Electrochemical ortho functionalization of 2-phenylpyridine with perfluorocarboxylic acids catalyzed by palladium in higher oxidation states

Article abstract of DOI:10.1021/om400492g

The electochemical oxidation of palladium acetate or palladium perfluoroacetate in the presence of 2-phenylpyridine promotes catalytic ortho C-H substitution reactions. As possible intermediates, Pd(II) metallacycles with Pd-bound acetate, perfluoroacetat

Full text of DOI:10.1021/om400492g

Article
pubs.acs.org/Organometallics
Electrochemical Ortho Functionalization of 2‑Phenylpyridine with
Perfluorocarboxylic Acids Catalyzed by Palladium in Higher
Oxidation States
Yulia B. Dudkina,† Dmitry Y. Mikhaylov,† Tatyana V. Gryaznova,† Artem I. Tufatullin,†
Olga N. Kataeva,†,‡ David A. Vicic,§ and Yulia H. Budnikova*,†
†A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of Russian Academy of Sciences, 8 Arbuzov str.,
Kazan 420088, Russian Federation
‡Kazan Federal University, 18 Kremlevskaya str., Kazan 420008, Russian Federation
§Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States
S
* Supporting Information
ABSTRACT: The electochemical oxidation of palladium
acetate or palladium perfluoroacetate in the presence of 2-
phenylpyridine promotes catalytic ortho C−H substitution
reactions. As possible intermediates, Pd(II) metallacycles with
Pd-bound acetate, perfluoroacetate, and perfluoroheptanoate
substituents have been isolated and characterized: binuclear
[(PhPy)Pd(μ-OAc)]2 and [(PhPy)Pd(μ-TFA)]2 and mono-
nuclear [(PhPy)Pd(TFA)](CH3CN), [(PhPy)Pd(TFA)]-
(PhPy), and [(PhPy)Pd(PFH)](PhPy). The fluorinated deriv-
atives were found to exist in solvent-dependent equilibria
between mononuclear and binuclear forms. Cyclic voltammetry
was used to elucidate redox properties of the palladacycles and
the oxidation route to the final products.
oxidative functionalization reactions involve palladium(III)14−16
INTRODUCTION
■
and palladium(IV)17−19 intermediates that makes chemistry of
Transformation of C−H bonds into diverse functional groups is
a highly attractive strategy in organic synthesis. Mild and
selective, it is relevant in all chemical fields, including the
palladium in higher oxidation states a challenging and advanced
field. Understanding the properties of PdII, PdIII, and PdIV
complexes should prove useful for the development of novel
synthesis of pharmaceuticals, natural products, agrochemicals,
catalysts in these types of transformations.
polymers, and other different chemicals. Over the past decade,
the field of transition-metal-catalyzed C−H bond functionaliza-
tions has developed rapidly, providing new synthetic methods
With regard to new methods involving high-valent palladium
catalysts, one of the major problems is that the co-oxidants used
and detailed mechanistic studies.1−3 Due to the functional
mostly require stoichiometric transition-metal or organic
oxidants, such as Ag salts, organic peroxides, and electrophilic
group tolerance of palladium and the ability to install many
fluorine reagents, which are often either expensive to purchase
different types of bonds (C−O, C−Hal, C−N, C−S, C−C)
or difficult to separate from the product mixture.1,10,18
palladium complexes are particularly applicable catalysts for
such transformations,1−3 with palladium acetate being the most
Moreover, every time a new ligand or substrate or reaction
condition is used, extensive screenings are necessary in order to
commonly used. The chemically similar palladium trifluor-
identify the optimal oxidant. Potential-controlled electrolysis
oacetate (TFA) is less commonly utilized, though it is known
under mild conditions is a feasible alternative to the classical
to be an efficient catalyst and cocatalyst in rearrangements
methods that have been demonstrated by Kakiuchi20 in the
involving C−H bond activation.4,5 Moreover, in some cases
effective electrochemical halogenation of arylpyridine deriva-
tives.
Recently, we discovered a one-step catalytic method for
ortho fluoroalkylation of 2-phenylpyridine that employs higher
trifluoroacetic acid additive was found to be critical in achieving
a high efficiency of C−H functionalization6 and the
trifluoroacetate ligand was used to improve the solubilities
and catalytic activities of transition-metal complexes.7
oxidation states of nickel or palladium by electro-oxidation of
The catalitic cycles of Pd-catalyzed reactions are generally
stable MII precursors.21The aim of this work is to further
based on PdII/Pd0 shuttles which have been extensively
investigated for the past decades,2,8−12 including the known
contribution of electrochemistry to the mechanistic study.13
Received: May 30, 2013
However, a variety of Pd-catalyzed ligand-directed C−H
© XXXX American Chemical Society
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develop that methodology toward alternative approaches to
directed perfluorocarboxylation of C−H bonds of aromatic
compounds, to characterize fluorinated palladacyclic inter-
mediates, and to investigate the effect of changing acetate to
trifluoroacetate ligand for catalytic efficiency.
