Carbonylative Suzuki coupling reactions catalyzed by ONO pincer–type Pd(II) complexes using chloroform as a carbon monoxide surrogate

Article abstract of DOI:10.1002/aoc.5414

Benzoylhydrazone Schiff base–ligated three new ONO pincer–type palladium(II) complexes, [(PdL¹(PPh₃)] (1), [(PdL²(PPh₃)] (2), and [(PdL³(PPh₃)] (3), were synthesized by the reaction of the respective ligand, N-(2-hydroxybenzylidene)benzohydrazide (HL¹), N-(2-hydroxy-3-methoxybenzylidene)benzohydrazide (HL²), or N-(5-bromo-2-hydroxybenzylidene) benzohydrazide (HL³), with Pd(OAc)₂ and PPh₃ in methanol and isolated as air-stable reddish-orange crystalline solids in high yields (78%–83%). All three complexes were fully characterized by elemental analysis, Fourier-transform infrared spectroscopy, UV–Visible, ¹H nuclear magnetic resonance (NMR), ¹³C{¹H} NMR, and ³¹P{¹H} NMR spectroscopic studies. The molecular structure of all three complexes was established unambiguously by single-crystal X-ray diffraction studies which revealed a distorted square planar geometry of all three complexes. The ONO pincer–type ligands occupied three coordination sites at the palladium, while the fourth site is occupied by the monodentate triphenylphosphine ligand. The catalytic potential of all three complexes was explored in the carbonylative Suzuki coupling of aryl bromides and iodides with arylboronic acids to yield biaryl ketones, using CHCl₃ as the source of carbonyl. The reported protocol is convenient and safe as it obviates the use of carbon monoxide (CO) balloons or pressured CO reactors which are otherwise needed for the carbonylation reactions. The methodology has been successfully applied to the synthesis of two antineoplastic drugs, namely, phenstatin and naphthylphenstatin, in good yields (81% and 85%, respectively). Under the optimized reaction conditions, complex 2 exhibited the best catalytic activity in the carbonylative Suzuki couplings. The reported catalysts have wide reaction scope with good functional group tolerance. All catalysts could be retrieved from the reaction after completion and recycled up to three times with insignificant loss in the catalytic activity.

Full text of DOI:10.1002/aoc.5414

Received: 4 October 2019
DOI: 10.1002/aoc.5414
Revised: 12 November 2019
Accepted: 13 November 2019
F U L L P A P E R
Carbonylative Suzuki coupling reactions catalyzed by ONO
pincer–type Pd(II) complexes using chloroform as a carbon
monoxide surrogate
1
1
2
Samaresh Layek | Bhumika Agrahari | Rakesh Ganguly
|
1
1
Parthasarathi Das | Devendra D. Pathak
1
Department of Chemistry, Indian
Benzoylhydrazone Schiff base–ligated three new ONO pincer–type
Institute of Technology (ISM), Dhanbad,
1
2
826004, India
palladium(II) complexes, [(PdL (PPh )] (1), [(PdL (PPh )] (2), and
3
3
2
Division of Chemistry & Biological
Chemistry, Nanyang Technological
University, 639798, Singapore
3
[
(
(PdL (PPh )] (3), were synthesized by the reaction of the respective ligand, N-
3
1
2-hydroxybenzylidene)benzohydrazide
(HL ),
N-(2-hydroxy-3-meth-
2
oxybenzylidene)benzohydrazide (HL ), or N-(5-bromo-2-hydroxybenzylidene)
Correspondence
Prof. Devendra D. Pathak, Department of
Chemistry, Indian Institute of Technology
3
benzohydrazide (HL ), with Pd(OAc) and PPh in methanol and isolated as
2
3
air-stable reddish-orange crystalline solids in high yields (78%–83%). All three
(
Indian School of Mines), Dhanbad-
26004, Jharkhand, India.
complexes were fully characterized by elemental analysis, Fourier-transform
8
1
13
infrared spectroscopy, UV–Visible, H nuclear magnetic resonance (NMR),
C
Email: ddpathak@yahoo.com
1
31
1
{
H} NMR, and P{ H} NMR spectroscopic studies. The molecular structure of
all three complexes was established unambiguously by single-crystal X-ray dif-
fraction studies which revealed a distorted square planar geometry of all three
complexes. The ONO pincer–type ligands occupied three coordination sites at
the palladium, while the fourth site is occupied by the monodentate
triphenylphosphine ligand. The catalytic potential of all three complexes was
explored in the carbonylative Suzuki coupling of aryl bromides and iodides
with arylboronic acids to yield biaryl ketones, using CHCl as the source of
3
carbonyl. The reported protocol is convenient and safe as it obviates the use of
carbon monoxide (CO) balloons or pressured CO reactors which are otherwise
needed for the carbonylation reactions. The methodology has been successfully
applied to the synthesis of two antineoplastic drugs, namely, phenstatin and
naphthylphenstatin, in good yields (81% and 85%, respectively). Under the
optimized reaction conditions, complex 2 exhibited the best catalytic activity in
the carbonylative Suzuki couplings. The reported catalysts have wide reaction
scope with good functional group tolerance. All catalysts could be retrieved
from the reaction after completion and recycled up to three times with insig-
nificant loss in the catalytic activity.
K E Y W O R D S
carbonylation, catalysis, crystal structure, ONO pincer type, palladium
Appl Organometal Chem. 2020;e5414.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 14
https://doi.org/10.1002/aoc.5414

