Carbonylative Suzuki Coupling Reaction Catalyzed by a Hydrospirophosphorane Palladium Complex

Article abstract of DOI:10.1002/cctc.201700946

A Pd complex with the H-spirophosphorane ligand, PdCl₂{P(OCMe₂CMe₂O)OC₆H₄NH₂} (Cat. 1), was used as the catalyst in the carbonylative Suzuki coupling of substituted iodobenzenes with arylboronic acids and with sodium tetraphenylborate. Substituted diarylketones were obtained in good to excellent yields and a selectivity over 95 % under 1 atm of CO with 1.5 mol % of the catalyst. The promoting role of the H-spirophosphorane ligand in the catalytic process was evidenced. We used X-ray photoelectron spectroscopy, TEM, and ³¹P NMR spectroscopy to reveal that during the reaction Cat. 1 undergoes transformation into Pd complexes that bear the spirophosphorane ligand or other P ligands formed by its dealkylation and to Pd nanoparticles. All these Pd species contribute to the high productivity of the system.

Full text of DOI:10.1002/cctc.201700946

Accepted Article
Title: Carbonylative Suzuki coupling reaction catalyzed by H -
spirophosphorane (HP~N) palladium complex
Authors: Przemyslaw Wojcik, Labrini Sygellou, Andrzej Gniewek, Anna
Skarzynska, and Anna Trzeciak
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ChemCatChem
10.1002/cctc.201700946
Carbonylative Suzuki coupling reaction catalyzed by
H – spirophosphorane (HP~N) palladium complex
[
a]
[b]
[a]
[a]
*[a]
P. Wójcik, L. Sygellou, A. Gniewek, A. Skarżyńska, and A. M. Trzeciak
[
a] Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie Str., 50-383 Wrocław Poland.
E-mail: anna.trzeciak@chem.uni.wroc.pl
[
b] Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation of Research and
Technology-Hellas (FORTH/ICE-HT), Patras, Greece
Abstract
Palladium complex with H-spirophosphorane ligand, PdCl {P(OCMe CMe O)OC H NH }
2
2
2
6
4
2
(Cat. 1), was used as the catalyst in the carbonylative Suzuki coupling of substituted
iodobenzenes with arylboronic acids and with sodium tetraphenylborate. Substituted
diarylketones were obtained in good to excellent yield and selectivity over 95% already at
1
atm of CO, using 1.5 mol% of the catalyst. The promoting role of the H-spirophosphorane
3
1
ligand in the catalytic process was evidenced. XPS, TEM and P NMR studies revealed that
during reaction Cat. 1 undergoes transformation to palladium complexes bearing
spirophosphorane ligand or other P-ligands formed by its dealkylation and to Pd NPs. All
these palladium species contribute to high productivity of the system.
Key words: palladium, carbonylative Suzuki coupling, diarylketones, nanoparticles, H-
spirophosphoranes
1
. Introduction
Palladium-catalyzed carbonylative Suzuki cross-coupling reaction appears to be direct and
convenient method for the synthesis of diaryl ketones, which constitute a structural motif
found in a large number of natural products and biologically active small molecules. Such
structures are key components in the production of anti-inflammatory drugs and
[
1]
agrochemicals. The first carbonylative Suzuki cross-coupling was reported by Kojima in
[
2]
1
986. The survey of the literature reveals that there are many efficient palladium based
[
3-5]
catalytic systems utilized in the carbonylative cross-coupling reaction.
For example, in situ
generated palladium NPs were applied as effective catalysts for the carbonylative Suzuki
[
6]
[2, 8, 11,
reaction. However, vast majority of the known systems requires a long reaction time
1
6, 17, 19-21, 22, 23, 27]
[2, 8, 11, 17, 19, 20]
,
unattractive amount of the catalyst.
and relatively high
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ChemCatChem
10.1002/cctc.201700946
[
21-23]
pressure of carbon monoxide.
Among them there is a large group of catalysts containing
[
7-23]
phosphorus ligands mainly of phosphine type.
Catalytic systems incorporating phosphines
In contrast, there are only few examples
[
24-31]
along with palladium precursor were also used.
[
19-21, 23]
of catalytic systems consisting of phosphite based ligands.
Hydrospirophosphoranes (HSPs), a class of phosphorus(III) compounds with unique
[
32-34]
P-H bond,
constitute a group of ancillary ligands of choice in modern homogenous
Palladium complexes with H-spirophosphoranes have been applied as efficient
[
35]
catalysis.
and regioselective catalysts in Heck and Hiyama cross-coupling reactions. It was emerged
that the presence of spirophosphorane ligand in coordination sphere of palladium is essential
[
36-39]
and the structure of the complex plays a significant role in the catalytic process.
It was
demonstrated that the kind of HSP ligand coordinated to palladium influences the selectivity
of arylation of n-butyl acrylate and the yield of substituted stilbenes in the coupling reaction
of aryl halides with substituted styrenes and styrylsilanes. Despite of their promising
properties in catalytic systems, H-spirophosphoranes remain still not too much explored group
of the ligands.
Thus, the design of new palladium H-spirophosphorane complexes which promote
high catalytic activity of metals in mild conditions (atmospheric pressure of carbon
monoxide) is still becoming an attractive challenge. Inspired by the fact, that palladium
complexes with phosphite ligands have been successfully applied in Suzuki cross-coupling
[
40-41]
reactions,
we decided to design new palladium complex with H-spirophosphorane, as a
catalyst in carbonylative Suzuki cross-coupling reaction. Until now, catalytic activity of
palladium complexes incorporating hydrospirophosphorane ligand in carbonylative Suzuki
cross-coupling reactions of phenylboronic acid with aryl iodide have been never studied. It
was expected, that the presence of P- and N- donor groups in the ligand molecule would offer
an efficient stabilization of catalytically active palladium intermediates and prevent
uncontrolled decomposition to palladium black. Moreover, the coordination mode of
P,N-ligand could be easily switched between mono- and chelating one facilitating creation of
the new place for substrate coordination.
2
2
. Experimental
.1. Reagents
All commercially available reagents were purchased from Sigma-Aldrich and used without
[
42]
purification. Solvents were carefully dried and deoxygenated by standard methods.
The
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ChemCatChem
10.1002/cctc.201700946
[
36]
ligand HP(OCMe CMe O)(OC H NH), (HP~N), has been obtained according literature.
2
2
6
4
[
43]
The precursor PdCl (cod) has been synthesized as previously reported.
2
2
.2. Instrumental methods
1
13
1
H NMR and C NMR were recorded on a Bruker Avance 500 MHz spectrometer ( H NMR
1
3
5
00 MHz, C NMR 125 MHz) using TMS as internal reference. Chemical shifts (δ) are
1
31
reported in ppm. H NMR and P NMR spectra of the complex were obtained on a Bruker
Avance III spectrometer (300 MHz). Chemical shifts (δ) are given in ppm towards the TMS
1
31
1
(
H) and H PO ( P) using deuterated solvents as lock and reference ( H), respectively. IR
3 4
and FIR measurement were performed in KBr pellets with a Nicolet FTIR Impact 400.