We were unable to isolate nonfluorinated counterparts of
complex 3 and 4 using the same procedures. However,
nonfluorinated mononuclear palladacycles similar to 3 and 4
were recently proposed as intermediates in Pd(OAc)2-catalyzed
C−H activation reactions.23
Crystals of complexes 3−5 were grown, and the results of the
X-ray diffraction studies are provided in Figures 1−3. The
RESULTS AND DISCUSSION
■
Acetic acid, trifluoroacetic acid, and longer chain perfluoroalkyl
carboxylic acids can be used as substrates for the substitution
and functionalization of C−H bonds.1,14,21 We therefore set out
to investigate the differences in the nature and electrochemical
properties of acetate- and perfluoroacetate-bridged pallada-
cycles that would be competent as reaction intermediates in the
functionalization of a C−H bond of phenylpyridine using
CH3COOH and CF3COOH. The palladium acetate dimer 1 is
already known and was synthesized using the method described
by Ritter and co-workers (eq 1).14
Figure 1. ORTEP representation of the molecular complex [(PhPy)-
Pd(TFA)](CH3CN) (3). Thermal ellipsoids are drawn at the 50%
probability level. Selected bond distances (Å): Pd−C = 1.928; Pd−N1
= 1.995; Pd−O = 2.147; Pd−N2 = 2.015.
We found that synthesis of the palladium trifluoroacetate
(TFA) derivatives was not as straightforward as the preparation
of the nonfluorinated analogues. Treatment of palladium
trifluoroacetate with 2-phenylpyridine (PhPy) yields, depend-
ing on the solvent employed, either the dimeric or monomeric
palladacycles as described in Scheme 1. If dichloromethane is
Scheme 1. Solvent-Dependent Reactivity of Palladium(II)
Trifluoroacetate with 2-Phenylpyridine
Figure 2. ORTEP representation of the molecular complex [(PhPy)-
Pd(TFA)](PhPy) (4). Thermal ellipsoids are drawn at the 50%
probability level. Selected bond distances (Å): Pd−C = 1.970; Pd−N1
= 2.055; Pd−O = 2.141; Pd−N2 = 2.012.
palladium centers in 3−5 all adopt a square-planar geometry,
with the two nitrogen ligands bound trans to each other. The
trifluoroacetate was always found to be bound trans to the
carbon of the aryl ligand.
Bimetallic palladium complexes are known to occur in
temperature -dependent equilibrium with mononuclear species
in the presence of nitrogenous ligands. The equilibrium was
first reported in 1987 by Ryabov24 in terms of bridge-splitting
equilibria between chlorine- or acetate-bridged cyclopalladated
complexes and free N donor ligands. Subsequently it was
shown that the dinuclear palladium complex is favored and
exhibits redox synergy in oxidative catalysis.25,26 We found that
used as solvent, the previously reported dinuclear complex 222
is produced in 64% yield. However, if acetonitrile or acetone is
used as the reaction medium, the dimer formation is suppressed
and mononuclear 3 or 4 could be produced in 85 and 86%
yields, respectively. Complex 5, which has a structure similar to
that of 4 but with a longer fluoroalkyl chain, can be obtained as
a product of 2-phenylpyridine treatment with perfluorohepta-
noic (PFH) acid in acetonitrile (88% yield).
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solvent effects to have major consequences for the redox
chemistry of the palladium complexes. Figure 4 shows scans of
Figure 3. ORTEP representation of the molecular complex [(PhPy)-
Pd(PFH)](PhPy) (5). Thermal ellipsoids are drawn at the 50%
probability level. Selected bond distances (Å): Pd−C = 1.973; Pd−N1
= 2.017; Pd−O = 2.141; Pd−N2 = 2.041.
Figure 4. Cyclic voltammograms of [(PhPy)Pd(μ-OAc)] (1; red) and
[(PhPy)Pd(μ-TFA)]2 (2; blue) in CH2Cl2.
monomer−dimer equilibria for trifluoroacetate derivatives
exhibit strong solvent dependence. Thus, the solvent governs
the formation of various palladium complexes and the
substitution of reaction media involves a change in the
environment (Scheme 2). Evidence is obtained for these
acetate-bridged 1 and trifluoroacetate-bridged 2 dimers in
CH2Cl2. Cyclic voltammograms of acetate dimer 1 and
trifluoroacetate monomer 3 in acetonitrile solvent are
represented in Figure 5. The oxidation potentials of complexes
Scheme 2. Solvent-Dependent Monomer−Dimer
Equilibrium of 2-Phenylpyridyl Palladium(II)
Trifluoroacetate
Figure 5. Cyclic voltammograms of [(PhPy)Pd(μ-OAc)]2 (1; red)
and [(PhPy)Pd(TFA)](CH3CN) (3; blue) in CH3CN.
1
interconversions by H NMR spectroscopy on the isolated
crystalline complexes 2−5 in different solvents. For complexes
2 and 3, fast interconversions are observed. When complex 2 is
dissolved in CD3CN solvent, quantitative conversion to 3 is
observed. However, solutions of 3 in CD2Cl2 exhibit two
groups of signals for both dinuclear 2 and mononuclear 3,
supporting a ratio of 1:0.15 for 2 and 3. Palladacycles 4 and 5
were found to persist in noncoordinatng solvents such as
chloroform and acetone, showing insignificant dimerization.