2
of 14
LAYEK ET AL.
1
| INTRODUCTION
Rational design and judicious choice of suitable
ligands play a key role in the development and fine-
tuning of the catalytic properties and stereoselectivities
of metal complexes. The steric and electronic properties
imparted by the ligand greatly influence the nature of
Palladium catalysts and reagents are becoming extremely
versatile and indispensable in modern organic synthe-
sis. Development of efficient methods for the C–C bond
[
1]
formation is one of the enduring research theme in
the reactive species and thus the outcome of a homoge-
[2–4]
[38]
organic chemistry.
Transition metal-catalyzed
neously catalyzed reaction.
In this regard, one of the
carbonylative Suzuki cross-coupling reaction is one of the
powerful methods to generate C–C bonds which are an
integral part of structural skeleton of a wide variety of
natural products, pharmaceutical drugs, agrochemicals,
thriving strategies is to use tridentate ligands, such as
pincer ligands, which are well known to form well-
[
39,40]
defined stable complexes.
The tridentate coordina-
tion of pincer ligands provides strong binding to a
metal center by invoking chelate effect and results in
high stability of the pincer metal complexes. The
robustness of a complex is of special interest for the
design of catalysts that need to be stable enough to
cope with the harsh reaction conditions. Therefore, it
was thought worthwhile to synthesize pincer complexes
and use them as catalysts. Complexes having different
pincer skeleton such as CCC, NNN, CNC, NCN, PNP,
[
5–10]
fine chemicals, and organic functional materials
Figure 1).
A review of the literature reveals that the direct car-
bonyl insertion reactions involve the use of carbon mon-
(
[11–14]
oxide (CO) as a highly toxic gas
and suffer from one
or more drawbacks, such as rigorous reaction conditions,
high temperature, difficulty in handling of reactants, and
limited utility of high pressurized toxic CO gas, preclud-
ing their large-scale industrial applications. To avoid
SNS, ONO, and SCS have been used in catalysis and
[
15–18]
[19]
[41–48]
these drawbacks, metal carbonyls,
formic acid,
materials science.
In particular, the ONO pincer–
[20,21]
[22]
acid chloride,
formamide (DMF),
silacarboxylic acids,
dimethyl-
type palladium complexes have proved to be very effec-
[23,24]
[13]
[49]
COgen and SilaCOgen,
etc.
tive catalysts for Suzuki–Miyaura coupling,
C-2
[
50]
were used as alternative CO surrogates. Although most of
these precursors have their own pros and cons, many of
arylation of quinoline scaffolds,
Domino reaction of
and synthe-
etc. In recent years, hydrazone-
[
51]
benzoyl chloride with arylboronic acids,
[25–27]
[52]
them have some limitations.
Recently, the use of
sis of tetrazoles,
chloroform as a CO surrogate has grabbed the attention
of organic chemists for the synthesis of a variety of
based ONO pincer complexes have also been reported
to exhibit interesting biological and catalytic
properties.
[28]
[53,54]
targeted organic molecules.
The in situ-generated CO
from CHCl and a base has been used in various organic
Prompted by these reports and our ongoing interest in
3
[29]
[55,56]
reactions such as carboxylation,
carbonylative
Heck-type
palladium chemistry,
we herein report the synthesis
[30,31]
[32,33]
coupling,
aminocarbonylations,
and characterization of three new palladium(II) com-
plexes containing hydrazone-based ONO pincer–type
ligands and their application in carbonylative Suzuki cou-
pling reactions for the synthesis of a series of biaryl
ketones, using chloroform as a cheap and convenient
source of CO. To the best of our knowledge, this is the
first report on the application of isolated and structurally
characterized palladium(II) pincer–type Schiff base
complexes in the carbonylative Suzuki coupling reaction
(Scheme 1).
[34]
domino cyclization,
and carbonylative Sonogashira
[
35]
coupling.
Recently, one of us has elegantly demon-
strated the application of this methodology (chloroform
as CO surrogates) in aminocarbonylation of 7-azaindole
and imidazopyridines, leading to the synthesis of
biologically important aminocarbonyl-functionalized
[
36,37]
7
-azaindole and 2-amidoimidazo[1,2-a]pyridines.
However, all catalysts were generated in situ and high
loading of the catalyst was required.
FIGURE 1 Some important molecules containing the
biaryl ketone functionality

CARBONYLATIVE SUZUKI COUPLING REACTIONS
3 of 14
SCHEME 1 Previous and proposed
strategies for the synthesis of biaryl ketones
using chloroform as the carbonyl source. See
[
30,31]
also.
PEG-400, polyethylene glycol 400
2
| EXPERIMENTAL SECTION
.1 | Materials and physical
vacuum over anhydrous CaCl . The details of experimen-
2
tal procedure for these Schiff base ligands and their
spectroscopic data and spectra are given in the
Supporting Information.
2
measurements
All chemicals and solvent were purchased from Sigma
Aldrich, Merck (India) Limited, Alfa Aesar and
TCIchemicals. Silica gel (100–200 mesh, Merck) was used
for column chromatography. The thin-layer chromatog-
raphy was performed on Merck-pre-coated silica gel 60-
F254 plates andvisualized by UV-light. The FT-IR spectra
were recorded on a Perkin Elmer Spectrometer (Model:
Cary660) in the range of 400–4000 cm-1 using KBr pellets
in which MCT used as a detector with scan number 20,
and resolution 4 cm-1. Electronic absorption spectral
analysis was recorded on a Shimadzu UV-1800 Spectro-
photometer in the wave length range of 250–800 nm. The
NMR spectra of complexes were recorded in CDCl3/
DMSO-d6on a Bruker 75 AvIII HD-400 MHz spectrome-
ter using TMS as the internal standard.
2.2.1 | Schiff base ligand N-(2-
hydroxybenzylidene)benzohydrazide (HL )
1
Benzoylhydrazine (0.136 g, 1.0 mmol), salicylaldehyde
(0.122 g, 1.0 mmol), and ethanol (10 mL) afforded the
title compound as a white crystalline solid.
Yields: 0.246 g, 87%, Selected Fourier-transform infra-
−1
red (FT-IR; KBr), cm : 3440 (ν OH), 3264 (ν NH), 1671
1
(ν C=O), 1623 (ν C=N),1539 (ν C=C); H nuclear mag-
netic resonance (NMR) (dimethyl sulfoxide [DMSO]-d ,
6
ꢀ
25 C, 400 MHz): δ 12.020 (s, 1H, phenolic OH), 11.212
(s, 1H, NH), 8.547 (s, 1H, –N=CH–), 7.842 (d, 3H, Ar
H),7.521–7.435 (m, 4H, Ar H), 6.850–6.798 (m, 2H, Ar H).
2
.2.2 | Schiff base ligand N-(2-hydroxy-3-
2
.2 | General procedure for the synthesis
methoxybenzylidene)benzohydrazide
(HL )
1
3
2
of ligands HL –HL
All three ligands were synthesized by reported
Benzoylhydrazine (0.136 g, 1.0 mmol), ortho-vaniline
(0.152 g, 1.0 mmol), and ethanol (10 mL) afforded the
title compound as an off-white crystalline solid.
[57–59]
methods.
In brief, in a round-bottomed flask,
ethanolic solutions (5 mL) of benzoylhydrazine
−
1
(
1.0 mmol) and the corresponding aldehyde (1.0 mmol)
Yields: 0.225 g, 83%, Selected FT-IR (KBr), cm : 3369
ꢀ
were stirred and refluxed at 80 C in ethanol (5 mL) for
(ν OH), 3212 (ν NH), 1652 (ν C=O), 1602 (ν C=N),1570 (ν
1
ꢀ
7
hr. Upon cooling to room temperature, a solid precipi-
C=C); H NMR (DMSO-d , 25 C, 400 MHz): δ 11.976 (s,
6
tate was formed from the reaction mixture. After this, the
solid was filtered, washed with cold ethanol, and dried in
1H, phenolic OH), 10.848 (s, 1H, NH), 8.547 (s, 1H, –
N=CH–), 7.830 (d, 2H, Ar H), 7.523–7.418 (m, 3H, Ar H),