Elemental analyses were performed on a 2400 CHNS VarioEL III apparatus. The sizes of Pd
nanoparticles were characterized on Transmission Electron Microscopy (TEM). TEM
2
measurements were carried out using a FEI Tecnai G 20 X-TWIN electron microscope
(TEM) operating at 200 kV. Analytical gas chromatographic (GC) analyses for the
carbonylative Suzuki coupling products were performed on Hewlett-Packard 5890 Series II
gas chromatograph connected to HP 5971A mass selective detector. Separation was achieved
on capilary HP-5 column coated with a diphenyl (5%) dimethylsiloxane (95%) copolymer
film. Helium was used as the carrier gas. The XRD measurements were carried out with a
Bruker D8 Advance diffractometer equipped with the Vantec detector and operating with Cu-
Kα radiation line.
The photoemission experiments were carried out in an ultra-high vacuum system (UHV)
which consists of a fast entry specimen assembly, a sample preparation and an analysis
−
9
chamber. The base pressure in both chambers was 1×10 mbar. Unmonochromated Al-Kα
line at 1486.6 eV and an analyzer pass energy of 97 eV, giving a full width at half maximum
(FWHM) of 1.7 eV for the Au 4f 7/2 peak, were used in all XPS measurements. The XPS
core level spectra were analyzed using a fitting routine, which can decompose each spectrum
into individual mixed Gaussian-Lorentzian peaks after a Shirley background subtraction.
Regarding the measurement errors, for the XPS core level peaks we estimate that for a good
signal to noise ratio, errors in peak positions are of about ±0.1 eV. The binding energy (BE)
scale was calibrated by assigning the main C1s peak at 284.7 eV. The calibration of the
analyzer’s kinetic energy scale was done according to ASTM-E 902-88.
Single crystals of PdCl {P(OCMe CMe O)OC H NH }, Cat. 1, suitable for X-ray
2
2
2
6
4
2
measurements were obtained by slow decreasing temperature of the saturated CH CN
3
solution. Subsequently, the crystals were mounted on glass fibers and the diffraction data
were collected on a KM-4 CCD diffractometer with graphite monochromated Mo-Kα
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ChemCatChem
10.1002/cctc.201700946
radiation (λ = 0.71073 Å). The structure was solved using direct methods and developed by
2
full leasts-quares refinement on F . Structural solution and refinement was carried out using
[
44]
SHELX suite of programs.
Analytical absorption corrections were performed with
[
45]
CrysAlis RED. C, N, O, P, Cl and Pd atoms were refined anisotropically. All H atoms in
Cat. 1 were positioned geometrically and refined isotropically using a riding model with a
common fixed isotropic thermal parameter. The molecular structure plots were prepared using
[
46]
ORTEP-3 program. The supplementary crystallographic data for this paper can be obtained
from The Cambridge Crystallographic Data Centre via
https://www.ccdc.cam.ac.uk/structures/. CCDC reference code: 1551370.
2
.3. General procedure for the carbonylative Suzuki cross-coupling reaction
The reactions were carried out in 50 ml Schlenk tube equipped with a magnetic stirrer under
carbon monoxide atmosphere. In a typical experiment, the Schlenk was charged the reagents:
-5
catalyst (1.5×10 mol), 4-iodoanisole (1 mmol), phenylboronic acid (1.2 mmol) and K CO (3
2
3
mmol) in 1,4-dioxane (5 ml) containing dodecane (0.076 ml) as the internal standard. Then
the yellow mixture was frozen in liquid nitrogen the air from the Schlenk flask was evacuated.
Next the Schlenk tube was filled with carbon monoxide from a balloon (three times). The
balloon was disconnected. The mixture was heated at 100ºC and stirred for 2 h. After that, the
reaction mixture was cooled and the organic products were separated by extraction with n-
hexane (3 times with 7 ml) and analyzed by GC with dodecane as the internal standard. Next,
the solvents were evaporated, the crude product was purified by column chromatography on
silica gel using n-hexane: ethyl acetate (5:1) as the eluent, the corresponding diaryl ketones
were obtained.
2
.4. Procedure for catalyst recycling
After first catalytic cycle, the organic products were separated by extraction (with hexane).
The liquid phase with the reagents was moved to another glass flask using a stainless steel
tube (catheter tubing) and only the palladium residue was left in the Schlenk flask. Then, the
palladium residue was washed with hexane and dried in vacuum. After that, the Schlenk flask
was charged again with reagents: 4-iodoanisole (1 mmol), phenylboronic acid (1.2 mmol),
K CO (3 mmol), 1,4-dioxane (5 ml) and dodecane (0.076 ml). The yellow mixture was
2
3
frozen in liquid nitrogen, the air from the Schenk flask was evacuated. Next the Schlenk tube
was filled again with carbon monoxide from a balloon (three times). The balloon was
disconnected. The mixture was heated at 100ºC and stirred for 2 h. After that, the reaction
mixture was cooled and the organic products were separated by extraction with n-hexane (3
times with 7 ml) and analyzed by GC with dodecane as the internal standard.
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2
.5. Procedure for the carbonylative Suzuki reaction at higher pressure
In a typical experiment: 1,4-diiodobenzene (1 mmol), catalyst Cat. 1 (1.5 mol%),
phenylboronic acid or sodium tetraphenylborate (0.3-2.2 mmol) and 1,4-dioxane (5 ml) were
transferred under an inert atmosphere into a stainless steel autoclave. It was charged with
carbon monoxide (5-10 atm) and heated with stirring at 100ºC for 2-5 h. After that time, the
autoclave was cooled down and the organic products were extracted with n-hexane (3 times
with 7 ml)) and analyzed by GC with dodecane as the internal standard (0.076 ml).
2
.6. Synthesis of PdCl {P(OCMe CMe O)OC H NH }, Cat. 1
2
2
2
6
4
2
Cl
Cl
Pd
O
P
0
O
NH
O
CH CN, 1h, 40 C
3
H
+ PdCl (cod)
P
NH2
2
O
O
-
cod
O
HP(OCMe CMe O)(OC H NH) L1
PdCl {P(OCMe CMe )OC H NH } Cat.1
2
2
6
4
2
2
2
6
4
2
Scheme 1.
Under the nitrogen atmosphere the complex [PdCl (cod)] (0.16 g, 0.58mmol) was added to a
2
solution of the ligand HP(OCMe CMe O)(OC H NH) L1 (0.17 g, 0.67 mmol) in 8 ml of
2
2
6
4
acetonitrile. The yellow solution was stirred for 1h at 40ºC. Then, the product was
precipitated with diethyl ether, filtered, washed with diethyl ether, hexane and dried in vacuo.
Yield: 0.22 g (88%). Single crystals suitable for X-ray analysis were obtained from
acetonitrile-toluene solution by slow cooling.
Anal. Calc. for C H Cl N O PPd [%]: C = 33.32; H = 4.19; N = 3.24; Found: C = 33.61; H
12
18
2
2
2
=
4.17; N = 3.27.
