Synthesis and catalytic application of Pd complex catalysts: Atom-efficient cross-coupling of triarylbismuthines with haloarenes and acid chlorides under mild conditions

Article abstract of DOI:10.1002/aoc.3591

Palladium-catalysed cross-coupling reactions are some of the most frequently used synthetic tools for the construction of new carbon–carbon bonds in organic synthesis. In the work presented, Pd(II) complex catalysts were synthesized from palladium chloride and nitrogen donor ligands as the precursors. Infrared and ¹H NMR spectroscopic analyses showed that the palladium complexes were formed in the bidentate mode to the palladium centre. The resultant Pd(II) complexes were tested as catalysts for the coupling of organobismuth(III) compounds with aryl and acid halides leading to excellent yields with high turnover frequency values. The catalysts were stable under the reaction conditions and no degradation was noticed even at 150°C for one of the catalysts. The reaction proceeds via an aryl palladium complex formed by transmetallation reaction between catalyst and Ar₃Bi. The whole synthetic transformation has high atom economy as all three aryl groups attached to bismuth are efficiently transferred to the electrophilic partner.

Full text of DOI:10.1002/aoc.3591

Received: 1 June 2016
DOI 10.1002/aoc.3591
Revised: 18 July 2016
Accepted: 31 July 2016
F U L L PA P E R
Synthesis and catalytic application of Pd complex catalysts: Atom‐
efficient cross‐coupling of triarylbismuthines with haloarenes and
acid chlorides under mild conditions
*
B.D. Jadhav | S.K. Pardeshi
Department of Chemistry, Savitribai Phule Pune
University (formerly University of Pune),
Ganeshkhind, Pune 411007, India
Palladium‐catalysed cross‐coupling reactions are some of the most frequently used
synthetic tools for the construction of new carbon–carbon bonds in organic synthe-
sis. In the work presented, Pd(II) complex catalysts were synthesized from palla-
Correspondence
S. K. Pardeshi, Department of Chemistry,
Savitribai Phule Pune University (formerly
University of Pune), Ganeshkhind, Pune 411007,
India.
1
dium chloride and nitrogen donor ligands as the precursors. Infrared and H NMR
spectroscopic analyses showed that the palladium complexes were formed in the
bidentate mode to the palladium centre. The resultant Pd(II) complexes were tested
as catalysts for the coupling of organobismuth(III) compounds with aryl and acid
halides leading to excellent yields with high turnover frequency values. The cata-
lysts were stable under the reaction conditions and no degradation was noticed even
at 150°C for one of the catalysts. The reaction proceeds via an aryl palladium com-
Email: skpar@chem.unipune.ac.in
plex formed by transmetallation reaction between catalyst and Ar Bi. The whole
3
synthetic transformation has high atom economy as all three aryl groups attached
to bismuth are efficiently transferred to the electrophilic partner.
KEYWORDS
atom efficient, cross‐coupling, organobismuth(III) compounds, palladium complex
catalysts, transmetallation
1
|
INTRODUCTION
These reactions require organometallic reagents in stoi-
chiometric amounts and suffer from low atom economy. In
Palladium‐catalysed cross‐coupling reactions have greatly
improved the possibilities for synthetic organic chemists to
synthesize various useful chemical compounds. Cross‐
coupling reactions between organic halides and organometal-
lic reagents, alkenes or alkynes comprise an important class of
C─C bond forming reactions especially for the preparation of
this context, organobismuth(III) compounds are gaining
momentum as multicoupling organometallic compounds
and can react with three equivalents of electrophilic reagents.
In addition, they are nontoxic and show high reactivity due to
[
5]
the low Bi─C bond energy. Barton et al., Tanaka and
[
6]
[7]
[8]
co‐workers, Rao et al. and Suzuki and Matano have
demonstrated the use of organobismuth compounds in C─C
coupling reactions with routinely use palladium precursors
[
1]
a variety of biaryls and disubstituted alkenes and alkynes.
Since the synthetic utility of these reactions has been demon-
strated to allow the syntheses of complex molecules and func-
tional materials including pharmaceuticals, agrochemicals,
fine chemicals, electric devices, etc., the Nobel Prize in
Chemistry in 2010 was awarded to three pioneers, E. Negishi,
A. Suzuki and R. Heck, who developed cross‐coupling reac-
[
9]
such as Pd(OAc) , Pd(PPh ) and PdCl (PPh ) as catalysts.