4
of 14
LAYEK ET AL.
2
7
3
.206–6.923 (m, 2H, Ar H), 6.762 (t, 1H, Ar H), 3.709 (s,
H, –OCH3).
2.3.2 | [Pd(L )(PPh )] (2)
3
Yield: 515.90 mg, 81%. Elemental analysis (%) calculated
for C H N O PPd: C, 62.22; H, 4.27; N, 4.40. Found
33
27 2 3
2
.2.3 | Schiff base ligand N-(2-hydroxy-5-
(%) C, 62.45; H, 4.13; N, 4.59. Selected IR bands (KBr, ν
3
−1
bromobenzylidene)benzohydrazide (HL )
in cm ): 1594 (ν C=N), 1537 (ν C=C), 532 (ν Pd–O),
−
1
−1
5
08 (ν Pd–N); UV–Vis [λmax (nm), ε (L mol cm )]:
1
Benzoylhydrazine
-bromosalicylaldehyde (0.201 g, 1.0 mmol), and ethanol
10 mL) afforded the title compound as an off-white crys-
talline solid.
(0.136
g,
1.0
mmol),
320 (15700), 347 (11000), 404 (5700); H NMR (CDCl ,
3
5
(
400 MHz, δ ppm): 8.35 (d, J = 16 Hz, 1H, –CH=N–),
7.88–7.83 (m, 8H, Ar H), 7.58–7.54 (m, 3H, Ar H),
7.50–7.42 (m, 5H, Ar H), 7.48–7.40 (m, 1H, Ar H),
7.38–7.29 (m, 3H, Ar H), 7.11 (d, 8 Hz, 1H, Ar H), 6.88
−1
Yields: 0.252 g, 81%, Selected FT-IR (KBr), cm : 3429
(
(
(
ν OH), 3223 (ν NH), 1649 (ν C=O), 1605 (ν C=N),1542
(d, 6.4 Hz, 1H, Ar H), 6.67 (t, 7.8 Hz, 1H, Ar H), 3.69 (s,
1
ꢀ
13
ν C=C); H NMR (DMSO-d , 25 C, 400 MHz): δ 12.063
3H, –OCH3); C NMR (CDCl , 100 MHz, δ ppm):
6
3
s, 1H, phenolic OH), 11.552 (s, 1H, NH), 8.522 (s, 1H, –
173.34, 153.74, 151.95, 134.77, 134.65, 131.62, 131.07,
N=CH–), 7.952 (d, 2H, Ar H),7.595–7.483 (m, 4H, Ar H),
.342 (d, 1H, Ar H), 6.872 (d, 1H, Ar H).
130.16, 130.03, 129.44, 129.34, 128.97, 128.52, 127.98,
31
7
125.97, 119.68, 114.81, 56.49; P NMR (CDCl , 160 MHz,
3
δ ppm): 17.69.
2
.3 | General procedure for the synthesis
3
of palladium(II) complexes 1–3
2.3.3 | [Pd(L )(PPh )] (3)
3
A solution of the Schiff base ligand HL –3 (1.0 mmol) in
3
1
Yield: 534.93 mg, 78%. Elemental analysis (%) calcu-
lated for C H BrN O PPd: C, 56.04; H, 3.53; N, 4.08.
mL methanol was added dropwise to the 5 mL
32
24
2 2
methanolic solution of Pd(OAc) (0.224 g, 1.0 mmol) with
constant stirring at room temperature. After stirring the
Found (%) C, 56.25; H, 3.84; N, 4.19. Selected IR bands
(KBr, ν in cm ): 1596 (ν C=N), 1516 (ν C=C), 531 (ν
2
−
1
solution for 5 min, PPh (0.264 g, 1.0 mmol) was added as
Pd–O),− 499 (ν Pd–N); UV–Vis [λmax (nm),
ε
3
1
−1
a co-ligand to the reaction mixture. After 8 hr of stirring
at room temperature, the resultant colored precipitate
was filtered, washed with cold methanol, and dried in
(L mol cm )]: 312 (16900), 339 (11600), 410 (9300);
1
H NMR (CDCl , 400 MHz, δ ppm): 8.26 (d, J = 16 Hz,
3
1H, –CH=N–), 7.95 (d, J = 8 Hz, 2H, Ar H), 7.79 (t,
J = 10 Hz, 6H, Ar H), 7.57–7.48 (m, 10H, Ar H), 7.42
(t, J = 8 Hz, 1H, Ar H), 7.35 (t, J = 6 Hz, 2H, Ar H),
7.27 (d, J = 12 Hz, 1H, Ar H), 6.69 (d, J = 12 Hz, 1H,
vacuum over anhydrous CaCl . The precipitate was rec-
2
rystallized from DMF. Suitable single crystals for X-ray
crystallography were obtained on slow evaporation over
a period of 2–3 weeks from a concentrated solution of the
complex in DMF.
13
Ar H); C NMR (CDCl , 100 MHz, δ ppm): 161.45,
3
145.77, 134.77, 134.58, 134.47, 131.15, 128.73, 128.67,
31
1
1
28.57, 128.05, 106.62, 106.33;
60 MHz, δ ppm): 21.24.
P NMR (CDCl3,
1
2
.3.1 | [Pd(L )(PPh )] (1)
3
Yield: 504.57 mg, 83%. Elemental analysis (%) calculated
for C H N O PPd: C, 63.32; H, 4.15; N, 4.62. Found
2.4 | General procedure for
carbonylative Suzuki–Miyaura coupling
32
26 2 2
(
%) C, 63.69; H, 4.23; N, 4.38. Selected IR bands (KBr, ν in
−
1
cm ): 1603 (ν C=N), 1527 (ν C=C), 531 (ν Pd–O), 506 (ν
Pd–N); UV–Vis [λmax (nm),
A 10-mL sealed tube was charged with arylboronic
acid (1.2 mmol), aryl halide (1.0 mmol), KOH
−
1
−1
ε
(L mol
cm )]:
1
3
4
7
7
7
14 (15500), 339 (10400), 402 (9080); H NMR (CDCl ,
(8.0 mmol, 0.49 g), CHCl (4.0 mmol, 326 μL), toluene
3
3
00 MHz, δ ppm): 8.52 (d, J = 15.6 Hz, 1H, –CH=N–),
.67 (d, J = 24 Hz, 2H, Ar H), 7.62–7.61 (m, 7H, Ar H),
.58–7.52 (m, 11H, Ar H), 7.50 (t, J = 7.6 Hz, 1H, Ar H),
.48 (t, J = 7.2 Hz, 1H, Ar H), 6.62–6.56 (m, 2H, Ar H);
(2 mL), and palladium(II) complex (1 mol%). The tube
was sealed tightly and stirred vigorously at 80 C for
ꢀ
8–10 hr. After completion of the reaction, the reaction
mixture was allowed to cool to room temperature and
the combined organic layer was extracted with ethyl
acetate, washed with water, dried over Na SO , and
concentrated under reduced pressure over a rotary
evaporator. The residue was purified by column
1
3
C NMR (CDCl , 100 MHz, δ ppm): 162.20, 146.97,
3
1
1
1
34.63, 134.52, 133.72, 132.69, 131.77, 133.02, 129.16,
28.68, 128.01, 121.19, 119.70, 115.56; P NMR (CDCl3,
60 MHz, δ ppm): 21.11.
2
4
31