However the NMR spectrum of complex 4 dissolved in
CD3CN reveals broad signals attributed to initial complex 4,
complex 3, and free 2-phenylpyridine resulting from fast
exchange processes in the solution. Such behavior is also
observed for compound 5.
1−3 are collected in Table 1. It should be noted that the
oxidation potential of free PhPy is 2.15 V in CH2Cl2 or 2.08 V
in CH3CN, and no significant peaks for Pd(TFA)2 could be
observed under the applied conditions.
The electrochemistry of [(PhPy)Pd(μ-OAc)]2 and [(PhPy)-
Pd(μ-TFA)]2 in CH2Cl2 and CH3CN with 0.1 M TBAPF6 as
Table 1. Oxidation Potentials (in V) for Palladacycles 1−3
CH2Cl2 solvent
CH3CN solvent
complex
E1
E2
E3
E1
E2
a
a
1
2
3
0.89
1.18
1.27
1.67
1.46
0.88
1.82
turns into 3
a
A cyclic voltammetry study performed in dichloromethane or
acetonitrile solution containing TBATFB as the supporting
electrolyte and Ag/AgCl as the reference electrode reveals the
turns into 2
1.72
1.90
a
For irreversible waves Epa is given.
C
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the supporting electrolyte was described recently.22 The report
shows clearly that acetate-bridged 1 is more easily oxidized than
TFA-bridged 2, which is consistent with our data (Figure 4 and
Table 1). However, the shapes and the number of observed
peaks in the cyclic voltammograms are somewhat different from
those observed under our conditions. In our study, not only do
we rationalize the solvent effect behavior but we also derive a
different number of electrons for the oxidation stages (see
below). The electron number was determined using methods27
including chronoamperometry and steady-state voltammetry at
a microelectrode.
In CH2Cl2 solution the acetate derivative 1 (Figure 4, red
curve) undergoes three reversible oxidations, which are
assigned to PdII−PdII/PdIII−PdIII, PdIII−PdIII/PdIII−PdIV, and
PdIII−PdIV/PdIV−PdIV couples (the absolute value of the
number of electrons consumed in the first oxidation step was
found to be 2.2). A proposed rationale for the redox behavior of
1 is provided in Scheme 3. In CH3CN 1 is oxidized successively
Figure 6. ESR spectrum obtained upon oxidation of [(PhPy)Pd(μ-
OAc)]2 (1) (CH2Cl2 solution, room temperature, 0.9 V vs Ag/AgCl).
Thus, this feature probably indicates one-dimensional PdIII
metallacyclic wire formation with oxidation of complex 1.31
The unique feature of trifluoroacetate derivatives to form
mono- or dinuclear palladacycles allowed for an investigation of
the activity of different catalytic forms. A set of electrochemical
oxidative C−H functionalization reactions of 2-phenylpyridine
was explored (Table 2).
Scheme 3. Proposed Mechanism for the Electrooxidation of
1 in CH2Cl2
Table 2. Pd-Catalyzed Functionalization of 2-Phenylpyridine
with Perfluoroalkyl Carboxylic Acids
entry
catalyst
solvent
acid
product (yield, %)
in reversible and irreversible steps conforming to PdII−PdII/
PdIII−PdIII and PdIII−PdIII/PdIV−PdIV couples, respectively
(Figure 5, red curve). In both solvents 1 exists in a dimeric
form.
The cyclic voltammetry data confirm the solvent-dependent
nuclearity transformation of the trifluoroacetate derivatives.
The voltammogram for dimer 2 features two quasi-reversible
two-electron (measured: 2.1) waves, assigned to PdII−PdII/
PdIII−PdIII and PdIII−PdIII/PdIV−PdIV couples (Figure 4, blue
curve). The mononuclear complex 3 exhibits two very closely
spaced one-electron irreversible oxidations assigned as PdII/
PdIII and PdIII/PdIV couples (Figure 5, blue curve). Thus,
conversion of the dimer into a mononuclear form reveals
irreversibility in electrochemical oxidation.
a
1
Pd(OAc)2
Pd(TFA)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
Pd(OAc)2
Pd(TFA)2
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
CH2Cl2
CH3CN
CH3CN
CH3CN
C6F13COOH
C6F13COOH
C6F13COOH
HC4F8COOH
HC4F8COOH
CH3COOH
HC4F8COOH
CF3COOH
6a (65)
a
2
6a (62)
b
3
6a (15) + 6b (37)
7a (73)
a
4
a
5
7a (14)
a
6
9a (trace)
7a (68)
a
7
a
8
8a (trace)
a
9
CF3COOH
a
The current strength was 100 mA h−1. 2 F of electricity was passed
b
per PhPy. The current strength was 250 mA h−1. 3 F of electricity was
passed per PhPy.