2
3 4
2
3 2
10]
[
In a recent study, Rao and Dhanorkar
reported the
threefold cross‐coupling reactivity of sterically demanding
bulky triarylbismuths with various aryl iodides under task‐
specific Pd–Cu dual catalytic conditions. The catalytic role
of palladium in the cross‐coupling reaction via oxidative
addition, transmetallation and reductive elimination steps
forms the main catalytic cycle. Moreover involvement of cop-
per in the catalytic cycle was monitored by in situ generation
[
2]
[3]
tions using organozinc reagents, organoboron reagents
[
4]
and alkenes. These innovations led to the development of
various palladium catalysts, ligands, and other types of transi-
tion‐metal‐catalysed reactions.
Appl. Organometal. Chem. 2016; 1–11
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
JADHAV AND PARDESHI
of arylcopper from arylbismuth and its participation in aryl
exchange during the transmetallation step in the Pd–Cu dual
catalytic cycle. An efficient protocol for the cross‐coupling
of functionalized aryl‐ and heteroarylbismuth reagents with
activation in a palladium‐catalysed reaction, there is a scar-
city of detailed studies concerning the mechanism and effi-
[
14]
ciency of this reduction process.
In the majority of studies, mechanistic investigations for
cross‐coupling reactions have not been performed in the con-
text of a real catalytic cycle, i.e. starting from the Pd(II) or
Pd(0) precursor or from the actual arylpalladium(II) complex
involved in the nucleophilic attack. Amatore et al. reported
catalytic systems which are commonly used in Heck
reactions, consisting of Pd(OAc) (PPh ) or a mixture of
2‐halo or 2‐triflyl pyridines, pyrimidines, pyrazines and
pyridazines bearing reactive moieties was reported by Petiot
[
11]
and Gagnon.
They utilized formylphenyl group‐
containing organobismuth(III) compounds in an atom‐
efficient manner to synthesize 2‐aryl heterocycles under
ordinary conditions. Bismuth salt‐catalysed addition of silyl
nucleophiles and allylstannane to carbonyl compounds and
imines under ordinary conditions and with microwave assis-
tance to synthesize biologically important allyl alcohols was
2
3 2
Pd(OAc) and nPPh (n ≥ 2) which spontaneously and quan-
2
3
titatively generate in situ a zero‐valent palladium complex
able to activate aryl iodides via an oxidative addition. They
[12]
31
reported by Ollevier.
established by means of analytical techniques such as
P
The cross‐coupling of tiarylbismuth compounds with
various organic electrophiles, catalysed by palladium, is a
mild, versatile reaction which can tolerate a wide variety of
functional groups on either coupling partner. It is a stereospe-
cific and regioselective reaction which furnishes high yields
of product, and is therefore used for the synthesis of complex
organic molecules. In addition it offers several additional
advantages, such as being unaffected by the presence of
moisture and the inorganic by‐product of the reaction is
nontoxic and easy to remove from the reaction mixture,
thereby making the cross‐coupling suitable for laboratories
and also for industrial processes. To expand the scope and
reactivity of triarylbismuthines, the development of new and
efficient catalytic protocols is in demand.
NMR spectroscopy and cyclic voltammetry that PPh was
able to reduce Pd(OAc)2 into a zero‐valent palladium
3
complex,
from
the
Pd(OAc) (PPh )
complex,
2
3 2
triphenylphosphine being oxidized to triphenylphosphine
[
15]
oxide.
They also elaborated the mechanistic aspect from
kinetic investigations and reported data on reactivity of
[
16]
Pd(0) complex in oxidative addition with iodobenzene.
Due to the instability of the Pd(0) complex generated from
the mixture of Pd(OAc) and 2PPh , kinetic investigations
2
3
were conducted in the presence of excess of
triphenylphosphine (n > 2).
In another study, Amatore and Jutand elaborated the
reduction of Pd(II) complex into Pd(0) species by assessing
the intermediates formed by cyclic voltammetry. They also
highlighted the role of various innocent ions and added bases
on the formation of intermediates and their cross‐coupling
The active state of the catalyst in cross‐coupling reactions
involves Pd(0) as active oxidation state which is not quite sta-
ble, and hence is generated in situ during the reaction. The
efficiency of the catalysts depends on the ease of interchange
between Pd(II), which is the most stable oxidation state, and
Pd(0), the formation of which in turn depends on the nature
of the ligands employed, in terms of how they would stabilize
the metal by chelation. Undoubtedly, the compatibility of pal-
ladium complexes with a variety of functional groups and the
facile interchange between the two important stable oxidation
states, Pd(II) and Pd(0), are the characteristics and appealing
features held responsible for the popularity gained by
[
17]
ability towards the Heck reaction.
Osawa et al. reported
that a complex of Pd(OAc) combined with 3.0 equiv. of
2
the Binap ligand afforded spontaneously and quantitatively
a zero‐valent palladium complex, Pd(0)(Binap) , and Binap
2
[
18]
oxide.