1
H NMR (500 MHz, CD CN):  1.57 (s, 6H, CH ), 1.65 (s, 6H, CH ), 5.76 (bs, 2H, NH ),
3
3
3
2
1
3
7
.17 (m, 1H, Ph-H), 7.25 (m, 1H, Ph-H), 7.36 (m, 1H, Ph-H), 7.38 (m, 1H, Ph-H). C NMR
(
(
(
500 MHz, CD CN):  24.6 (CH ), 24.7 (CH ), 25.0 (CH ), 25.1 (CH ), 87.3 (C(CH ) ) 87.4
3
3
3
3
3
3 2
C(CH ) ), 121.4 (arom. CH), 122.7 (arom. CH), 123.4 (arom. CH), 125.1 (arom. CH), 127.6
3
2
31
-1
arom. CH), 129.5 (arom. CH). P NMR (200 MHz, CD CN):  114.1 IR (KBr, cm ): 290vs
3
and 331s ν(Pd–Cl); 454m ν(Pd–P) 894vs, 970vs, 1132s ν(P-O-C); 3158m, 3257m ν(NH2).
. Results and discussion
3
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10.1002/cctc.201700946
3
.1. Synthesis and X-ray structure of PdCl {P(OCMe CMe O)OC H NH } (Cat. 1)
2 2 2 6 4 2
complex.
The palladium complex of the formula PdCl {P(OCMe CMe O)OC H NH } (Cat. 1) was
2
2
2
6
4
2
obtained by substitution of 1,5-cyclooctadiene (cod) ligand in PdCl (cod) precursor. The
2
1
13
coordination of P,N-spirophosphorane ligand to palladium was confirmed by H, C NMR
3
1
and P NMR spectroscopy.
Single crystal X-ray diffraction analysis revealed that the stereochemistry of complex Cat. 1 is
almost identical as expected from the spectroscopic studies. The molecular structure is
presented in Fig. 1 and list of selected bond lengths and bond angles are given in Table 1.
Fig. 1. Molecular structure of complex Cat. 1. Displacement ellipsoids are drawn at the 30%
probability level and H atoms are shown as small spheres of arbitrary radii.
Table 1.Relevant bond distances (Å) and angles (°) for complex Cat. 1
Bond lengths
Pd1-Cl11
Pd1-Cl12
Pd2-Cl21
Pd2-Cl22
2.3801(13)
2.3008(13)
2.3631(12)
2.2894(13)
Pd1-P11
Pd2-P21
Pd1-N11
Pd2-N21
2.1970(14)
2.1922(13)
2.040(3)
2.057(4)
Bond angles
Cl12-Pd1-Cl11
Cl22-Pd2-Cl21
P11-Pd1-Cl12
P21-Pd2-Cl22
P11-Pd1-Cl11
P21-Pd2-Cl21
95.88(4)
94.50(5)
90.74(5)
89.99(5)
172.21(5)
94.50(5)
N11-Pd1-P11
N21-Pd2-P21
N11-Pd1-Cl12
N21-Pd2-Cl22
N11-Pd1-Cl11
N21-Pd2-Cl21
87.73(11)
88.79(11)
90.74(5)
178.43(11)
85.68(11)
86.80(11)
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The X-ray analysis showed that the coordination geometry of the palladium is square-planar
with slight distortions. The structure of Cat. 1 is comprised of two crystallographically
independent molecules of the complex and two solvent molecules in an asymmetric unit. The
donor atoms, P and N, affect the Pd-Cl bonds. The Pd-Cln1 bond lengths of 2.3801(13) and
2
.3631(12) Å are longer than those of Pd-Cln2 bonds 2.3008(13) and 2.2894(13) Å (where n
1, 2). These differences of bond length are caused by a stronger trans effect of the
phosphorus atoms compared to the nitrogen atoms. The Pd-Nn1 bond lengths of 2.040(3) and
=
2
.057(4) have a similar values like the complexes containing chelate (P~N) ligand and two
[
38, 67]
atoms of chlorine.
3
.2. Catalytic activity of Cat. 1 in carbonylative Suzuki reactions
For the studies of the catalytic activity of Cat. 1 the coupling of 4-iodoanisole with
phenylboronic acid under CO atmosphere was selected as a model reaction (Scheme 2).
I
O
B(OH)
Cat. 1
,4-dioxane, 2h, 100 C
+
MeO
+
+ CO
0
1
MeO
OMe
1
2
Scheme 2.
A series of experiments was carried out to optimize reaction parameters such as: solvent,
base, temperature, time and the amount of the catalyst. The obtained results are summarized
in Table 2.
Initially, the effect of the solvent and the base was studied (Table 2, entry 1-11). The highest
yield of the desired product, 4-methoxybenzophenone (81%), was obtained in 1,4-dioxane
with K CO as the base (Table 2, entry 4). An effect of the cation present in the base was
2
3
observed and lower yields of 4-methoxybenzophenone were obtained with other carbonates,
such as Cs CO or NaHCO (Table 2, entry 5 and 6). In contrast, application of a stronger
2
3
3
base (KOH) or ethylene glycol as the solvent favored the formation of Suzuki product, 4-
methoxybiphenyl (Table 2, entry 3 and 7). Similarly, Et N provided low yield of the diaryl
3
ketone (Table 2, entry 8) whereas dimethyl carbonate and propylene carbonate were not
suitable solvents for this reaction (Table 2, entry 10 and 11).
The high ability of ethylene glycol to promote Suzuki-Miyaura coupling was also observed
for other palladium complexes [64, 70, 71]. It was related to the fact that ethylene glycol
facilitated reduction of Pd(II) to Pd(0). Similarly, when Cat. 1 was used as a catalyst,
formation of the black palladium was much faster in ethylene glycol than in other solvents.
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Palladium nanoparticles formed in this process catalyze efficiently Suzuki-Miyaura coupling
but not carbonylative coupling. The possible reason of such reactivity could be relatively
strong connection of ethylene glycol to palladium, preventing CO access [70, 71]. Thus,
under carbonylative Suzuki-Miyaura conditions activation of CO was not efficient as a result
of blocking of palladium environment by ethylene glycol or by products of its
dehydrogenation [71].
Further studies showed that the amount of K CO has strong influence on the catalytic system
2
3
productivity (Table 2, entry 4, 12, 13). The yield of 4-methoxybenzophenone was equal to,
respectively, 42 and 61% when 2 or 4 equivalents of base were used, significantly lower than
with 3 equivalents (Table 2, entry 4). When the amount of the catalyst was concerned (Table
2
, entry 14-16), 85% of 4-methoxybenzophenone was formed at the presence of 1.5 mol% of
the catalyst and an increase of this amount did not influence the yield (Table 2, entry 16).
Examination of the effect of temperature and reaction time (Table 2, Entry 17-21), allowed to
conclude that the 92% yield of 1 could be obtained at 100ºC in 2h, whereas longer time was
needed to get the similar yield at 80ºC (Table 2, Entry 21).
ab
Table 2. Optimization of the carbonylative Suzuki cross-coupling reaction.
Entry Amount of
catalyst
Solvent
Base
Amount
of base
[mmol]
Temp.