Under spectroelectrochemical conditions, we were able to
observe an ESR spectrum of the intermediate at the first
oxidation peak of dimer 1 in CH2Cl2 (Figure 6). As an ESR
spectrum for the PdIII−PdIII dimer could not be obtained due
to the fact that the two PdIII centers are strongly
antiferromagnetically coupled;8 the signal (g = 2.14 and ΔH
= 40 Hz) was consistent with a paramagnetic mixed-valence
PdII−PdIII state formed in the comproportionation reaction
between the initial PdII−PdII and final PdIII−PdIII species
(Scheme 3). A similar reaction was previously observed under
ESR spectroelectrochemical conditions for another two-
electron process with NiIIbpyX2 (bpy = 2,2′-bipyridine).28,29
Our effort to obtain a low-T spectrum of 1 failed because the
signal disappeared, the solution being frozen (150 K). The
decrease in intensity of the spectrum with decreasing
temperature, in contrast with Curie law, is an attribute for
dimerization and polymerization processes in the solution.30
Pd(OAc)2 and Pd(TFA)2 were used as catalysts and
dichloromethane or acetonitrile was the reaction medium.
Experiments using Pd(TFA)2 in CH2Cl2 were not performed,
owing to its low solubility. As seen in Table 2, the C−H
functionalization products were obtained in good or moderate
yields, despite the complete conversion of PhPy. Gratifyingly,
we note that the electrochemical oxidations of the palladium
complexes indeed lead to perfluorocarboxylate or perfluoroalkyl
derivatives as described in Table 2. In the case of long-chain
perfluorocarboxylic acids, it is possible to obtain perfluoroalkyl
products by increasing the current strength and the amount of
passed electricity.
It is noteworthy that both Pd(OAc)2 and Pd(TFA)2 catalysts
are equally efficient in CH3CN. We suggest that functionaliza-
tion of PhPy with perfluorocarboxylic acids in CH3CN runs
through a mononuclear fluorinated palladium(II) complex
formation for both Pd(OAc)2 and Pd(TFA)2 catalysts. In
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support of this, the characterized mononuclear complex 3,
resulting from carboxylic acid exchange, was obtained as the
product of complete conversion of 1 treated with trifluoroacetic
acid in acetonitrile (eq 2).
oxidation, suggesting binuclear PdIII−PdIII complex formation,
and then again to a yellow solution after completion of the
reaction. These color changes are consistent with previous
studies.14
In CH2Cl2 solution, complete conversion of acetate-bridged
binuclear complex 1 into a perfluorocarboxylate-bridged dimer
occurred by treatment with an excess of perfluorocarboxylic
acid, as can be seen from the CV experiment (Figure 7).
Addition of perfluoroheptanoic acid to 1 suspended in
acetonitrile resulted in a soluble material, suggesting
fragmentation of 1 to a mononuclear PdII complex (Scheme
5). Electrochemical oxidation of this solution presumably leads
to a mononuclear PdIV complex, further treatment of which
with phenylpyridine afforded 6a (67%).
To get more insight into the monomer−dimer trans-
formations during the C−H functionalization reactions, a
sequence of electrochemical oxidation and further stoichio-
metric reactions with 2-phenylpyridine with [(PhPy)Pd(μ-
OAc)]2 (1) was performed.
CONCLUSIONS
■
A convenient approach to C−H substitution products of 2-
phenylpyridine using perfluoroalkyl carboxylic acid containing
substituents is provided and is based on the electrochemical
generation of palladium in high oxidation states. The
fluorinated acid substituents make the oxidation of the dimeric
palladium cycles more difficult in comparison to nonfluorinated
compounds. We have also been able to record a signal in the
ESR spectrum during the electro-oxidation of the dimer
[(PhPy)Pd(μ-OAc)]2 and have tentatively assigned this signal
to result from a mixed-valent PdII−PdIII state. We also report
that phenylpyridine palladium(II) trifluoroacetate derivatives
were found to exist as mononuclear or binuclear complexes in
the presence or absence of nitrogen donor ligands. The NMR
and electrochemical data support the notion that C−H
functionalization reactions of 2-phenylpyridine with perfluor-
ocarboxylic acids proceed through mononuclear PdII inter-
mediates in acetonitrile or binuclear fluorinated PdII
intermediates in dichloromethane.
Scheme 4. Conversion of 1 into 9a in CH2Cl2
EXPERIMENTAL SECTION
■
General Considerations. All syntheses were carried out under a
dry argon atmosphere. Yields refer to isolated and purified
compounds. NMR experiments were carried out with Bruker
AVANCE-400 spectrometers (400.1 MHz (1H), 100.6 MHz (13C),
376.5 MHz (19F)). Chemical shifts are reported on the δ (ppm) scale
relative to the residual solvent signals for H and 13C and to external
1
C6F6 (−164.9 ppm) for 19F NMR spectra. NMR data are reported as
follows: resonance, multiplicity (singlet (s), doublet (d), triplet (t),
quartet (q), quintet (quin), sextet (sext), multiplet (m), broad
resonance (br)), coupling constants in Hz, integration. The ESR
spectrum was recorded in degassed dichloromethane solution at room
temperature on a Bruker EMX spectrometer. The supporting salt
Et4NBF4 was prepared by mixing an aqueous solution of Et4NOH
(30%) and HBF4 until a neutral pH value was achieved. The
precipitate that formed (Et4NBF4) was filtered off, recrystallized from
ethanol, and dried in a vacuum desiccator at 100 °C for 48 h. Bu4NBF4
was purchased from Aldrich and was used without further purification.