CARBONYLATIVE SUZUKI COUPLING REACTIONS
5 of 14
1
–3
chromatography through silica gel using ethyl acetate–
petroleum ether as eluent to afford the desired benzo-
phenone product. The isolated organic products were
Schiff base complexes of the type [Pd(L )(PPh )] were
3
synthesized as shown in Scheme 2. The reaction of the
Schiff base ligands precursor with palladium acetate in a
1:1 molar ratio with PPh as a co-ligand in methanol at
1
13
1
characterized with H and C{ H} NMR spectra (see
3
Figures S18–S53).
room temperature for 10 hr afforded the corresponding
1
2
complexes, [Pd(L )(PPh )] (1), [Pd(L )(PPh )] (2), and
3
3
3
[
Pd(L )(PPh )] (3) in 78%–83% yields as air-stable,
3
2.5 | Crystallographic studies
reddish-orange crystalline solids. The complexes were
found to be soluble in polar solvents such as CH Cl ,
2
2
Diffraction quality crystals of complexes 1, 2, and 3 were
obtained over a period of few days from a concentrated
solution of the complex in DMF at room temperature
and the structure of the complexes was elucidated by
single-crystal X-ray diffraction. The X-ray diffraction
intensity data were measured with a Bruker Kappa dif-
fractometer equipped with a charge-coupled device
detector, employing Mo-Kα radiation (λ = 0.71073 Å),
with the SMART suite of programs. All data were
processed and corrected for Lorentz and polarization
effects with SAINT and for absorption effects with
SADABS. Structural solution and refinement were
carried out with the SHELXTL suite of programs. The
structures were refined (weighted least squares
CH CN, DMF, and DMSO and sparingly in nonpolar sol-
3
vents such as n-hexane and diethyl ether. Single crystals
of complexes 1, 2, and 3, suitable for X-ray crystallogra-
phy, were grown over a period of few days on standing a
concentrated solution of the complex in DMF. All com-
plexes were characterized by elemental analysis, FT-IR,
1
13
1
31
1
UV–Visible, H NMR, C{ H} NMR, and P{ H} NMR
spectroscopic techniques and their structures were also
confirmed by single-crystal X-ray diffraction studies.
3.2 | Catalysts characterization
3.2.1 | FT-IR analysis
2
refinement on F ) to convergence.
1
–3
The FT-IR spectra of Schiff base ligands HL and their
corresponding palladium(II) complexes 1, 2, and 3 are
presented in Figures S1–S3, respectively. The FT-IR spec-
tra of the ligands displayed broad bands in the range of
3
3
| RESULTS AND DISCUSSION
.1 | Catalysts synthesis
−
1
3369–3440 cm
3
attributed to ν OH and bands at
212–3264 cm− due to ν NH, respectively. In addition to
1
1
2
3
Benzoylhydrazone-based ONO pincer–type three Schiff
these, ligands HL , HL , and HL exhibited sharp and
high-intensity bands at 1671, 1652, and 1649 cm ,
1
–3
−1
base ligands (HL ) were synthesized by the reported
[
57–59]
methods and isolated as white crystalline solids.
respectively, owing to ν C=O. The existence of ν NH and
1–3
Using these ligands, three new pincer-type palladium(II)
ν C=O clearly indicated that the ligands HL
existed
SCHEME 2 Synthesis of ONO pincer–type
Pd(II) Schiff base complexes 1–3. RT, room
temperature