ºC]
Time
[h]
Total
conver
Yield Yield
Selectivity
to 1
[
1
2
[
mol%]
sion
[%]
[%]
[%]
b
[
%]
Effect of solvent
1
2
3
1
1
1
DMF
NMP
K CO3
3
3
3
80
80
80
2
2
2
69
45
65
41
7
4
4
94
91
7
2
K CO3
2
ethylene
glycol
K CO3
2
100
93
4
1
1,4-dioxane
K CO3
2
3
80
2
82
81
1
99
Effect of base
5
6
7
8
9
1
1
1
1
1
1
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
Cs CO3
3
3
3
3
3
3
80
80
80
80
80
80
2
2
2
2
2
2
66
20
86
34
0
54
20
31
34
0
12
0
82
100
37
100
0
2
NaHCO3
KOH
55
0
Et N
3
KH PO3
0
2
1
0
dimethyl
carbonate
propylene
carbonate
0
0
0
0
1
1
1
1,4-dioxane
3
80
2
0
0
0
0
Effect of amount of base
1
1
2
3
1
1
1,4-dioxane
1,4-dioxane
K CO3
2
80
2
2
42
65
40
61
2
4
95
94
2
K CO3
2
4
80
Effect of amount of catalyst
1
1
1
4
5
6
0.5
1.5
2
1,4-dioxane
1,4-dioxane
1,4-dioxane
K CO3
3
3
3
80
80
80
2
2
2
75
87
88
74
85
86
1
2
2
99
98
98
2
K CO3
2
K CO3
2
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Effect of temperature and time
1
1
1
2
2
7
8
9
0
1
1
1
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
K CO3
3
3
3
3
3
80
8
4
2
2
2
96
84
29
87
94
94
82
28
85
92
2
2
1
2
2
98
98
96
98
98
2
K CO3
2
80
1
K CO3
2
60
1.5
1.5
K CO3
2
80
K CO3
2
100
a
Reaction conditions: 4-IC H OCH (1 mmol), PhB(OH) (1.2 mmol), Cat. 1 (1.5 mol%), solvent (5 ml), base
3 mmol), CO (1atm.), Conversion was determined by GC using dodecane as an internal standard.
6
4
3
2
b
(
Having optimized reaction condition in hand we tested other palladium precursors in order to
estimate an effect of P,N-spirophosphorane ligand on the reaction course (Table 3).
ab
Table 3. The carbonylative Suzuki coupling reaction of 4-iodoanisole with phenylboronic acid.
Entry
Catalyst
Reaction
time
Total
conv.
(%)
1
(%)
2
(%)
b
[
h]
1
2
Cat. 1
Cat. 1
1
2
61
94
58
92
3
2
3
4
5
6
PdCl (cod)
2
2
2
2
57
60
35
55
45
59
35
54
12
1
2
PdCl (cod) + L1
2
PdCl2
0
PdCl + L1
1
2
7
8
9
Pd(dba)2
Pd(OAc)2
2
2
2
2
48
48
52
49
42
19
52
49
6
29
0
PdCl {P(OPh) }
2
3 2
1
0
PdCl (PPh )
0
2
3 2
a
4
-IC H OCH (1 mmol), PhB(OH) (1.2 mmol), [Pd] (1.5 mol%), 1,4-dioxane (5 ml), K CO (3 mmol), CO
6 4 3 2 2 3
0
b
(
1atm.), 100 C. Conversion was determined by GC using dodecane as internal standard.
In the studied series Cat. 1 was found the most catalytically active, forming 58% of 4-
methoxybenzophenone after 1h and 92% after 2h (Table 3, entry 1 and 2). On the other hand,
two other complexes containing P-ligands, PdCl (P(OPh) ) and PdCl (PPh ) , produced only
2
3 2
2
3 2
5
2 and 49%, respectively, under the same conditions (Table 3, entry 9 and 10). When
phosphine-free precursors, PdCl (cod), PdCl , Pd(dba) and Pd(OAc) , were applied, the yield
2
2
2
2
of 4-methoxybenzophenone was lower than 50% (Table 3, entry 3, 5, 7, 8). Nevertheless,
upon addition of our P,N-ligand to PdCl (cod) or to PdCl the yield of diaryl ketone increased
2
2
from 45 to 59% for PdCl (cod) and from 35 to 54% for PdCl (Table 3, entry 4, 6). The
2
2
positive effect of the H-spirophosphorane ligand was therefore evidenced. It is worth to note
that the pre-made complex gave much better results than that formed in situ.
3
.3. Attempts of palladium catalyst recycling
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Before studying the nature of the catalytically active palladium species in our system, we
decided to get some knowledge about its stability under reaction conditions. Although
recycling of a soluble catalyst is rather problematic from technical point of view, attempts to
test few subsequent runs with the recovered catalyst were undertaken.
For this purpose, the carbonylative Suzuki cross-coupling reaction catalyzed by Cat. 1 was
carried out, yielding 88% of 4-methoxybenzophenone. In the end of the reaction the color of
the solution turned black, possibly due to the formation of colloidal palladium. The organic
products were extracted with hexane, while palladium solid black residue was suspended in
the remaining liquid phase. The liquids were transferred to second flask by using a stainless
steel tube (catheter tubing), a new portion of reactants was added to the palladium residue and
reaction was carried out for next 2 h. This process was repeated 4 times and the conversion
decreased from 91% in the first experiment to 70% in the last one (Table 4). In the same time,
the selectivity to 4-methoxybenzophenone decreased whereas the amount of Suzuki product,
4
-methoxybiphenyl, increased from 3 to 34%. Clearly, the longer use of the catalyst resulted
in the change of its reactivity. In particular, the ability to activate CO and to form diaryl
[
38]
ketone decreased.
We assumed that the black residue used in this experiment contained
mainly Pd NPs, however the presence of some amount of precipitated palladium complex is
also possible. In the subsequent runs (Table 4) palladium complex could undergo
transformation to Pd NPs. The reverse process, of palladium leaching from Pd NPs with
formation of soluble palladium species, could be also expected. Participation of all these
palladium forms in the catalytic resulted the change of reaction selectivity in time.
Table 4. Recycling of the catalyst in carbonylative Suzuki coupling reaction of 4-iodoanisole with
ab
phenylboronic acid.
Run
Total
Yield of
product 1
(%)
Yield of
product 2
(%)
Selectivity
to 1
conv.
b
(
%)
[%]
1
2
3
4
5
91
95
96
70
70
88
71
68
45
36
3
97
75
71
65
52
24
28
30
34
a
4
-IC H OCH (1 mmol), PhB(OH) (1.2 mmol), 1,4-dioxane (5 ml), K CO (3 mmol), CO (1atm.), 2h, 100ºC.
6 4 3 2 2 3
b
Conversion was determined by GC using dodecane as an internal standard.
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3
.4. Filtration test – catalytic activity of soluble palladium species.