Anhydrous solvents were obtained either by distillation over
Figure 7. Cyclic voltammograms of [(PhPy)Pd(μ-OAc)]2 in the
absence (red) and in the presence (6 equiv) of H(CF2)4COOH
(violet) in CH2Cl2 in comparison with [(PhPy)Pd(μ-TFA)]2 (blue).
Dimer 1 was electrochemically oxidized in the presence of
acetic acid in dichloromethane (Scheme 4). Treatment of this
oxidized material with 2-phenylpyridine afforded the product
9a in 61% yield. During this sequence the reaction mixture
changed from yellow to brownish red upon electrochemical
Scheme 5. Conversion of 1 into 6a in CH3CN
E
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phosphoric anhydride (dichloromethane, acetonitrile, ethyl acetate) or
by distillation over sodium/benzophenone (ether, pentane, hexane).
Trifluoroacetic, perfluoroheptanoic, and 5H-perfluoropentanoic acids
(purchased from P&M Invest), 2-phenylpyridine (Acros Organics),
acetic acid (Acros Organics), and palladium acetate (Alfa Aesar) were
used as received.
CH3CN. The reaction mixture was stirred for 4 h, washed with water,
and dried over MgSO4, and then the solution was concentrated in
vacuo to afford 1.53 g of a colorless crystalline solid (88% yield). H
1
NMR (400.0 MHz, CDCl3): δ 9.38 (ddd, J = 5.7, 1.5, 0.9 Hz, 1H);
8.41 (ddd, J = 5.7, 1.5, 0.9 Hz, 1H); 8.10 (dd, J = 7.9, 1.5 Hz, 2H);
7.95 (td, J = 7.8, 1.6 Hz, 1H); 7.76 (td, J = 7.8, 1.6 Hz, 1H); 7.65 (ddd,
J = 7.9, 1.2, 0.6 Hz, 1H); 7.56 (br d, J = 7.9 Hz, 1H); 7.44−7.37 (m,
4H); 7.31 (dd, J = 7.7, 1.3 Hz, 1H); 7.09 (ddd, J = 7.3, 5.8, 1.4 Hz,
1H); 6.99 (td, J = 7.5, 1.0 Hz, 1H); 6.81 (td, J = 7.6, 1.4 Hz, 1H); 6.03
(dd, J = 7.8, 0.9 Hz, 1H). 13C NMR (100.6 MHz, CDCl3): δ 165.01;
Electrochemistry. Cyclic voltammograms were recorded with a
BASi Epsilon potentiostat (USA) at room temperature in dichloro-
methane (5 × 10−3 substrate concentration) or acetonitrile (10−3
substrate concentration) solution. Bu4NBF4 (0.1 M) was used as the
supporting electrolyte, and a glassy-carbon electrode was used as the
working electrode. The auxiliary electrode was a platinum rod. All
potentials are referenced against the Ag/AgCl redox couple. The scan
rate was 100 mV s−1. Preparative electrolyses were carried out using a
B5-49 dc source at a current strength of 100 mA h−1 in a 30 mL three-
electrode cell. The potential of the working electrode was detected by
a V7−27 dc voltmeter in reference to Ag/AgCl electrode. The surface
area of the platinum cylindrical anode used as the working electrode
was 20.0 cm2. A ceramic plate with a pore size of 10 μm was used as a
membrane. A platinum grid served as a cathode, and the catholyte was
a saturated solution of the background used in the anolyte in the
corresponding solvent.
2
161.98 (t, JCF = 25.1 Hz); 161.68; 153.97; 150.87; 149.47; 145.30;
140.04; 138.81; 138.45; 133.14; 129.58; 129.17; 128.94; 128.78;
128.74; 128.37; 126.08; 124.46; 123.10; 123.01; 122.09; 118.29; CF3
and CF2 groups could not be resolved. 19F NMR (376.5 MHz,
(CD3)2CO): δ −81.69; −116.50; −122.24; −122.90; −123.37;
−126.70.
Synthesis of Ortho-Substituted 2-Phenylpyridine. In an
electrochemical cell, 7 mmol of 2-phenylpyridine, 14 mmol of
substrate acid, and 0.7 mmol of the corresponding Pd salt were
dissolved in 20 mL of solvent. Electrolysis was conducted in a divided
cell at a platinum anode with the use of Et4NBF4 (0.1 M, 0.43 g) or
Bu4NBF4 (0.1 M, 0.66 g) as background electrolyte in CH3CN or
CH2Cl2, respectively, with stirring under a constant stream of argon.
At the end of the electrolysis, the reaction mixture was placed in a 50
mL flask, and the solvent was removed on a rotary evaporator. The
residue was then washed with water and extracted with benzene (three
times with 50 mL). The organic layer was dried over MgSO4, and then
the solvent was removed. The product was purified by silica gel
column chromatography (ethyl acetate−hexane). Yields of products
are provided below.