They performed the reaction in the presence of
known amounts of added water, the presence of which was
necessary for the formation of the Pd(0) complex. Wei et al.
reported studies which provide an in‐depth understanding of
the in situ generation of {L Pd(0)} (n = 1 or 2) catalysts
n
[
19]
under the standard conditions for the Miyaura borylation
[13]
palladium and its compounds in catalysis.
In addition to
where they utilized an air‐stable catalyst, the palladium pre-
palladium, other metals and their complexes utilized for
cross‐coupling reactions are platinum, nickel, cobalt, rho-
dium, iridium and ruthenium complexes which have been
shown to be active catalysts; however, most research focuses
on palladium catalysts. The reason is that in almost every
form palladium has been tried and tested for these reactions
and found to be active in varying degrees.
cursor [(Cy P) Pd(OAc) ].
More recently a study by Buchwald and co‐workers
indicated that in several cases water can promote the reduc-
3
2
2
[
20]
tion of [L Pd(OAc) ] to {L Pd(0)} and phosphine oxide.
n
2
n
[
21]
Other reported methods
for the generation of active
L Pd(0) include the treatment of [L PdCl ] with aqueous
n
n
2
alkaline base (KOH) to give phosphine oxide and the
A key step in the catalytic cycle of a cross‐coupling reac-
tion is the activation step, which remains poorly understood;
however, it directly impacts the overall rate and performance
of a Pd(0)‐catalysed transformation. This step involves the
reduction of a stable Pd(II) precursor to an active, zero‐valent
palladium catalyst and must occur prior to entering the cata-
lytic cycle. Even though the clear inference of catalyst
{L Pd(0)} species. There are a few reports in the literature
n
where, besides phosphine‐mediated reduction, nucleophilic
[
22]
coupling partners such as organolithium reagents,
[
23]
[24]
[25]
organostannanes,
arylboronic acids
and amines
have also been used for the reduction of Pd(II) to Pd(0).
Contemporary research on cross‐coupling reactions is
devoted towards minimization of catalyst cost and reduction

JADHAV AND PARDESHI
3
in catalyst deactivation. One approach for reducing the cata-
lyst cost and deactivation is to design catalysts that have
higher activity and stability and thus can give very high cou-
pling ability. Efforts in this direction have led to the develop-
ment of palladacycle catalysts, which have turned out to be
some of the most active catalysts employed for cross‐cou-
pling reactions of bromoarenes and chloroarenes in terms of
cross‐coupling ability and isolated yields of biaryls. A num-
ber of metal‐catalysed cross‐coupling reactions are performed
in homogeneous media in which the catalyst is located in the
organic phase. This has the advantage of efficient mass trans-
fer between substrates and catalyst and therefore mixing is
palladium tetraphenylphosphine were purchased from Sigma
and used without further purification. The inorganic and
organic salts, solvents and amine ligands were procured from
commercial sources (Hi Media and Loba Chemicals, India)
and used as received. Solvents were distilled before use.
Infrared (IR) spectra were recorded with a Shimadzu 8400
1
13
FT‐IR spectrometer and Bruker Advance. H NMR and
C
NMR spectra were measured with a Varian 300 MHz spec-
trometer with CDCl as the solvent and tetramethylsilane as
3
the internal standard. GC–MS data were collected with a
Shimadzu QP5050 gas chromatograph mass spectrometer.
Elemental analysis of the complexes was carried out with
Shimadzu CHNS‐O EA1108 elemental analyser. The analy-
sis of the contents of the reaction mixture was done using
an Agilent 6850 Series II gas chromatograph and CV mea-
surements were performed with a CHI 6054E electrochemi-
cal analyser. The reactions were monitored by TLC, and
visualized with UV light followed by development in an
iodine chamber. Preparative column chromatography was
carried out on a 10 cm × l.5 cm column with silica gel 60/
120 mesh as stationary phase while petroleum ether and ethyl
acetate mixed in various proportions constituted the mobile
phase.
[
26]
not a problem.
Development of homogeneous catalysts
has proceeded quickly, and sophisticated ligand systems have
been reported that add functionality and selectivity to the
[27]
metal that is central to the catalytic process.
The overall
performance of a catalytic system really depends on the struc-
ture, nature and three‐dimensional orientation of the ligands
in terms of selectivity and efficiency. Palladium is possibly
the most versatile and most widely applied metal in the field
of catalysis due to its high selectivity and activity in a large
number of organic transformations.
