature for the cross-
coupling of 4-picolinic acid and 2,4-dimethoxy bromobenzenea
Catalyst
Temp. loading Yieldb
Entry Base
Solvent
(°C)
(mol%) (%)
1
2
K2CO3 DMA
K2CO3 NMP
K2CO3 DMSO
K2CO3 DME
K2CO3 DMF
K2CO3 DMA–DMSO (3 : 1)
K2CO3 DMA–DME (7 : 3)
K2CO3 DMA–DME–DMSO (3 : 3 : 1) 130
K2CO3 DMA–DME–DMSO (7 : 2 : 1) 130
CeCO3 DMA–DME–DMSO (7 : 2 : 1) 130
Na2CO3 DMA–DME–DMSO (7 : 2 : 1) 130
130
130
130
120
130
130
130
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
1
2
1
48
34
Trace
22
29
36
40
55
78
Trace
30
N.R.
58
46
15
Trace
54
78
3
4c
5
Scheme 3 Reactions of 2- and 3-picolinic acids with 2,4-dimethoxy
bromobenzene. Reaction conditions: PdCl2 (2 mol%), bdppmapy (2 mol%),
K2CO3 (1.2 mmol) and Cu2O (0.6 mmol) in DMA–DME–DMSO = 7 : 2 : 1
(2 mL), a nitrogen atmosphere, 150 mg 3 Å MS, 24 h at 130 °C.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
from 1% to 2%. However, the yield of 4a could not be
improved using 5 mol% and 10 mol% of catalyst loading. To
this end, the optimized reaction conditions could be fixed as
follows: PdCl2 (2 mol%), bdppmapy (2 mol%) in DMA–DME–
DMSO = 7 : 2 : 1 (2 mL) and in the presence of K2CO3
(1.2 mmol) and Cu2O (0.6 mmol) at 130 °C. In order to explore
the reactivity of this catalytic system, 2- and 3-picolinic acids
(2a and 2b) were introduced into the model system to react
with 2,4-dimethoxy bromobenzene under the optimized con-
ditions (Scheme 3). Low yields (25% for 4b and ∼0% for 4c)
were obtained, which implied that bdppmapy in this system
was highly selective for 4-picolinic acid. To this end, only
4-picolinic acid was utilized in this work.
The substituent effects of aryl bromides on this reaction
were investigated under the above optimized conditions
(Table 4). The p-substituted aryl bromides provided the desired
products in moderate yields no matter whether the substituted
groups were electron-donating groups or electron withdraw
groups (3a, 5a–5c, 5j, 5k). In the case of strong electron with-
drawing groups like CF3 and F, lower yields were obtained (5j,
5k). Furthermore, the positions of aryl bromides did affect the
reactions. Those with m-substituted aryl bromides had low
yields while those with o-mono-substituted aryl bromides
offered good yields (5b–5e). However reactions with the o-di-
substituted aryl bromide gave a negative result in the same
yield with m-disubstituted aryl bromide because the steric
effect of bdppmapy was the key factor in this situation
(Table 4, 5g, 5h). To expand the scope of this reaction, we
employed some other substrates (5l–5n), which gave medium
yields (6i–6n).
NEt3
K3PO4
DMA–DME–DMSO (7 : 2 : 1) 130
DMA–DME–DMSO (7 : 2 : 1) 130
K2CO3 DMA–DME–DMSO (7 : 2 : 1) 120
K2CO3 DMA–DME–DMSO (7 : 2 : 1) 110
K2CO3 DMA–DME–DMSO (7 : 2 : 1) 140
K2CO3 DMA–DME–DMSO (7 : 2 : 1) 140
K2CO3 DMA–DME–DMSO (7 : 2 : 1) 140
K2CO3 DMA–DME–DMSO (7 : 2 : 1) 140
75
a Reaction conditions: 2 (0.4 mmol), 3a (0.6 mmol), a solution of PdCl2
(178 mg) and bdppmapy (492 mg) in DMA (10 mL) as the catalyst, base
(1.2 mmol), solvent (2 mL), Cu2O (0.6 mmol), 24 h, a nitrogen
atmosphere, 150 mg 3 Å MS. b Isolated yields based on 2. c Pure DME
could not reach 130 °C.