6
of 14
LAYEK ET AL.
[
60]
mainly in keto form in the solid state.
The bands at
were attributed to the
azomethine (ν C=N) group of the corresponding ligands
, 2, and 3. However, the FT-IR spectra of complexes 1, 2,
not display any signal corresponding to NH protons. The
disappearance of the –NH resonance in the complexes
clearly indicated that ligands had undergone keto to enol
−
1
1
623, 1602, and 1605 cm
1
tautomerism before complexation
followed by
−1
and 3 exhibited bands at 1603, 1594, and 1596 cm ,
respectively, assignable to the ν C=N stretching fre-
quency. A comparison of the FT-IR of the free ligands
and their corresponding complexes clearly indicated that
the ν C=N stretching frequency was shifted to a lower
wave number by Δν 20, 8, and 9 cm in complexes 1, 2,
and 3, respectively. The shifting of azomethine stretching
frequency to a lower wave number may be taken as an
evidence for the coordination of the nitrogen atom to the
metal. In the FT-IR spectra of complexes, the vanishing
of ν OH peak and appearance of peaks in the range of
deprotonation. In addition, the disappearance of OH
1
protons (phenolate) in the H NMR of the complexes
indicated the deprotonation of the hydroxy groups on
coordination. However, in complexes 1, 2, and 3, the res-
onance corresponding to the azomethine proton was
observed at δ 8.52, δ 8.35, and δ 8.26, respectively. The
upfield shift of the azomethine proton in all three com-
plexes as compared with the free ligands supported the
coordination of the nitrogen of the azomethine group to
the metal ion. The upfield shifting of azomethine protons
−
1
may be due to the sterical and geometrical constraints
−
1
[62]
5
31–532 and 499–508 cm assignable to ν Pd–O and ν
experienced by the ligand upon complex formation.
In
Pd–N, respectively, support the formation of the
complexes.
complex 2, the –OCH protons of the o-vaniline moiety
3
[61]
were observed as a singlet at δ 3.695. In the free ligand
2
HL , these protons were observed at the same position,
which clearly reflected that the methoxy group remains
unperturbed during complexation.
3.2.2 | UV–Visible analysis
13
1
The C{ H} NMR spectra of the complexes 1, 2, and
3 (Figures S7, S11, and S15) exhibited a sharp signal at δ
162.20, δ 173.34, and δ 161.45, corresponding to
azomethine carbon (–CH=N–) of the coordinated ligand,
respectively. The signals that appeared in the region of δ
106.33–δ 153.74 were assigned to various aromatic car-
bons of the complexes. In complex 2, the methoxy carbon
The electronic spectra of palladium(II) complexes (1–3)
−4
were recorded in DMF solutions (10 M) in the wave-
length range of 250–800 nm and are shown in Figure S4.
All spectra are very similar and exhibit three strong
absorption bands in the following regions: 312–320 nm,
3
39–347 nm, and 402–410 nm. The strong and
high-energy peaks in the region 312–320 nm were attrib-
uted to the π–π* transition of the aromatic fragment of
the ligand. In all complexes, the n–π* transition
corresponding to the azomethine groups was observed in
the range of 339–347 nm. The lower energy absorptions
between 402 and 410 nm for all three complexes were
due to ligand-to-metal charge transfer.
(–OCH ) of the o-vaniline moiety was observed at δ
3
56.49.
31
1
The P{ H} NMR spectra of complexes 1, 2, and
3 (Figures S8, S12, and S16) showed a sharp singlet at δ
21.11, δ 17.70, and δ 21.24, respectively. The free PPh3
ligand exhibited a resonance at δ –6.4. It is evident from
3
1
the
triphenylphosphine ligand experienced a downfield shift
Δ = 24–27 ppm) which clearly confirms the coordina-
tion of the PPh ligand through the P atom to the metal
P
NMR data of the complexes that the
(
3.2.3 | NMR analysis
3
[
63]
center.
The H NMR spectra of all ligands (HL1–3) were recorded
1
in DMSO-d at room temperature (Figures S5, S9, and
6
1
S13). The H NMR spectra of all three Schiff base ligands
3.2.4 | X-ray crystal structure
determinations
1
showed a singlet at δ 12.02, δ 11.98, and δ 12.06 for HL ,
2
3
HL , and HL , respectively, due to the OH proton. The
other characteristic resonance that appeared at δ 11.21, δ
Single crystals of complexes 1, 2, and 3, suitable for X-ray
crystallography, were grown over a period of few days on
a standing concentrated solution of the respective com-
plex in DMF. The mode of coordination of the ONO
pincer–type ligands to palladium ion in all three com-
plexes was determined from the single-crystal X-ray
structure and the ORTEP representations of the three
new palladium(II) complexes including the non-C and
non-H atoms numbering scheme (Figures 2–4). X-ray
1
0.85, and δ 11.55 as singlet was assigned to the –NH pro-
1
2
3
ton for ligands HL , HL , and HL , respectively. The
azomethine (HC=N–) proton in ligands HL , HL , and
HL was observed as a sharp singlet at δ 8.55, δ 8.55, and
δ 8.52, respectively. The H, C{ H}, and P{ H} spectra
1
2
3
1
13
1
31
1
of all complexes were recorded in CDCl3 and their
1
spectral data are summarized in Table S1. The H NMR
spectra of complexes 1–3 (Figures S6, S10, and S14) did

CARBONYLATIVE SUZUKI COUPLING REACTIONS
7 of 14
FIGURE 2 ORTEP (Oak Ridge Thermal Ellipsoid Plot)
diagram of complex 1 with the non-C and non-H atoms labeling
scheme (thermal ellipsoids are drawn at the 50% probability level)
FIGURE 4 ORTEP (Oak Ridge Thermal Ellipsoid Plot)
diagram of complex 3 with the non-C and non-H atoms labeling
scheme (thermal ellipsoids are drawn at the 50% probability level)
Two rings, one five-membered and one six-membered
chelating rings, were formed when the pincer-type ligand
coordinated through the O, N, and O donors to the
palladium(II). A summary of the crystallographic and
refinement data of complexes is furnished in Table 1 with
selected bond lengths and bond angles given in Table S2.
The N(5)–Pd(1)–O(3), N(5)–Pd(1)–O(5), O(3)–Pd(1)–P(2),
and O(4)–Pd(1)–P(2) chelate bite angles of the complexes
1
are 94.83(19), 80.21(19), 93.83(14), and 171.33(15),
respectively. The N(1)–Pd(1)–O(1), N(1)–Pd(1)–O(2), O
1)–Pd(1)–P(1), and O(2)–Pd(1)–P(1) chelate bite angles
of complexes 1 and 2 are in the range of 93.66(9)–94.47
11), 79.92(11)–80.09(9), 85.38(7)–94.09(5), and 92.16(6)–
00.30(7), respectively. The Pd–O, Pd–N, and Pd–P bond
(
(
1
FIGURE 3 ORTEP (Oak Ridge Thermal Ellipsoid Plot)
diagram of complex 2 with the non-C and non-H atoms labeling
scheme (thermal ellipsoids are drawn at the 50% probability level)
lengths in all complexes are in the range of 1.926(6)–
1.993(2) Å, 1.968(5)–1.975(3) Å, and 2.2837(9)–2.2965
(
7) Å, respectively, which are in the normal range and
similar to other structurally characterized analogous
[
64–66]
investigation revealed that complexes 1 and 2 crystallized
palladium(II) complexes.
in the monoclinic system in space groups P2 /c and C2/c,
1
respectively. However, complex 3 crystallized in the
orthorhombic system in the space group Pbca. The
molecular structures of these complexes were found to be
almost similar and revealed that the Pd(II) center in all
complexes has a distorted square planar geometry, as
reflected in the bond parameters around the metal center
3.3 | Catalytic studies: carbonylative
Suzuki coupling reaction
After the synthesis and characterization of the complexes
1–3, the catalytic potential of all three complexes was
explored in the synthesis of a wide range of biaryl
ketones via the carbonylative Suzuki–Miyaura coupling
reaction of various aryl bromides/iodides with different
arylboronic acids using CHCl3 as the source of CO
(Scheme 3).
(Table S2). Distortion is mainly caused by the presence of
the double deprotonated tridentate Schiff base ligand. In
all complexes, the metal centre Pd(II) is surrounded by
the ONO pincer–type ligand occupying three coordina-
tion sites and a monodentate triphenylphosphine ligand.