As commercially available PdCl possesses a polymeric
2
structure and is almost insoluble in organic solvents, it is
unsuitable as a direct precursor to divalent palladium
reagents. It is very important to prevent the palladium from
precipitating in the reaction medium. In this connection,
ligands play a crucial role in holding the palladium in a stable
state. Palladacycles do this by binding to palladium through
phosphorus and carbon atoms while complexes with nitrogen
donor ligands via nitrogen atom lone pair electron interac-
tions. Since Pd(II) can easily be reduced to Pd(0) by suitable
reductants, in the work reported here, we synthesized new
Pd(II) complexes in the presence of suitable phosphine,
amine and bidentate nitrogen donor ligands which bind the
palladium centre effectively. Catalytic activities of all the
synthesized Pd(II) complexes were tested in coupling
reactions of aryl halides and acid chlorides with
organobismuth(III) reagents as atom‐efficient cross‐coupling
partners under mild conditions. Optimization of reaction
conditions was conducted by screening of solvents, bases,
triarylbismuthines and aryl halides for cross‐coupling reac-
tion with the catalytic system. Moreover we have elaborated
the catalyst activation step of Pd(II) into Pd(0) using cyclic
voltammetry (CV) in order to detect and characterize the
transient palladium species generated in situ from the
synthesized palladium precursors.
2
.2 | Synthesis of dichloropalladium(II) complex
catalyst with nitrogen donor ligands
In a 50 ml round‐bottom flask containing 10 ml of methanol
was added 0.133 g (0.65 mmol) of PdCl2 followed by
0.65 mmol of appropriate donor ligand. The mixture was
stirred at room temperature for 6 h. When a solid precipitated
it was filtered and washed with methanol and dried under
vacuum (Scheme 1).
Yields of the complexes were 88–95%. These synthesized
Pd catalysts were characterized using IR and NMR spectros-
copies and elemental analysis. For all the complexes, it was
found that the elemental analysis values were in good agree-
[
28]
ment with reported ones.
The major IR frequencies were
at 1600 cm− for N─H stretching and 1125 cm for C─N
1
−1
1
stretching. By inspecting the H NMR spectra of the Pd com-
plexes it was clear that there was shift in δ values towards the
2
2
|
EXPERIMENTAL
.1 | General
All reactions and manipulations were conducted under nitro-
gen atmosphere. Palladium acetate, palladium chloride and
SCHEME 1 General synthetic pathway for the synthesis of Pd(II) com-
plexes (A to E) from nitrogen donor ligands

4
JADHAV AND PARDESHI
considering three aryls from BiAr .Thus, 0.75 mmol of
3
product is 100% yield.
2
.5 | General procedure for coupling of acyl chlorides
with triarylbismuthines
Under a nitrogen atmosphere, a two‐neck round‐bottom flask
was charged with Pd catalyst A (60 mg, 0.015 mmol),
triarylbismuth (0.75 mmol), 1,4‐dioxane (3 ml) and acid
chloride (2.5 mmol) and the reaction mixture was stirred at
9
0°C. The reaction was monitored by GC in the presence of
dodecane (0.17 ml, 0.75 mmol) as an internal standard after
2 h, 24 h or at specific reaction time. After cooling to room
SCHEME 2 General synthetic pathway for the synthesis of Pd(II) com-
plexes (F and G) from salen ligands
1
temperature, 3 N HCl was added and the organic product was
extracted with ethyl acetate. The organic layer was dried over
downfield side in comparison to the respective ligands
suggesting bonding through nitrogen with palladium thereby
causing deshielding. Following this procedure, Pd catalysts A
to E were synthesized by changing the N donor ligand, while
symmetric Pd(II) salen complexes F and G were synthesized
MgSO and the solvent was removed in a vacuum. Column
4
chromatography on silica gel afforded the desired aromatic
ketone. All isolated products were characterized using IR,
1
13
H NMR and C NMR spectroscopic techniques.
[29]
as per Scheme 2 with minor modification.
It has been found that the OH band in IR spectra at
3
| RESULTS AND DISCUSSION
1
1
3
1
439 cm− for free ligand and H NMR signal at ca
3.4 ppm were absent for both types of complexes, thereby
With these Pd catalysts in hand, we envisioned to apply them
in the construction of C─C bonds via cross‐coupling reac-
tions. We initiated our studies by optimization of reaction
conditions for cross‐coupling using catalyst A, where 4‐
bromotoluene and triphenylbismuthine were selected as
model coupling partners (Scheme 3).
supporting the deprotonation of phenoxide oxygen atoms
and their subsequent coordination with the Pd(II) centre.