Cu(I) or Ag(I) salts including Ag2O, Ag2CO3, Cu2O, CuI, CuBr,
CuCl and CuCO3 (Table 2, entries 6 and 10–15) as additives
were introduced into the reaction system. With the optimized
Pd source (PdCl2) and the phosphine ligand (bdppmapy),
Cu2O clearly exhibited the best activity. Ag(I) salts were not
suitable in this reaction due to the trace yields of the expected
products, though some Pd/Ag co-catalyzed decarboxylative
cross-coupling reactions were also reported.7b–d
The model reaction was further explored to optimize the
solvent, base, reaction temperature and catalyst loading. As
indicated in entries 1–7 in Table 3, analogous reactions in
pure DMA, N-methylpyrrolidone (NMP), DMSO, DME and
DMF as well as mixed DMA–DMSO could only give low yields.
Su et al. reported that during the decarboxylative cross-coup-
ling reactions, addition of DME into the polar solvents such as
NMP and DMF could improve the yield of 4a by nearly 30%,
and prevent the formation of side-products derived from
proto-decarboxylation.7b,8g Therefore we introduced DME into
our system to make a mixture of DMA–DME–DMSO = 7 : 2 : 1.
The best yield (78%) was attained (entry 9) in such a mixed
solvent system. In this solvent system, several bases were also
examined (entries 9–13), and K2CO3 was proved to be most
suitable. In addition, the temperature was examined in the
range of 110–140 °C (entries 9 and 14–16). Obviously, when
the temperature was raised, the yield of 4a increased from 15%
(110 °C) to 78% (130 °C), but decreased to a trace at 140 °C,
which may be due to the decomposition of the phosphine
ligand at higher temperature.16e The influence of the catalyst
loading was also explored (entries 9 and 17–19). The yield of
4a increased to 78% when the catalyst loading was enhanced
As an expansion of this work, chlorobenzene was used to
replace aryl bromides. It was quite inactive and only a trace
product (6o) was monitored by LC-MS in the reaction (5o in
Table 4). In addition, iodobenzene was also employed and the
yield of 6o was found to be only 18% as a large number of by-
products were observed which might be due to the homo-coup-
ling of iodobenzene (5p in Table 4).
Conclusions
In the work reported here, we demonstrated the design, syn-
thesis and structural characterization of a new mononuclear
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Table 4 The palladium-catalyzed decarboxylative cross-coupling reac-
tions of 4-picolinic acid with aryl bromidesa
Table 4 (Contd.)
Substrate
Product
Yieldb (%)
48
Substrate
Product
Yieldb (%)
78
58
44
62
83
54
71
68
63
42
75
40
54
Trace
18
44
a Reaction conditions: 2 (0.4 mmol), 5a (0.6 mmol), a 0.08 mL solution
of PdCl2 (178 mg) and bdppmapy (492 mg) in DMA (10 mL), K2CO3
(1.2 mmol), DME (1.4 mL), DMA (0.32 mL) and DMSO (0.2 mL), Cu2O
(0.6 mmol), 24 h, a nitrogen atmosphere, 150 mg 3 Å MS. b Isolated
yields based on 2.
9792 | Dalton Trans., 2014, 43, 9786–9794
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Pd(II) complex 1. Complex 1, assisted by a Cu2O additive, was
employed as an efficient catalyst for the decarboxylative cross-
coupling reactions of 4-picolinic acid with aryl bromides. Under
the optimized conditions, the yield of the resulting 4-aryl-
pyridines (compound 6a) was up to 83% with only 2% catalyst
loading. The method reported in this work provides an effective
route to synthesize 4-aryl-pyridines from commercially economi-
cal and stable 4-picolinic acid. It is anticipated that more P–N
hybrid ligands could be used to design and synthesize other
Pd(II) complexes with new structures and higher catalytic activi-
ties. Further studies in this direction are currently underway.
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Acknowledgements
The authors thank the National Natural Science Foundation of
China (21271134, 21171124, 21373142 and 21101109) and the
State Key Laboratory of Organometallic Chemistry of Shanghai
Institute of Organic Chemistry (201201006) for financial
support. J. P. Lang also highly appreciate the support from the
Qin-Lan Project and the “333” Project of Jiangsu Province (no.
BRA2012139), the Priority Academic Program Development of
Jiangsu Higher Education Institutions, and the “SooChow
Scholar” Program of Soochow University. The authors also
greatly thank the editor and the reviewers for their helpful
comments.
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