In order to explain the reason of the reaction selectivity change observed during recycling, the
filtration test has been performed. The standard reaction was stopped after 20 min. and the
small sample was analyzed by GC showing 32% conversion of 4-iodoanisole to 4-
methoxybenzophenone (Table 5). Next, the liquid phase was transferred to another flask by
using a stainless steel tube (catheter tubing) to separate the solid materials. The liquid phase
was next heated for 2h under CO (1 atm., 100ºC). After that time, the yield of 4-
methoxybenzophenone increased to 38% and 4-methoxybiphenyl (8%) was also formed
(Table 5).
Table 5. Results of filtration test for the carbonylative cross-coupling of 4-iodoanisole with
ab
phenylboronic acid catalyzed by Cat. 1.
Total conv.
1
(%)
2
(%)
b
(
%)
Analysis before catalyst separation (20 min reaction)
Analysis after catalyst separation (120 min reaction)
32
32
38
0
8
46
a
4
-IC H OCH (1 mmol), PhB(OH) (1.2 mmol), Cat. 1 (1.5 mol%), 1,4-dioxane (5 ml), K CO (3 mmol), CO
6 4 3 2 2 3
º
b
(
1atm.), 100 C. Conversion was determined by GC using dodecane as internal standard.
The results of filtration test showed that the reaction is catalyzed by different forms of
palladium, namely soluble and solid ones, originating from the Cat. 1 precursor. It is also
worth to note that conversion of 4-iodoanisole obtained in solution (46%) after separation of
the insoluble palladium forms was significantly lower than in the standard carbonylative
Suzuki reaction (94%). Thus, contribution of the insoluble palladium forms in the catalytic
process was very important.
3
.5. Poisoning tests
To gain better understanding of the mechanism of the reaction, several attempts have been
undertaken to define the nature of the true active species. To this aim, a variety of poisons
[
55, 56]
(
Hg(0), CS , TMTU, TBAB) have been used.
The Hg(0) test is most commonly used in
2
order to confirm the role of Pd NPs in the catalytic reaction. It is expected that formation of
[
57-
Pd/Hg amalgam from Pd NPs and Hg(0) would result in inhibition of the reaction course.
6
0]
Such an effect was observed in our system. When Hg(0) was introduced at the beginning
of the reaction, the conversion of 4-iodoanisole was only 13%, considerably lower than in the
reference experiment performed without Hg(0), which enabled to reach 94% conversion
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ChemCatChem
10.1002/cctc.201700946
(Table 6, Fig. 2). In the second experiment Hg(0) was added after 30 min from the beginning
of the catalytic run and the reaction stopped at 44% conversion (Fig. 2). Accordingly, these
[
57-60]
results indicated participation of Pd NPs in the catalytic process.
To exclude possible
reaction between Cat. 1 and Hg(0) the solution containing the both species was examined
3
1
using P NMR. The only signal was observed at 114.1 ppm confirming the presence of
unaffected Cat. 1.
Next, an effect of CS poison on the reaction course was studied (Table 6, entry 4-6). It is
2
accepted in the literature that inhibition of the reaction course at the ratio [CS ]/[Pd] < 1 is
2
[58]
indicative for heterogeneous reaction mechanism.
Our system produced 41% of 2-
methoxybenzophenone at the presence of 0.65 mol% of CS (Table 6, entry 4). The inhibiting
2
effect was stronger after addition of higher amount of CS (Table 6, entry 5 and 6), however,
2
the catalyst was still active. The observed inhibiting effect can be explained by the
31
interactions of CS with Pd NPs, rather than with Cat. 1. In agreement with that, the P NMR
2
spectrum of Cat. 1 has not changed after CS addition (= 114.2 ppm).
2
Recently, the quantitative poisoning method involving tetramethylthiourea (TMTU) was
[61]
developed as an alternative for CS2. We decided also test TMTU in our system (Table 6,
entry 7-9). The inhibiting effect was weaker than that caused by CS and increased with
2
3
1
increase of TMTU amount. Analysis of the P NMR spectrum revealed some changes,
namely appearance of the new broad signal at 67.6 ppm after addition of TMTU to the
solution of Cat. 1. It can be therefore assumed that TMTU coordinated to Cat. 1 precursor,
similarly as it was reported for other palladium complexes. As a result, activation of
substrates by the catalyst is more difficult and conversion decreased.
The last experiment was based on running catalytic reactions in the presence of TBAB
(tetrabutylammonium bromide). On the one hand, addition of TBAB improves the
[
62]
stabilization of Pd NPs, increasing their reactivity. On the other hand, another effect should
[
63]
also be considered: dissolution of Pd NPs while heating with PhI in the presence of TBAB.
The inhibiting effect of TBAB was noticed by us recently in the carbonylative Suzuki
[
64]
coupling with other catalyst. The strong retard of the reaction course was observed after
TBAB introduction (Table 6, entry 10), whereas with 20-fold excess of TBAB the catalytic
3
1
system was completely inactive (Table 6, entry 11). Interestingly, the P NMR data indicated
the reaction of Cat. 1 with TBAB, because a new signal at 117.6 ppm appeared after
introduction of TBAB to the solution of Cat. 1. In this case TBAB is responsible for
transformation of the catalyst to its less active form.
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Table 6. The carbonylative Suzuki cross-coupling reaction of 4-iodoanisole with phenylboronic acid
ab
catalyzed by Cat. 1 – effect of Hg(0), CS , TMTU and TBAB.
2
b
Entry
Poison additive
Total conv.
%)
Yield 1
%)
Yield 2
(%)
2
(
(
1
2
None
94
15
92
13
Hg added at the beginning of the
reaction
2
3
4
5
6
7
8
9
Hg added after 30 min
44
41
32
26
81
70
64
38
0
40
41
32
25
81
70
64
38
0
4
0
0
1
0
0
0
0
0
CS , 0.65 mol%
2
CS , 1 mol%
2
CS , 1.5 mol%
2
TMTU, 0.65 mol%
TMTU, 1 mol%
TMTU, 1.5 mol%
TBAB, 15 mol%
TBAB, 30 mol%
1
1
0
1
a
4
-IC H OCH (1 mmol), PhB(OH) (1.2 mmol), Cat. 1 (1.5 mol%), 1,4-dioxane (5 ml), K CO (3 mmol), CO
6
4
3
2
2
3
b
b
(
1atm.), 2h, 100ºC. [Hg]/[Pd] = 240. Conversion was determined by GC using dodecane as internal
standard.
1
00
90
80
70
60
50
40
30
20
10
0
8
8
68
58
45
47
3
8
Hg(0) added at t = 30 min
Hg(0) added at t = 0 min
3
0
40
27
1
9
1
0
13
0
20
40
60
80
100
120
Reaction time [min]
Fig. 2. Reaction profile study of the carbonylative Suzuki cross-coupling of 4-iodoanisole with
phenylboronic acid catalyzed by Cat. 1 at presence of Hg(0).
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The results obtained with various catalyst poisons allow to consider the contribution of
different forms of palladium in the reaction course, namely soluble palladium complexes as
well as Pd NPs.