2
.3 | Synthesis of triarylbismuthines
We synthesized triphenylbismuth (72%, m.p. 78°C), tri‐p‐
tolylbismuth (85%, m.p. 119°C) and tri‐p‐methoxy-
phenylbismuth (65%, m.p. 185°C) by metathesis reaction
between bismuth trichloride and appropriate Grignard
We started the reaction in toluene and a low conversion of
desired cross‐coupled product (Table 1, entry 1) encouraged
us to look for a suitable solvent for this reaction. A screening
[30]
reagent as previously reported.
They were characterized
using UV, IR and NMR spectroscopies, GC–MS and elemen-
tal analysis and were used as representative nucleophilic
cross‐coupling partners with aryl halides under palladium
catalysis.
SCHEME 3 PdCl
triphenylbismuthine
2
(bipy)‐catalysed cross‐coupling of 4‐bromotoluene and
2.4 | General procedure for catalytic cross‐coupling of
aryl halides with triarylbismuthines
TABLE 1 Screening of solvents for cross‐coupling of 4‐bromotoluene with
triphenylbismuthine using catalyst A
a
An oven‐dried round‐bottom flask was charged with aryl
halide (3.3 equiv., 0.825 mmol), followed by triarylbismuth
b
Entry
Solvent
CH
Time (min)
Conversion (%)
(
1.0 equiv., 0.25 mmol), K CO (6.0 equiv., 1.50 mmol),
1
2
3
4
5
6
7
8
C
6
H
5
3
120
120
120
120
120
120
120
120
15
69
65
60
20
45
2
3
Pd(II) catalyst (0.1 equiv., 0.0225 mmol) and 1,4‐dioxane
3 ml) under nitrogen atmosphere. The reaction mixture
was stirred in a preheated oil bath maintained at 90°C for
to 8 h. The contents of the mixture were quenched with
Dioxane
DMSO
DMF
(
4
MeCN
THF
water (10 ml) and extracted with ethyl acetate (3 × 15 ml).
The combined organic extract was washed with water
c
DMF–H
2
O
35
(2 × 10 ml) and brine (10 ml), dried over anhydrous Na SO4
H
2
O
0
2
and concentrated to afford a crude product. The crude prod-
uct was subjected to silica gel column chromatography
using 1–2% ethyl acetate–petroleum ether as eluent to obtain
a biaryl. Isolated yield of biaryl was calculated by
a
Reaction conditions: catalyst (0.1 equiv.), triphenylbismuthine (1.0 equiv.), 4‐
2 3
bromotoluene (3.3 equiv.), K CO (6.0 equiv.), solvent (3 ml) at 90°C.
b
Conversions determined by GC analysis of crude reaction mixture.
Tetrabutylammonium bromide was added.
c

JADHAV AND PARDESHI
5
of various solvents was carried out to find the best medium
for the reaction. The detailed optimization results are summa-
rized in Table 1.
Among the various solvents studied, polar aprotic sol-
vents 1,4‐dioxane and dimethylsulfoxide (DMSO) give 69
and 65% conversions at 90°C in 120 min (Table 1, entries 2
and 3). Dimethylformamide (DMF) is another solvent of
choice which delivers 60% conversion under the optimized
protocol (Table 1, entry 4). Other solvents evaluated include
acetonitrile (MeCN) and tetrahydrofuran (THF) which pro-
When the reaction was conducted without addition of
base, prolonged heating was necessary and only 20% conver-
sion was achieved (Table 2, entry 10). This highlights the role
of base in the cross‐coupling mechanism. As the conversion
produced using K CO (Table 2, entry 2) is comparable with
2
3
that produced using TEA, due to the volatility and competi-
tive coordination ability of TEA with palladium complex cat-
alyst, we selected K CO3 as optimized base for further
2
reactions. The use of strong bases like NaOH and KOH gives
less than 5% product formation. Both these bases lead to the
dehalogenation of the halide.
vide low conversions (Table 1, entries 5 and 6). DMF–H O
2
system requires the addition of the phase transfer catalyst
tetrabutylammonium bromide which gives 35% conversion
After the optimization of solvent and base for cross‐cou-
pling with catalyst A, we checked the catalytic performance
of the other synthesized palladium catalysts for the model
reaction under similar reaction conditions with the same Pd
content under optimized conditions. The corresponding
results are summarized in Table 3. It is evident that the
highest conversion of cross‐coupled product is achieved
when the reaction is conducted using catalyst A (Table 3,
entry 2). Catalysts B and C afford 55 and 50% conversions,
respectively (Table 3, entries 4 and 5). Catalysts D and E lead
to 45 and 40% conversion of cross‐coupled product (Table 3,
entries 6 and 7) which are lower than for catalysts A, B and
C. Among the synthesized and tested catalysts, catalyst A is
found to be the most effective. This is possibly because under
the set of reaction conditions it undergoes dissociation and
facilitates oxidative addition which is the key step in cross‐
coupling reactions. It is very interesting to note that the cata-
lytic activities of symmetric Pd(II) salen complexes F and G
are comparable with that of catalyst A which deliver 70 and
65% conversions in 120 min (Table 3, entries 8 and 9). In
the case of complex F, higher cross‐coupling activity was
noticed in comparison with complex G, because the salen
ligand in complex F stabilizes the active Pd species and
avoids the formation of inactive palladium black in compari-
son to the salen ligand in complex G.