8
of 14
LAYEK ET AL.
1
2
3
TABLE 1 Crystallographic and refinement data for [(PdL (PPh
3
)] (1), [(PdL (PPh
3
)] (2), and [(PdL (PPh
3
)] (3) complexes
1
2
3
Complex
[(PdL (PPh
3
)]
[(PdL (PPh
3
)]
[(PdL (PPh
3
)]
CCDC number
Chemical formula
Formula weight
Temperature
Wavelength
Crystal size (mm)
Crystal habit
Crystal system
Space group
Unit cell dimensions
a (Å)
1915066
1526697
1862709
C
32
H
25
N
2
O
2
PPd
C
33
H
27
N
2
O
3
PPd
32 2 2
C H24BrN O PPd
607.92 g/mol
293 K
636.93 g/mol
103(2) K
685.81 g/mol
153(2) K
0.71073 Å
0.71073 Å
0.24 × 0.32 × 0.40
Block, red
Monoclinic
C2/c
0.71073 Å
0.08 × 0.18 × 0.21
Block, red
0.14 × 0.18 × 0.22
Block, red
Monoclinic
orthorhombic
Pbca
1
P2 /c
14.7005(8)
10.3418(6)
17.4996(9)
90
33.1424(16)
9.5982(5)
18.3665(9)
90
22.8515(8)
8.5215(3)
27.7953(10)
90
b (Å)
c (Å)
α (degrees)
β (degrees)
91.953(5)
90
107.7741(10)
90
90
γ (degrees)
90
3
3
V (Å )
2658.9(3)
4
5563.6(5)
8
5412.6(3) Å
Z
8
1.519 mg m–
0.791 mm−
1236.0
3
1.521 g cm–3
0.763 mm−1
2592
1.683 g cm–3
2.255 mm−1
2736
Density (calculated)
Absorption coefficient
F(000)
1
2
Goodness-of-fit on F
1.013
1.046
1.020
R
int
0.0565
0.0929
0.0657
R
1
[I > 2δ(I)]
wR (all data)
Largest diffraction peaks and hole
0.0658
0.0582
0.0328
2
0.1704
0.48 and −0.66 eÅ−
0.1542
1.088 and −1.255 eÅ−3
0.0795
1.803 and −0.732 eÅ−3
3
summarized in Table 2. As shown in Table 2, when the
reaction was carried out in the presence of 0.5 mol. % of
ꢀ
complex 1, in toluene (2 mL) as solvent at 80 C for 8 hr,
the desired product was obtained in only 58% yield
(
Table 2, Entry 1). The reaction with complexes 2 and 3,
SCHEME 3 Palladium(II) complex–catalyzed carbonylative
Suzuki reactions
under identical conditions, afforded the product in 70%
and 62% yield (Table 2, Entries 2 and 3). Because the best
yield was obtained with complex 2, we focused our atten-
tion to further increase the yield of the carbonylative
products. Therefore, on increasing the catalyst 2 loading
from 0.5 to 1 mol%, the corresponding product was
obtained in 85% yield (Table 2, Entry 4). However, fur-
ther enhancement in the catalyst loading beyond 1.0 mol
% did not give any fruitful result on the yield (Table 2,
Entry 5). To see the effect of temperature and reaction
time, the reaction was also carried out at different tem-
peratures and reaction times (Table 2, Entries 6–9). It can
be seen from the table that the best yield of the product
Our investigation began by carrying out the
carbonylative Suzuki–Miyaura coupling reaction of
iodobenzene (1.0 mmol) with phenylboronic acid
(
(
1.2 mmol) in the presence of CHCl (4 equiv.) and KOH
8 equiv.) in a sealed tube. This reaction was conducted
3
in various solvents at different temperatures in the pres-
ence of varying amounts of complexes in order to opti-
mize the reaction parameters such catalyst loading,
solvent, time, and temperature. The results are