Preparation of Palladium Complexes. Palladium Trifluoroa-
cetate. The procedure reported by Wilkinson et al. was used, without
further modification.32 [(PhPy)Pd(μ-OAc)]2 (1) was synthesized
utilizing the method described by Ritter.14 The 1H and 13C NMR data
for 1 matched those reported in the reference.14
[(PhPy)Pd(μ-TFA)]2 (2). Palladium trifluoroacetate (1.04 g, 3.1
mmol, 1 equiv) was added to a solution of 2-phenylpyridine (0.48 g,
3.1 mmol, 1 equiv) in CH2Cl2 (30 mL). After it was stirred for 4 h at
40 °C, the yellow solution was concentrated in vacuo and the product
was precipitated with Et2O (7 mL). The solid was isolated by filtration,
washed with Et2O (2 × 3 mL), and dried in vacuo to afford 0.99 g of a
yellow powder (65% yield). The 1H and 13C NMR data for 1 matched
those reported in ref 22. 19F NMR (376.5 MHz, CD2Cl2): δ −74.81.
[(PhPy)Pd(TFA)](CH3CN) (3). Palladium trifluoroacetate (1.04 g, 3.1
mmol, 1 equiv) was added to a solution of 2-phenylpyridine (0.48 g,
3.1 mmol, 1.00 equiv) in CH3CN (30 mL). After it was stirred for 4 h
at 21 °C, the yellow solution was concentrated in vacuo and left
overnight, and then the colorless crystalline solid was precipitated from
the mixture and filtered, and dried in vacuo to afford 1.08 g of the
product (84% yield). 1H NMR (400.0 MHz, CD3CN): δ 8.30 (br d, J
= 5.2 Hz, 1H); 7.96 (ddd, J = 8.1, 7.5, 1.5 Hz, 1H); 7.79 (br d, J = 7.9
Hz, 1H); 7.51 (dd, J = 7.6, 1.3 Hz, 1H); 7.26 (ddd, J = 7.3, 5.7, 1.4 Hz,
1H); 7.19 (br, 1H); 7.16 (td, J = 7.5, 1.1 Hz, 1H); 7.07 (td, J = 7.5, 1.5
Hz); 1.96 (s, 3H). 13C NMR (100.6 MHz, CD3CN): δ 166.15; 161.06
Yields of 2-(Pyridin-2′-yl)phenyl Perfluoroheptanoate (6a) under
Different Reaction Conditions. Yield: 2.35 g (65%), catalyst
Pd(OAc)2, solvent CH3CN, current strength was 100 mA h−1, 2 F
of electricity was passed per PhPy; 2.24 g (62%), catalyst Pd(TFA)2,
solvent CH3CN, current strength was 100 mA h−1, 2 F of electricity
was passed per PhPy; 0.54 g (15%), catalyst Pd(OAc)2, solvent
CH3CN, current strength was 250 mA h−1, 3 F of electricity was
passed per PhPy. 1H NMR (400.0 MHz, CDCl3): δ 9.05 (br d, J = 5.1
Hz, 1H); 8.26 (td, J = 7.9, 0.9 Hz, 1H); 7.97 (dd, J = 8.1, 0.8 Hz, 1H);
7.93−7.90 (m, 2H); 7.70−7.67 (m, 1H); 7.59−7.54 (m, 2H). 13C
2
NMR (100.6 MHz, CDCl3): δ 162,08 (t, JCF = 25.1 Hz); 154.02;
143.91; 143.63; 132.02; 131.64; 129.38 (2C); 127.90 (2C); 124.41;
1
2
124.12; 117.12 (qt, JCF = 288.3 Hz, JCF = 33.2 Hz); 110.96 (tquin,
1JCF = 270.5 Hz, 2JCF = 33.0 Hz); 110.96 (tquin, 1JCF = 270.5 Hz, 2JCF
= 33.0 Hz); 110.41 (tquin, JCF = 269.2 Hz, JCF = 33.3 Hz); 109.10
1
2
1
2
1
(tt, JCF = 266.5 Hz, JCF = 31.7 Hz); 108.43 (tsext, JCF = 273.8 Hz,
2
2JCF = 35.6 Hz). 19F NMR (376.5 MHz, CDCl3): δ −81.22; −117.57;
(q, JCF = 34.5 Hz); 150.22; 150.08; 146.40; 141.25; 135.27; 130.40;
126.51; 124.92; 123.99; 120.26; CN and solvent peaks overlap; CF3
peak could not be resolved; 1.68. 19F NMR (376.5 MHz, CD3CN): δ
−75.35.
−121.95; −122.71; −123.01; −126.38.
Yields of 2-(2′-Perfluorohexylphenyl)pyridine (6b) under Differ-
ent Reaction Conditions. Yield: 1.23 g (37%), catalyst Pd(OAc)2,
solvent CH3CN, current strength was 250 mA h−1, 3 F of electricity
was passed per PhPy. 1H NMR (400.0 MHz, (CD3)2CO): δ 8.94 (d, J
= 4.4 Hz, 1H,); 8.33 (dt, J = 8.1, 1.7 Hz, 1H); 8.21 (br d, J = 8.2 Hz,
1H); 8.13 (m, 2H); 7.75 (ddd, J = 7.5, 5.3, 1.2 Hz, 1H); 7.58 (m, 2H).