(Table 1, entry 7) and suggests the probable role of water as
reaction medium which shuts down the rate of coupling. This
was further checked by conducting the reaction in neat water
where no coupling is observed (Table 1, entry 8).
To select the appropriate base for cross‐coupling reac-
tions, we carried out the model reaction in the presence of
various bases under optimized reaction conditions. The
results are summarized in Table 2. Among the various bases
evaluated, triethylamine (TEA) is found to be the best, deliv-
ering 90% conversion (Table 2, entry 7) which was superior
to inorganic bases such as K CO , Na CO , NaOAc,
2
3
2
3
NaHCO and Cs CO . This may be due to the partial inho-
3
2
3
mogeneity of inorganic bases with the organic substrate,
reagent and solvent, which lowers the conversion and
increases the reaction times compared to organic bases. The
inorganic base K CO provides 85%, while 25% conversion
2
3
is obtained for NaHCO (Table 2, entries 2 and 6). Potassium
3
bases give better conversions compared to their sodium coun-
terparts. For example, amongst all the bases screened, KOAc
and Cs CO give 60% conversions (Table 2, entries 1 and 5),
2
3
which may be attributed to the better solubility of KOAc and
Cs CO in the organic solvents as compared to sodium bases.
2
3
K PO and Na CO afford moderate conversions (Table 2,
3
4
2
3
entries 3 and 4).
In order to compare the catalytic efficiency of our synthe-
sized Pd catalysts with commercially available palladium
salts like Pd(OAc) , generated in situ by adding PPh ligand
2
3
TABLE 2 Effect of bases on cross‐coupling of 4‐bromotoluene with
Pd(OAc) /PPh , PdCl and Pd(PPh ) , we performed the
a
2
3
2
3 4
triphenylbismuthine in 1,4‐dioxane at 90°C
model reaction without altering the catalyst amount. The
results indicate that yields of corresponding cross‐coupled
product lie in the range 25–55% (Table 3, entries 10–13). A
blank experiment confirmed that no biaryl was formed in
the absence of Pd catalyst (Table 3, entry 14).
All the synthesized Pd catalysts were screened for their
cross‐coupling activity and were found to be active for the
cross‐coupling reactions. The activity of these catalyst pre-
cursors was compared based on their initial turnover frequen-
cies (TOFs) calculated for the first 10–20 min of the reaction.
TOFs of corresponding catalytic runs are presented in
Table 3.
b
Entry
Base
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
KOAc
2
2
3
2
2
3
2
3
3
4
60
85
45
35
60
25
90
<5
<5
20
K
K
2
3
CO
3
PO
4
Na
2
CO
3
Cs
2
CO
3
NaHCO
3
TEA
NaOH
KOH
1
0
No base
a
TOFs increase with a decrease in the amount of Pd cata-
lyst A (Table 3, entries 1–3). This clearly indicates that deac-
tivation of catalytically active species occurs to a relevant
Reaction conditions: catalyst (0.1eqiv.), triphenylbismuthine (1.0 equiv.), 4‐
bromotoluene (3.3 equiv.), base (6.0 equiv.), 1, 4‐dioxane (3 ml).
b
Conversions based on GC analysis of crude reaction mixture.

6
JADHAV AND PARDESHI
a
TABLE 3 Influence of various Pd catalysts on cross‐coupling of 4‐bromotoluene with triphenylbismuthine in 1,4‐dioxane at 90°C
b
−1 c
Entry
Catalyst
(bipy)
Catalyst loading (mmol)
Time (min)
Conversion (%)
TOF (h )
1
2
3
4
5
6
7
8
9
PdCl
2
A
A
A
B
C
D
E
F
0.1
120
120
120
120
120
120
120
120
120
120
120
120
120
120
70
80
315
390
445
375
130
320
300
425
410
145
415
72
0.05
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
75
PdCl
PdCl
PdCl
PdCl
2
2
2
2
(PPh
(phen)
(py)
(aniline)
3
)
2
55
50
2
45
2
40
PdL6
PdL7
70
G
65
10
11
12
13
14
Pd (OAc)
Pd (OAc)
2
2
30
/PPh
3
55
PdCl
Pd (PPh
No catalyst
2
25
3
)
4
45
110
—
None
a
2 3
Reaction conditions: Pd catalyst (0.1 equiv.), triphenylbismuthine (1.0 equiv.), 4‐bromotoluene (3.3 equiv.), K CO (6.0 equiv.), 1,4‐dioxane (3 ml).