3
3
.6. Analysis of the catalyst transformations
.6.1. TEM experiments
In order to verify the presence of Pd NPs, the post-catalytic reaction mixture was examined by
TEM (Transmission Electron Microscopy). In fact, the analyzed mixture contained Pd NPs
with ca. 5 nm in diameter (Fig. 3).
a)
b)
5
0
5
0
3
6
5
9
6
0
4
5
7
Diameter of Pd NPs [nm]
c)
d)
Fig. 3. Pd NPs after a catalytic reaction: (a) TEM micrographs,(b) the size of Pd NPs, (c) energy
dispersive X-ray spectrum (with the use of Cu grid) of post reaction solution and (d) electron
diffraction pattern of Pd NPs.
Interestingly, the TEM analysis of the post-reaction residue after the fifth recycle gave similar
result. Thus, Pd NPs of ca. 7 nm in diameter were found (Fig. 4), only slightly bigger than
after the first run. Fortunately, under the reaction conditions agglomeration of Pd NPs was
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ChemCatChem
10.1002/cctc.201700946
limited, most probably as a result of the stabilizing effect of the phosphorus ligand present in
the system.
a)
b)
4
0
3
2
3
0
2
1
2
0
5
9
3
0
5
6
7
8
9
10
Diameter of Pd NPs [nm]
c)
d)
Fig. 4. Pd NPs after 5 runs of a catalytic reaction: (a) TEM micrographs,(b) The size of Pd NPs, (c)
energy dispersive X-ray spectrum (with the use of Cu grid) of fifth recycling and (d) electron
diffraction pattern of Pd NPs.
3
.6.2. XRD experiment
An additional evidence of Pd NPs presence in the post-reaction mixture was obtained from
the XRD (X-ray powder diffraction) method. A diffractogram of the residue showed four
characteristic reflections (Fig. 5), which correspond to Pd(0) crystallizing in Fm3m group
(JCPDS card number 5-681). Broadening of the diffraction lines indicated nanocrystaline
nature of the sample.
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(
111)
(
200)
(
220)
(
311)
30
40
50
60
70
80
90
2
 [deg]
Fig. 5. XRD powder diffractogram of Pd NPs.
Complementary studies were performed using the XPS method, which enables to determine
oxidation state of palladium.
3
.6.3. XPS experiments
The first XPS measurement was carried out for the Pd(II) complex as-prepared. The binding
energy (BE) and the FWHM (Full Width at Half Maximum) were used as a reference in the
deconvolution of the solution and black precipitate samples. Figure 6 shows the (a) Pd3d, (b)
Cl2p, (c) N1s and (d) P2p. The BE of Pd3d5/2 is at 338.8 eV and is assigned to Pd–N bonded
[47]
palladium, like in Pd[(NH ) ]Cl .
P2p spectrum, fitted with one doublet with spin-orbit-
3
4
2
splitting of 0.9 eV and at BE of P2p 133.6±0.1 eV is assigned to phosphonate based
3
/2
[
48]
molecules. Cl2p spectrum is fitted with one doublet of spin-orbit-splitting 1.7 eV and at BE
[
49]
Cl2p3/2 198.8 eV. Since the BE of Cl in PdCl is at 198.0 eV
and the covalent chlorine
2
[
50]
species at 200.2 eV , the peak at 198.8 eV is attributed in part of chlorine forming the Pd-
3
Cl bonds. N1s spectrum consists of one peak at 400.6 eV assigned to sp (N-C) type bonding
[
51]
states. The relative atomic concentration Pd:Cl:P:N in the analyzed region was calculated
by dividing the peak area Pd3d, Cl2p P2p and N1s by the appropriate relative sensitivity
factor after correcting it for the experimentally determined EA10 analyzer transmission
[
52]
characteristics (effectively Average Matrix RSF) and using the appropriate equations. The
results are: Pd:Cl:P:N= 1:1.75:1.04:1.05 which are close to the nominal within experimental
error of 10%.
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XPS
AlKα
Pd3d
XPS
AlKα
P2p
(a)
(b)
3
50
348
346
344
342
340
338
336
334
332
206
204
202
200
198
196
194
192
Binding Energy EB / eV
Binding Energy EB / eV
(c)
XPS
AlKα
N1s
(d)
XPS
AlKα
Cl2p
4
08
406
404
402
400
398
396
394
392
142
140
138
136
134
132
130
128
126
Binding Energy EB / eV
Binding Energy EB / eV
Fig 6. XPS peaks (a) Pd3d, (b) Cl2p, (c) N1s and (d) P2p of Cat. 1.
Next, the reaction of Cat. 1 with CO (1 atm) was studied. After 30 min heating at 100ºC in
dioxane the black precipitate containing Pd NPs was formed. The presence of Pd NPs with
diameter ca. 3-4 nm was confirmed by TEM, whereas reduction of Pd(II) to Pd(0) was
evidenced by XPS by the presence of the signal at BE 335.2±0.1eV. Examination of the
solution by the XPS method showed the presence of unchanged Cat. 1 complex. A similar
result, namely evidence of palladium reduction, was also obtained when Cat. 1 was treated
with CO (1 atm) at the presence of K CO base (Scheme 3).
2
3
Cl
Cl
Pd
CO
O
Pd NPs
P
NH2
O
O
Scheme 3.
The important function of Pd(0), both in the form of the complex as well as nanoparticles, in
[
58]
the catalytic cycle of carbonylative coupling is well accepted.
Therefore it was important
to confirm that CO contributes significantly in formation of the catalytically active Pd(0).
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Additional evidence of the fact that the presence of CO facilitates transformations Cat. 1 was
31
obtained from the P NMR spectra when reaction of Cat. 1 with 4-iodoanisole at the presence
of K CO under CO atmosphere at 100ºC was monitored in time. After 10 min the unchanged
2
3
Cat. 1 was observed (signal at 114.2 ppm) together with small amount of phosphonate ligand
formed by dealkylation of L1. After 50 min. the signal of Cat. 1 disappeared and a new broad
[
37, 65]
signal at 80 ppm appeared. It could be attributed to the palladium dimer (Scheme 4).
O
P
O
P
Cl
Cl
O
O
Pd
O
Cl
Cl
O
O
H
Pd
Pd
H
P
NH2
O
O
O
O
O
O
P
P
O
O
Scheme 4.
In an analogous experiment carried out without CO the Cat. 1 complex was still present in the
solution after 50 min.
The post-catalytic residue was also subjected to XPS analysis (Figure 7). The Pd3d peak
consists of two components at binding energies 335.2±0.1eV and 337.2±0.1 eV assigned to
[
50, 53]
metallic Pd(0) and Pd(II), probably PdOx.
The Cl2p (Figure 7b) is at BE 199.3 eV
The N1s (Figure 7c) is deconvoluted into two components
assigned to imine-like (–N=) and amine-like (–NH–) structures located at BE of 397.9 and
-1
[54]
assigned to Cl ionic bonds.
[
50]
3
99.6 eV, respectively. The P2p peak (Figure 7d) at binding energy 133.8 eV is assigned to
[
48]
phosphonate based molecules. The relative atomic concentration Pd:Cl:P:N was calculated.
The result is: Pd:Cl:P:N= 1:0.75:1.66:0.43.Chemical analysis of the particles surface showed
finally that the atomic ratio of Pd(0)/Pd(II) was 4.