CARBONYLATIVE SUZUKI COUPLING REACTIONS
9 of 14
a
TABLE 2 Optimization of the reaction conditions
ꢀ
b
Entry
Catalyst
Catalyst loading (mol%)
Solvents
Toluene
Base
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
NaOH
LiOH
Mg(OH)
Temp ( C)
Time (hr)
Yield (%)
1
2
3
4
5
6
7
8
9
Complex 1
Complex 2
Complex 3
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 2
Complex 3
–
0.5
0.5
0.5
1.0
1.2
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
–
80
80
80
80
80
60
100
80
80
80
80
80
80
80
80
80
80
80
80
80
8
8
58
70
62
85
85
74
85
70
86
45
30
42
25
54
45
30
80
70
n.r.
48
Toluene
Toluene
8
Toluene
8
Toluene
8
Toluene
8
Toluene
8
Toluene
6
Toluene
12
8
10
11
12
13
14
15
16
17
18
19
20
EtOH
Dimethylformamide
Dioxane
8
8
CH
3
CN
8
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
8
8
2
8
Cs(OH)ÁH
KOH
2
O
8
8
KOH
24
8
c
Pd(OAc)
2
1.0
KOH
a
3
Reaction conditions: iodobenzene (1.0 equiv.), phenyl boronic acid (1.2 equiv.), base (8.0 equiv.), CHCl (4.0 equiv.), and solvent (2 mL).
b
Yield after column chromatography.
mol% PPh was used as ligand.
c
5
3
ꢀ
was obtained with 1 mol% of catalyst 2 at 80 C for 8 hr.
reaction in presence of Pd(OAc) and PPh (as ligand)
2 3
The reaction was also carried out in EtOH, CH CN,
resulted in low yield of the desired products (Table 2,
Entry 20). Hence, the pre-formed complex as a catalyst
resulted in better yield as compared with in situ-
generated palladium complex. The enhanced catalytic
activity using the isolated palladium(II) complexes may
be ascribed to the stabilization of the reaction intermedi-
ates by chelation of ONO pincer–type Schiff base ligand,
in addition to the fine-tuning of electronic and steric
properties of the complex to facilitate the formation of
the desired product. Complex 2, containing a methoxy
group at o-position, exhibited the best catalytic activity
which may be rationalized in terms of strong electron-
donating properties of the methoxy group.
3
DMF, and dioxane (Table 2, Entries 10–13) to see the sol-
vent effect, but none of the solvent afforded higher yield
of the product. Therefore, toluene was found to be the
best solvent for the reaction.
Chloroform in presence of a base is known to
generate carbene which undergoes in situ hydrolysis to
[30]
produce CO.
Therefore, a number of bases, such as
NaOH, LiOH, Mg(OH) , and CsOHÁH O, were investi-
2
2
gated to optimize the generation of CO from chloroform.
Among all bases used, KOH provided the best yield of
the desired product (Table 2, Entries 14–17).
To demonstrate the role of the palladium complex as
catalyst, a blank reaction (without complex) was carried
out under identical conditions; however, the reaction did
not yield any product (Table 2, Entry 19). Further, the
After optimization of the reaction conditions, the
scope of palladium(II)-catalyzed carbonylative Suzuki
couplings was extended to a library of halides (aromatic

10 of 14
LAYEK ET AL.
a,b
TABLE 3 Carbonylative Suzuki coupling reactions of various halides with aryl boronic acids
a
Reaction conditions: Aryl halide (1) (1.0 mmol, 1.0 equiv.), boronic acid (2) (1.2 mmol, 1.2 equiv.), Complex 2 (1 mol%), KOH (8 equiv.), and CHCl
3
(4.0
ꢀ
equiv.) in toluene (2 mL) at 80 C.
b
Isolated Yields.
and heteroaromatic) and several substituted and
satisfactory yields (Table 3, Entries 3h, 3i, and 3q). Aryl
unsubstituted boronic acids in the presence of CHCl and
bromides in the place of iodides behaved well in the
carbonylative Suzuki coupling reactions, but produced
relatively low yields (Table 3, Entry 3a). The heteroaryl
iodobenzene, 2-iodopyridine, also produced the
corresponding biaryl ketone product in high yield, indi-
cating that catalyst poisoning does not occur during catal-
ysis. Unfortunately, the reaction of chloro substrates,
instead of bromo and iodo substrates, did not succeed
under the optimized conditions, presumably owing to
less reactivity of the C–Cl bond.
3
KOH and complex 2, and the results are summarized in
Table 3. In general, all the investigated reactions of differ-
ent types of iodobenzene having a variety of functional
groups such methyl, hydroxy, nitro, and trimethoxy on
the aryl ring with different boronic acids having ortho-,
meta-, and para-substituents on the aryl ring underwent
smooth carbonylative coupling under the optimized reac-
tion conditions to afford the corresponding biaryl ketones
in very good yields (Table 3) with negligible biaryl forma-
tion, which may be formed as a result of homocoupling
reaction of aryl boronic acids. The electron-withdrawing
Arylboronic acid bearing both electron-donating and
electron-withdrawing substituents reacted well to afford
the corresponding carbonylative coupling products in
–NO2 group containing iodobenzene also presented

CARBONYLATIVE SUZUKI COUPLING REACTIONS
11 of 14
TABLE 4 Comparison of catalytic activity of the palladium(II) complex with reported catalysts for the synthesis of biphenyl ketones
using CHCl
3
as the source of carbonyl
TOF (hr−
1
)
Reference
Entry
Reaction conditions
Yields (%)
TON
[30]
1
FeCl
Na
2 mL), 24 hr, 120 C
2
(10 mol%), NaI (50 mol%), CsOHÁH
2
O (5 equiv.),
CO (2 equiv.), PivOH (1.5 equiv.), PEG-400
94
9.4
0.40
2
3
ꢀ
(
[
31]
2
3
Pd(OAc)
toluene (2 mL), 12 hr, 80 C
2
(2 mol%), DMAP (20 mol%), KOH (5 equiv.),
75
85
37.5
85.0
3.13
ꢀ
Pd(II) pincer–type complex 2 (1 mol%), toluene (2 mL),
10.63
This work
ꢀ
8
hr, 80 C
TOF, turnover frequency; TON, turnover number; PEG-400, polyethylene glycol 400.
SCHEME 4 Plausible mechanism for
the synthesis of biaryl ketone
high yields (Table 3). Both m- or p-substituted
good cytotoxic properties and inhibited tubulin polymeri-
zation in cancerous cells.
[
67,68]
phenylboronic acids gave very good yields of the
corresponding benzophenone (Table 3, Entries 3b and 3c)
as compared with that of o-substituted arylboronic acid
Furthermore, we performed a comparison of the cata-
lytic activity of the synthesized palladium(II) complexes
[
30,31]
(Table 3, Entry 3m). The low yield with o-substituted aryl
with the state-of-the-art catalysts.
The results
boronic acids may be rationalized in terms of steric
hindrance. Under optimized conditions, the reaction of
hetero-arylboronic acid (i.e. thiophene-2-phenylboronic
acid) also proceeded smoothly to afford the
corresponding product in high yield (Table 3, Entry 3r).
Finally, to validate the practical utility of the devel-
oped catalytic system, the synthesis of two pharmaceuti-
cally important antineoplastic drugs (4-methoxyphenyl)
showed that complex 2 has better catalytic activity than
other catalysts in terms of yield, catalyst loading, reac-
tions time, turnover number (TON), turnover frequency
(TOF), and reusability as shown in Table 4.
A plausible mechanism of this catalytic system is
[
30,31]
proposed in Scheme 4, based on previous reports.
II/0/II
The proposed mechanism entails a Pd
catalytic
II
cycle. First, the catalyst [Pd (L)(PPh )] undergoes a two-
3
(
(
3,4,5-trimethoxyphenyl)methanone and (2-naphthyl)
3,4,5-trimethoxyphenyl)methanone (phenstatin scaf-
electron reduction in presence of arylboronic acid to pro-
0
[51]
duce [Pd (L)(PPh )] (A).
In the next step, (A) reacts
3
folds) was carried out. As reported in Entries 3u–3v
Table 3), the carbonylative Suzuki cross-coupling of
-iodo-1,2,3-trimethoxybenzene with 4-methoxy phe-
nylboronic acid and 2-naphthylboronic acid afforded
4-methoxyphenyl) (3,4,5-trimethoxyphenyl)methanone
and (2-naphthyl) (3,4,5-trimethoxyphenyl)methanone in
very good yields (81% and 85%, respectively). Both com-
pounds belong to the phenstatin family and exhibited
with any aryl halide to generate an arylpalladium(II)
intermediate (B) by an oxidative addition step. The inter-
mediate (B) undergoes insertion of CO (generated in situ
from chloroform and KOH) between the Pd(II) and car-
bon of the aryl group to form the intermediate (C). Sub-
sequently, intermediate (C) reacts with a phenyl anion
(nucleophile generated from arylboronic acid) and
undergoes transmetalation to form the intermediate (D).
(
5
(