b
Conversions based on GC analysis of crude reaction mixture.
c
TOF = moles of product per mole of catalyst per unit time.
extent with higher catalyst concentration. Therefore low cata-
lyst concentrations are favoured in order to reduce the deacti-
vation rates, as illustrated in this study.
reactivity of these complexes is dependent on the electron
density on the Pd centre. It has been observed that the catalyst
precursors with a higher electron density on Pd react faster
than those having lower electron density. The higher electron
density on Pd increases the rate of the oxidative addition step
Palladium N,N bidentate amine complex A gives the
highest rate of reaction and TOF value of 445 h− with
1
[
32]
0.025 mmol of catalyst (Table 3, entry 3). With same catalyst
which in turn leads to higher rate of reaction.
Overall it is
amount under similar reaction conditions, catalyst B shows a
decrease in the rate of cross‐coupling with a TOF value of
found that the catalysts with bidentate ligands give better rate
of reaction than those with monodentate ligands.
−
1
375 h which is lower in comparison with catalyst A
With the optimized conditions in hand we probed the
(Table 3, entry 4). This can be attributed to the comparative
general applicability of palladium complex A, PdCl (bipy),
2
ease of dissociation of bipy ligand as compared to phosphine
ligand to give the active catalytic species L2Pd(0). The rates
as well as TOF values are lower for complex C which
possesses a rigid bidentate phenanthroline nucleus (Table 3,
entry 5).
to the cross‐coupling between triarylbismuthines and aryl
halides as per Scheme 4.
Typical results are summarized in Table 4. It is clear
that the cross‐coupling reaction with aryl iodides, as
expected, is better than with the corresponding aryl bro-
mides and provides excellent yields of biaryls 4a and 4b
(Table 4, entries 1–4). The triarylbismuthines containing
electron‐donating groups at the para position, e.g. p‐
tolylbismuthine, give 87% yield of 4b while 79% yield is
recorded for 4c for which the bismuthine contains an elec-
tron‐withdrawing group at the para position (Table 4,
In the case of palladium complexes D and E possessing
amine ligands, a higher rate is achieved for complex D which
contains a pyridine nucleus than complex E where aniline
acts as ligand. The explanation for this activity may be due
to availability of electron density on the Pd centre. The cata-
lyst precursors with a higher electron density on the Pd react
faster than those having lower electron density. The higher
electron density on Pd increases the rate of the oxidative
entries
5
and 6). Heterocyclic aryl bromide 2‐
bromothiophene affords 60% yield of 4 g (Table 4, entry
10), but it requires prolonged reaction time. In the case of
2‐bromopyridine, the yield of 4 h is 55% (Table 4, entry
11) which might be due to complexation between lone pair
electrons on nitrogen of the pyridine nucleus and the Pd
[
31]
addition step which in turn leads to higher rate of reaction.
Palladium complexes F and G possess L6 and L7 as rigid
structural entities of imines which effectively coordinate to
the Pd centre thereby providing high electron density at the
Pd centre, and hence an increase in reaction rate and TOF
values are observed (Table 3, entries 6 and 7).
The evaluated palladium–phosphine catalyst, palladium–
amine catalyst, palladium–N,N bidentate amine catalyst and
Pd–salen catalyst systems afford better conversions in com-
parison with commercially available Pd salts. The order of
SCHEME 4 PdCl (bipy)‐catalysed cross‐coupling of aryl halides with
triarylbismuthines
2

JADHAV AND PARDESHI
7
a
TABLE 4 Cross‐coupling of aryl halides with triarylbismuthines in 1,4‐dioxane at 90°C catalysed by Pd complex A
b
Entry
R
1
X
R'
Time (h)
Product
Yield (%)
1
2
3
4
5
6
7
8
H
H
Br
I
H
H
H
6
6
6
6
6
6
6
6
8
8
8
8
8
6
4a
4a
4b
4b
4b
4c
75
85
75
80
87
79
4‐Me
H
Br
Br
I
4‐Me
4‐Me
4‐OMe
H
H
H
I
c
4‐COCH
4‐COCH
3
3
I
4d
4e
88
Br
Br
Br
Br
Br
Br
C1
4‐Me
H
70
65
60
55
70
65
60
9
1‐Naphthyl
2‐Thienyl
2‐Pyridyl
1‐Naphthyl
4‐Br
4f
c
10
11
12
13
14
H
4 g
4 h
4i
c
H
4‐Me
H
4j
4‐CN
H
4 k
a
3 2 3
Conditions for cross‐coupling reaction. Equivalent ratios with respect to BiAr (1.0 equiv.): aryl halide (3.3 equiv.), Pd catalyst A (0.1 equiv.), K CO (6.0 equiv.), 1,4‐
1
13
dioxane (3 ml). All products characterized using IR, H NMR, C NMR and elemental analyses.