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XPS
AlKα
Pd3d
(b)
XPS
AlKα
Cl2p
(a)
Pd0
Pd oxide
Cl ionic bonds
3
50
348
346
344
342
340
338
336
334
332
204
202
200
198
196
194
Binding Energy EB / eV
Binding Energy EB / eV
XPS
AlKα
N1s
phosphonates
XPS
AlKα
P2p
(c)
(d)
imine like
amine like
4
10
408
406
404
402
400
398
396
394
142
140
138
136
134
132
130
Binding Energy EB / eV
Binding Energy EB / eV
Fig 7. (a) Pd3d, (b) Cl2p, (c) N1s and (d) P2p XPS spectra of the post-catalytic residue diluted
in dioxane and spin-coated on Si plasma treated wafer.
This XPS experiment showed that after catalytic reaction Pd occured mostly as Pd(0) NPs
(80%) and a small amount of an oxide form was detected (20%). As expected, chlorides were
removed from chemical environment of catalyst during catalytic reaction. The P atom is in the
phosphonate form, the same as in complex Cat. 1. This means that chemical environment of
phosphorus was not changed during catalytic reaction, whereas the chemical environment of
nitrogen was modified. This may indicate that Pd-N bond was broken. The possible
transformation of the H-spirophosphorane ligand is presented in Scheme 4.
It is worth to note that in all the samples analyzed by XPS the P-ligand was found. It could be
assumed that ligand L1 or its derivatives are bonded to different forms of palladium present in
the system, namely palladium complexes and nanoparticles.
3
.7. The carbonylative Suzuki coupling reaction of substituted iodobenzenes with
phenylboronic acid
The versatility of the catalytic system was tested in the carbonylative Suzuki reaction of
substituted iodobenzenes with phenyl boronic acid (Table 7).
I
B(OH)2
O
[
Pd]
+
+ CO
0
1
00 C, 2h
R
R
3
a-i
Scheme 5.
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ChemCatChem
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The data presented in Table 7 confirmed high ability of the system to form diaryl ketones
from substituted iodobenzenes containing electron donating as well as electron withdrawing
groups. Moreover, the reaction with sterically hindered 2-methoxy-iodobenzene gave
satisfactory yield of ketone, 80%. The best results were obtained for 3-iodoanisole (95%) and
for 2-iodotiophene (97%). The only substrate that has nor reacted under applied conditions
was 4-iodophenol.
ab
Table 7. Palladium-catalyzed carbonylative Suzuki coupling of substituted iodobenzenes.
O
OMe O
O
MeO
b
c
b
c
b
c
conv. 90% , 3a 88% (81%)
conv. 84% , 3b 80% (72%)
conv. 96% , 3c 95% (90%)
O
O
O
O N
2
b
c
O
conv. 92% , 3d 92% (84%)
b
c
conv. 99% , 3e 94% (85%)
b
c
conv. 97% , 3f 95% (83%)
O
O
O
O
S
b
c
b
c
b
c
conv. 100% , 3i 99% (90%)
conv. 100% , 3g 97% (91%)
conv. 86% , 3h 80% (74%)
a
4
-IC H R (1 mmol), PhB(OH) (1.2 mmol), Cat. 1 (1.5 mol%), 1,4-dioxane (5 ml), K CO (3 mmol), CO
6 4 2 2 3
b
c
(
1atm.), 2h, 100ºC. Conversion was determined by GC using dodecane as internal standard. Isolated
yield.
3
.8. The carbonylative Suzuki coupling reaction of 4-iododanisole with arylboronic acids
The effect of the boronic acids on the studied reaction was investigated using 4-iodoanisole as
a substrate (Table 8).
I
B(OH)2
O
[
Pd]
+
+ CO
0
100 C
MeO
R
R
4a-h
OMe
Scheme 6.
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The reactions with arylboronic acids containing substituents at 4-position produced diaryl
ketones with a good yield and excellent selectivity (almost 100%). The best result was
obtained with the 4-tert-butylphenylboronic acid, which formed 88% of desired product. In
six cases (Table 8) elongation of the reaction time was necessary to obtain diaryl ketones in
reasonable yields. Interestingly 2-thiopheneboronic acid reacted quite well, while 2-
,
furanylboronic acid didn t form any coupling product.
ab
Table 8. Palladium-catalyzed carbonylative Suzuki coupling of arylboronic acid derivatives.
O
O
O
MeO
MeO
MeO
Br
b
d
b
conv. 78% , 4b 76% (69%)
conv. 61% , 4c 61%
b
d
c
d
conv. 88% , 4a 88% (80%)
conv. 76% , 4c 76% (66%)
O
Br
O
O
MeO
MeO
MeO
b
conv. 40% , 4d 38%
conv. 56% , 4d 55% (44%)
b
b
conv. 40% , 4e 39%
conv. 68% , 4e 58% (50%)
conv. 44% , 4f 43%
conv. 60% , 4f 52% (40%)
c
d
c
d
c
d
O
O
O
OMe
S
MeO
MeO
b
conv. 60% , 4g 59%
conv. 72% , 4g 71% (64%)
c
d
b
conv. 37% , 4h 37%
conv. 52% , 4h 52% (44%)
c
d
a
4
-IC H OCH (1 mmol), RB(OH) (1.2 mmol), Cat. 1 (1.5 mol%), 1,4-dioxane (5 ml), K CO (3 mmol), CO
6 4 3 2 2 3
b
c
d
(
1atm.), 2h, 100ºC. Conversion was determined by GC using dodecane as internal standard. 4 h
Isolated yield.
3
.9. The carbonylative Suzuki couplig of 1,4-diiodobenzene with phenylboronic acid
Encouraged by the very good results obtained with our catalyst for various substrates, we
decided to focus our research on a more difficult substrate:1,4-diiodobenzene.
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O
O
I
B(OH)2
+
[
Pd] (1 mol%)
+
CO +
I
5a
5b
O
1
,4-dioxane, K CO3
2
O
I
Ph
5c
Scheme 7.
As expected, under standard conditions a mixture of products was obtained at conversion of
the substrate equal to 57% (Table 9, entry 1). Interestingly, the double carbonylated product
was formed as the main one. Prolongation of the reaction time to 5 h resulted in conversion
increase to 95%, however, the selectivity decreased (Table 9, entry 2). On the other hand, at
the elevated CO pressure (10 atm), 1,4-diiodobenzene was converted with high selectivity to
product 5b which was obtained with the yield of 87% (Table 9, entry 3).
Table 9. Effect of CO pressure on carbonylative Suzuki coupling of 1,4-diiodobenzene with
ab
phenylboronic acid.
b
Entry
Pressure of CO
Time
[h]
2
Total Conv.
5a
[%]
10
20
7
5b
[%]
5c
[%]
1
[
atm.]
1
[%]
57
1
2
3
46
c
1
5
95
66 (57)
87
9
10
5
96
2
a
1
,4-I C H (1 mmol), PhB(OH) (2.2 mmol), Cat. 1 (1.5 mol%), 1,4-dioxane (5 ml), K CO (3 mmol), CO
2 6 4 2 2 3
b
(
c
1atm.), 2h, 100ºC. Conversion was determined by GC using dodecane as internal standard.