[(PhPy)Pd(TFA)](PhPy) (4). Palladium trifluoroacetate (0.50 g, 1.5
mmol, 1 equiv) was added to a solution of 2-phenylpyridine (0.47 g,
3.0 mmol, 2 equiv) in (CH3)2CO (50 mL). After it was refluxed for 4
h at 21 °C, the light yellow solution was concentrated in vacuo and the
product was precipitated with Et2O (5 mL), isolated by filtration, and
dried in vacuo to afford 0.68 g of a light yellow powder (86% yield).
1H NMR (400.0 MHz, CDCl3): δ 9.38 (dd, J = 5.7, 0.9 Hz, 1H); 8.40
(dd, J = 5.7, 0.8 Hz, 1H); 8.11−8.08 (m, 2H); 7.96 (td, J = 7.8, 1.6 Hz,
1H); 7.76 (td, J = 7.8, 1.5 Hz, 1H); 7.65 (d, J = 7.8 Hz, 1H); 7.56 (d, J
= 8.0 Hz, 1H); 7.44−7.38 (m, 4H); 7.31 (dd, J = 7.7, 1.3 Hz, 1H);
7.10 (ddd, J = 7.2, 5.8, 1.5 Hz, 1H); 7.00 (td, J = 7.5, 1.0 Hz, 1H); 6.83
(td, J = 7.5, 1.4 Hz, 1H); 6.08 (dd, J = 7.8, 0.9 Hz, 1H). 13C NMR
2
13C NMR (100.6 MHz, (CD3)2CO): δ 160.43 (t, JCF = 25.3 Hz);
154.27; 145.24; 142.79; 134.29; 130.83; 129.12; 128.20; 127.76;
124.22; 123.65; 118.66 (tt, 1JCF = 287.3 Hz, 2JCF = 33.4 Hz); 115−105
1
2
(m, 4C); 109.93 (qt, JCF = 237.6 Hz, JCF = 33.7 Hz). 19F NMR
(376.5 MHz, (CD3)2CO): δ −81.57; −117.71; −122.07; −123.89;
−123.21; −126.58.
Yields of 2-(Pyridin-2′-yl)phenyl 5H-Perfluoropentanoate (7a)
under Different Reaction Conditions. Yield: 2.04 g (73%), catalyst
Pd(OAc)2, solvent CH3CN, current strength was 100 mA h−1, 2 F of
electricity was passed per PhPy; 0.39 g (14%), catalyst Pd(OAc)2,
solvent CH2Cl2, current strength was 100 mA h−1, 2 F of electricity
was passed per PhPy; 1.90 g (68%), catalyst Pd(TFA)2, solvent
CH3CN, current strength was 100 mA h−1, 2 F of electricity was
passed per PhPy. 1H NMR (400.0 MHz, CDCl3): δ 8.90 (br d, J = 3.2
Hz, 1H); 8.27 (t, J = 7.6 Hz, 1H); 7.97 (d, J = 8.0 Hz, 1H); 7.86−7.84
(m, 2H); 7.67 (br t, J = 5.5 Hz, 1H); 7.54−7.47 (m, 2H); 6.25 (tt, J =
2
(100.6 MHz, CDCl3): δ 165.00; 161.80 (q, JCF = 35.6 Hz); 161.73;
153.91; 150.95; 149.41; 145.30; 140.03; 138.84; 138.56; 133.18;
129.62; 129.20; 128.90 (2C); 128.41 (2C); 126.11; 124.49; 123.22;
1
123.12; 122.13; 118.30;116.62 (q, JCF = 291.6 Hz). 19F NMR (376.5
MHz, (CD3)2CO): δ −75.46.
[(PhPy)Pd(PFH)](PhPy) (5). Palladium acetate (0.50 g, 2.2 mmol, 1
equiv), perfluoroheptanoic acid (0.81 g, 2.2 mmol, 1 equiv), and 2-
phenylpyridine (0.70 g, 4.5 mmol, 2 equiv) were mixed in 30 mL of
F
dx.doi.org/10.1021/om400492g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
52.0, 5.6 Hz, 1H). 13C NMR (100.6 MHz, CDCl3): δ 162.43 (t, 2JCF
=
0.0686 (I > 2σ(I)), maximum/minimum residual electron density
0.763/−0.595 e/Å3, GOF = 1.026.
25.3 Hz); 153.84; 144.25; 143.31; 131.83; 131.66; 129.46 (2C);
1
2
Crystal data for 5. formula C29H17F13N2O2Pd, crystal size 0.31 ×
0.24 × 0.18 mm3, M = 778.85, monoclinic, space group P21/c, a =
18.167(3) Å, b = 11.479(2) Å, c = 14.622(3) Å, β = 101.087(2)°, V =
127.91 (2C); 124.57; 124.24; 107.76 (tt, JCF = 253.1 Hz, JCF = 29.3
Hz); 108.93 (tt, 1JCF = 264.3 Hz, 2JCF = 29.8 Hz); 110.08 (tquin, 1JCF
=
263.5 Hz, 2JCF = 30.0 Hz); 110.79 (tquin, 1JCF = 266.5 Hz, 2JCF = 30.5
Hz). 19F NMR (376.5 MHz, CDCl3): δ −118.11, −125.83, −130.67,
−137.87.