12 of 14
LAYEK ET AL.
1
Finally, the intermediate (D) undergoes reductive elimi-
comparison of the H NMR spectra (Figure S17) of the
freshly prepared catalyst with the recovered catalyst
clearly showed that there was no significant changes in
complex 2.
nation to yield the carbonylative Suzuki-coupling product
II
(
biraryl ketone) and regenerate the [Pd (L)(PPh )] cata-
3
0
lyst by oxidation of [Pd (L)(PPh )] species.
3
3.3.1 | Catalytic recyclability
3.3.2 | Mercury poisoning test
The catalyst recyclability is an important step in cataly-
sis as it trims down the overall cost of the process. The
recyclability of complex 2 for the synthesis of biphenyl
ketones via carbonylative Suzuki coupling of
iodobenzene with phenylboronic acid was studied. The
complex showed good catalytic activity until the third
cycle with insignificant decrease in efficiency. The
results of recycling experiments with complex 2, with
respective yields after each run, are depicted in
Figure 5.
To confirm the homogeneous nature of the catalytic
[
69]
system, mercury poisoning test
was carried out.
Under optimized conditions, in presence of an excess of
mercury, the reaction of phenylboronic acid,
iodobenzene, and CHCl3 using palladium complex as
the catalyst proceeded smoothly to afford the benzophe-
none product without any significant effect on the yield
of the product. Therefore, it can be inferred from the
test result that the reaction follows a homogeneous
catalytic pathway.
A 10-mL sealed tube was charged with phenylboronic
acid (0.133 g, 1.2 mmol), iodobenzene (0.204 g, 1.0 mmol),
KOH (8.0 mmol, 0.49 g), CHCl (4.0 mmol, 326 μL), tolu-
4 | CONCLUSIONS
3
ene (2 mL), and palladium(II) complex (1 mol%). The
ꢀ
tube was sealed tightly and stirred vigorously at 80 C for
In summary, three new ONO pincer–type palladium(II)
Schiff base complexes 1, 2, and 3 were synthesized and
characterized by elemental analysis, FT-IR, UV–Visible,
8
hr. After completion of the reaction, the reaction mix-
ture was allowed to cool to room temperature and the
organic layer was extracted with ethyl acetate (3 ×
1
13
1
31
1
H NMR, C{ H} NMR, and P{ H} NMR and their
1
0 mL), washed with water, dried over anhydrous
molecular structures confirmed by single-crystal X-ray
diffraction studies. The catalytic activity of the com-
plexes was explored in the carbonylative Suzuki cou-
pling reactions of aryl halides with arylboronic acids,
Na SO , and concentrated on a rotary evaporator. The
2
4
desired carbonylative product and catalyst were sepa-
rated from the crude reaction mixture by column chro-
matography on silica using ethyl acetate and petroleum
ether as eluent. On the column, organic products moved
first, followed by catalyst. The recovered catalyst was
dried and utilized in successive cycles under the same
reaction conditions and results showed that the catalyst
exhibited good catalytic activity up to three cycles. A
using CHCl as the source of CO. Of the three com-
3
plexes, complex 2 exhibited the best catalytic activity.
The key feature of the reported methodology is that it
offers safe alternative to the use of CO balloons or pres-
sured CO reactors, which are otherwise required for the
carbonylation reactions. The practical utility of complex
2
is successfully demonstrated in the synthesis of two
anti-neoplastic phenstatin scaffolds. High values of TON
and TOF were achieved by complex 2, as compared with
the previous reported catalysts. The catalyst can be
recycled up to three times with insignificant loss in the
catalytic activity. The efficiency, reusability, and opera-
tional simplicity are the main features of the reported
catalytic system. Presently, efforts are in progress to
replace expensive palladium with alternative cheap and
earth-abundant metals for the carbonylative Suzuki cou-
pling reactions.
ACKNOWLEDGEMENTS
Authors are grateful to the SAIF Panjab University,
SAIF IISER Bhopal for the analysis of the samples and
to NTU, Singapore for the single crystal X-ray structure
determination. Samaresh Layek and Bhumika Agrahari
FIGURE 5 Recyclability of the catalyst

CARBONYLATIVE SUZUKI COUPLING REACTIONS
13 of 14
acknowledges the receipt of IIT (ISM) senior research
fellowship.
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2016, 18, 5782.
[31] P. Sharma, S. Rohilla, N. Jain, J. Org. Chem. 2017, 82, 1105.
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ORCID
Devendra D. Pathak https://orcid.org/0000-0002-2097-
4
975
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How to cite this article: Layek S, Agrahari B,
Ganguly R, Das P, Pathak DD. Carbonylative
Suzuki coupling reactions catalyzed by ONO
pincer–type Pd(II) complexes using chloroform as
a carbon monoxide surrogate. Appl Organometal
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