Isolated yield
3
.10. The carbonylative Suzuki coupling reaction of 4-iodoanisole and 1,4-diiodobenzene
with sodium tetraphenylborate
Since application of NaBPh in the carbonylative Suzuki coupling takes precedence in the
4
[
16, 66]
literature,
this compound can be used as an alternative to a phenyl boronic acid in
arylation reactions. First, we examined the catalytic system under the optimized conditions in
the carbonylative Suzuki coupling of 4-iodoanisole with sodium tetraphenylborate (Table 9).
O
I
[
Pd] (1.5 mol%)
+
MeO
+
NaBPh4 + CO
MeO
2
1
OMe
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ChemCatChem
10.1002/cctc.201700946
ab
Table 10. Carbonylative Suzuki coupling of 4-iodoanisole with sodium tetraphenylborate.
Base
Conv.
%]
1
[%]
34
2
[%]
65
[
K CO3
2
99
without base
87
33
54
a
4
-IC H OCH
3
(1 mmol), NaBPh4 (1.2 mmol), Cat. 1 (1.5 mol%), 1,4-dioxane (5 ml),
6
4
b
K CO (3 mmol), 2h, 100ºC. Conversion was determined by GC using dodecane as internal standard.
2
3
Under the optimized condition, the catalytic system showed conversion of 99%, but the yield
of 4-methoxybenzophenone was only 34% (Table 10). Surprisingly, when the reaction was
carried out without base, the results were similar although the amount of side product 2 was
smaller (Table 10).
The possibility of double carbonylation with Cat. 1 was studied in the carbonylative Suzuki
coupling of 1,4-diiodobenzene with sodium tetraphenylborate (Table 11). In this reaction five
products (5a-e) were observed.
O
O
+
I
I
5a
I
5b
O
[
Pd]
O
+
CO + NaB(Ph)4
1
,4-dioxane, K CO
+
I
2
3
Ph
5c
5d
5e
Scheme 8.
Using 2.2 mmol of NaBPh 100% conversion was obtained, but the main product was 5c
4
,
(biphenyl-4-yl(phenyl)methanone), formed by monocarbonylation (Table 11, entry 1). Next,
an effect of sodium tetraphenylborate amount on the reaction course was examined (Table 11,
entry 1-5). Along with the decrease of NaBPh amount, the conversion of 1,4-diiodobenzene
4
and the yield of product 5c decreased, while higher amount of the dicarbonylation product 5b
was formed. The highest yield of product 5b, 34%, was achieved at the presence of 0.6 mmol
of NaBPh (Table 11, entry 4).
4
An effect of the CO pressure was also investigated (Table 11, entry 6-9). At the elevated
pressure, 5 or 10 atm of CO, 1,4-diiodobenzene was converted mainly to products 5a and 5b
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ChemCatChem
10.1002/cctc.201700946
(Table 11, entry 6, 7 and 9). Under these conditions, the reaction without the base was more
selective to product 5a (Table 11, entry 7 and 8).
Table 11. Effect of amount of sodium tetraphenylborate and CO pressure on carbonylative Suzuki
ab
coupling of 1,4-diiodobenzene with sodium tetraphenylborate.
Entry
Amount of
NaBPh4
Pressure
of CO
[atm.]
Total
Conv.
[%]
5a
[%]
5b
[%]
5c
[%]
5d
[%]
5e
[%]
b
[
mmol]
Effect of amount of NaBPh4
d
1
2
2.2
1.2
1.2
0.6
0.3
1
1
1
1
1
100
0
1
6
3
2
21
22
25
34
16
55 (45)
0
1
3
2
1
24
37 (26)
17
d
99
38
26
12
7
c
3
77
4
5
64
13
34
8
Effect of CO pressure
6
7
8
1.2
5
87
31
47
7
1
1
1.2
1.2
0.6
10
10
10
84
81
80
30
37
31
53
43
48
1
1
1
0
0
0
0
0
0
c
9
a
1
,4-I C H (1 mmol), Cat. 1 (1.5 mol%), 1,4-dioxane (5 ml), K CO3 (3 mmol), 2h, 100ºC.
2 6 4 2
b
c
Conversion was determined by GC using dodecane as internal standard. Reaction without base.
Isolated yield
d
4
. Conclusions
It has been demonstrated that palladium complex [PdCl {P(OCMe CMe O)OC H NH }] with
2
2
2
6
4
2
H-spirophosphorane ligand appears to be highly efficient and selective precatalyst for
carbonylative Suzuki cross-coupling reactions of aryl iodides with arylboronic acid and with
sodium tetraphenylborate. The carbonylation reactions were performed under mild conditions
[
19, 21]
when compared to other palladium complexes with chelating phosphorus ligands.
Diarylketones were obtained with high yield in two hours, under atmospheric pressure of
carbon monoxide, at low amount of the catalyst. Interestingly, the solvent considerably
affected the yield of carbonylation products. The best results were obtained for 1,4-dioxane,
which favors generation of differently substituted benzophenones. In contrast, ethylene glycol
seems to be the solvent of choice for production of biphenyls, Suzuki reaction products.
Mechanistic studies carried out with application of the XPS and TEM methods
evidenced generation of Pd NPs under catalytic reaction conditions. It could be assumed that
the first cycle is a regular homogeneous catalysis, which then turns to other palladium forms,
including Pd NPs. The results obtained in recycling experiments indicated on the leaching of
palladium from Pd NPs with formation of soluble palladium species. Thus, the studied system
[68, 69]
can be described as a “cocktail”-type catalytic system.
In fact, the poisoning tests, XPS
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ChemCatChem
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3
1
and P NMR studies indicated on the presence of dynamic system containing different
palladium forms, namely Pd NPs and molecular complexes, forming a “cocktail”.
The presence of P-ligand, (H-spirophosphorane or its derivatives), in all the analyzed
samples confirmed its important role in the studied reactions. Performed studies revealed that
application of other chelating phosphorus ligands coordinated to palladium, gave worse
results, up to 50% of carbonylation product only. The H-spirophosphorane ligand plays a dual
role, stabilizing palladium in molecular complexes as well as Pd NPs formed in situ in the
2
catalytic system. It is worth noting that, the bidentate chelating κ -P,N propensity of
spirophosphoranes makes them good candidates to display hemilabile properties and facilitate
activation of the substrates on the palladium center. The spectroscopic studies indicated that
dealkylation of H-spirophosphorane ligand is possible and leads to phosphonate moiety which
may easily coordinate to palladium centre. Similar phosphonate complexes catalyzed the
[37, 65]
Heck coupling and methoxycarbonylation reaction
and they might also contribute to the
carbonylative Suzuki coupling. In summary, the studied “cocktail”-type system contained
different forms of palladium and phosphorus ligands, which contribute to the efficient
catalytic process.
Acknowledgement
Financial support of National Science Centre (NCN, Poland) with grant
2
014/15/B/ST5/02101 (P.W., A.G., A.M.T.) is gratefully acknowledged.
5
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