2992.3(9) Å3, Z = 4, ρcalcd = 1.729 g cm−3, μ = 0.732 mm−1, θmax
=
26.3°, 30998 reflections collected, 6030 independent (Rint = 0.0701)
and 4483 observed reflections (I > 2σ(I)), 424 refined parameters, R1
= 0.0338, wR2 = 0.0690 (I > 2σ(I)); maximum/minimum residual
electron density 0.825/−0.456 e/Å3, GOF = 0.949.
CCDC 940289−940291 contain supplementary crystallographic
data for this paper. These data can be obtained free of charge at www.
ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.;
fax +44(1223)336-033, e-mail deposit@ccdc.cam.ac.uk).
Yields of 2-(Pyridin-2′-yl)phenyl Trifluoroacetate (8a) under
Different Reaction Conditions. The product was observed in traces:
catalyst Pd(OAc)2, solvent CH3CN, current strength was 100 mA h−1,
2 F of electricity was passed per PhPy. 1H NMR (400.0 MHz, CDCl3):
δ 8.87 (d, J = 5.1 Hz, 1H); 8.23 (t, J = 7.8 Hz, 1H); 7.94 (d, J = 8.1 Hz,
1H); 7.86−7.83 (m, 2H); 7.63 (t, J = 6.4 Hz, 1H); 7.48−7.46 (m,
2
2H). 13C NMR (100.6 MHz, CDCl3): δ 161.41 (q, JCF = 36.3 Hz);
153.99; 143.74; 143.66; 132.27; 131.46; 129.34; 127.76; 124.28;
124.04;116.29 (q, 1JCF = 291.1 Hz). 19F NMR (376.5 MHz, CDCl3): δ
−75.45.
ASSOCIATED CONTENT
* Supporting Information
■
Yields of 2-(Pyridin-2′-yl)phenyl Acetate (9a). The product was
observed in traces: catalyst Pd(OAc)2, solvent CH2Cl2, current
strength was 100 mA h−1. 2 F of electricity was passed per PhPy.
S
CIF files giving X-ray structural information for 3−5 and figures
1
giving H, 13C, and 19F NMR spectra of new compounds. This
1
The H and 13C NMR data for 9a matched those reported in ref 14.
material is available free of charge via the Internet at http://
pubs.acs.org.
Reactions of Phenylpyridyl Palladium Acetate Dimer.
Equation 2. To a 0.7 mL suspension of 1 (10 mg, 0.016 mmol, 1
equiv) in CD3CN was added trifluoroacetic acid (7.3 mg, 0.064 mmol,
4 equiv). The reaction mixture immediately changed into a clear
AUTHOR INFORMATION
Corresponding Author
*E-mail for Y.H.B.: yulia@iopc.ru.
■
1
colorless solution. H NMR data revealed complete conversion of 1
into 3.
Scheme 4. A 20 mL solution of 1 (0.10 g, 0.16 mmol, 1 equiv),
acetic acid (0.05 g, 0.64 mmol, 4 equiv), and Et4NBF4 as a supporting
salt in CH2Cl2 was oxidized in an electrochemical cell (0.9 V vs Ag/
AgCl, 2 F of electricity per dimer); the solution changed from bright
yellow to brownish red. The reaction mixture was transferred via
cannula to a flask with 2-phenylpyridine33 (0.49 g, 3.1 mmol, 20 equiv)
and was heated under reflux for 4 h while the reaction mixture turned
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the RFBR grant 11-03-92662-
MCXa and by the Russian Ministry of Science and Education,
grant number 8446. D.A.V. thanks the U.S. NSF, grant CHE-
1124619, for support of this work.
1
clear yellow. According to H NMR data versus an internal standard,
9a was obtained in 61% yield.
Scheme 5. To a clear colorless solution obtained after addition of
perfluoroheptanoic acid (0.23 g, 0.64 mmol, 4 equiv) to 1 (0.10 g, 0.16
mmol, 1 equiv) suspended in 20 mL of acetonitrile was added
Et4NBF4 as a supporting salt. This mixture was electrochemically
oxidized (1.80 V vs Ag/AgCl, 2 F of electricity per original dimer).
The reaction mixture turned brownish red and was transferred via
cannula to a flask with 2-phenylpyridine33 (0.49 g, 3.1 mmol, 20 equiv)
and was heated under reflux for 4 h while the reaction mixture turned
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Details of X-ray Diffraction Measurements. Single-crystal X-ray
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APEX diffractometer and Mo Kα radiation (λ = 0.71073 Å). Programs
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dx.doi.org/10.1021/om400492g | Organometallics XXXX, XXX, XXX−XXX