b
Isolated yield obtained after column chromatography.
c
Heteroarenes, namely 2‐bromothiophene and 2‐bromopyridine, were used.
centre which in turn grabs the Pd catalyst and is thereby
least available for catalytic cycle.
system, treatment of acid chloride with organobismuth(III)
compounds in 1,4‐dioxane at 90°C in the presence of catalyst
A resulted in the desired ketone in high yield within 3 h
(Scheme 5).
Typical results are summarized in Table 5. Benzoyl chlo-
ride couples with triphenylbismuth efficiently under the
1‐Naphthyl
bromide
on
coupling
with
triphenylbismuthine provides 65% yield of 4f but requires
longer reaction time (Table 4, entry 9). Under similar reaction
conditions, with p‐tolylbismuthine it affords 4i in 70% yield
thereby signifying the role of p‐methyl substituent of
triarylbismuthine in cross‐coupling reactivity (Table 4,
entry 12). In the case of 4‐iodoacetophenone and
4‐bromoacetophenone, 88% yield of 4d is noticed while the
yield of 4e is 70% which is lower due to the p‐tolyl group
in bismuthine, thereby suggesting the effect of steric bulk
on cross‐coupling ability of bismuthine (Table 4, entries
SCHEME
5
Cross‐coupling reaction of acid chlorides with
organobismuth(III) compounds
7
and 8). In the case of 1,4‐dibromobenzene, a moderate yield
TABLE 5 Cross‐coupling of acid chlorides with triarylbismuthines
catalysed by Pd complex A
a
of 4j is observed (Table 4, entry 13). It is interesting to note
that the electron‐withdrawing cyano group at para position in
4‐chlorobenzonitrile enhances the cross‐coupling ability and
affords 4k in 60% yield (Table 4, entry 14). In all the above
reactions, besides cross‐coupled biaryls, formation of homo‐
coupled biaryls was noticed which is a known reactivity of
organobismuthine under metal catalysis. The detailed expla-
nation is given in Section 4 with the discussion of the
mechanism.
Yield (%)b
Entry
R
2
R'
Time (h)
Product
1
2
3
H
H
3 h
3 h
3 h
3 h
5a
5b
5c
5d
75
60
85
80
H
4‐Me
H
4‐F
After successful demonstration of the cross‐coupling
reaction with aryl halides, we further proceeded to find other
suitable applications of catalyst A and accordingly made an
attempt to use this catalyst for aryl ketone synthesis from acid
chloride. Very recently Gupta et al. used benzimidazole‐
based palladium–N‐heterocyclic carbene for cross‐coupling
c
4
CH
3
H
a
Conditions for cross‐coupling reaction: acid chloride (3.5 equiv.), BiAr
3
(1.0
equiv.), Pd catalyst A (0.1 equiv.), K
2
CO
3
1
(3.0 equiv.), 1,4‐dioxane (3 ml). All
products were characterized using IR, H NMR, C NMR and elemental
analyses.
13
b
Isolated yield obtained after column chromatography.
[33]
c
with boronic acid under ambient conditions.
In our
Aliphatic acid chloride was used.

8
JADHAV AND PARDESHI
present catalytic protocol, giving 75% yield of 5a (Table 5,
entry 1). It is found that the presence of an electron‐donating
group in the organobismuth compound, e.g. p‐
tolylbismuthine, retards the reaction and results in 60% yield
of 5b within 3 h (Table 5, entry 2). In the case of acid chlo-
ride with electron‐withdrawing group at the para position,
the rate of cross‐coupling is accelerated and affords 85%
yield of 5c (Table 5, entry 3). The aliphatic acid chloride ace-
tyl chloride provides an excellent yield of cross‐coupled
alkylaryl ketone 5d (Table 5, entry 4); however, it requires
excess equivalents due to its volatility.
we selected Pd(II) complex catalysts B and D as representa-
tive. This allows the characterization and titration of Pd(II)
or Pd(0) complexes at times shorter than those required to
obtain a first NMR spectrum.
A yellow solution is formed after addition of palladium
3
catalyst B (PdCl (PPh ) ; 1 mM) in 15 cm of DMF
2
3 2
containing nBu NClO (0.1 M) as supporting electrolyte.
4
4
The cyclic voltammogram of it exhibits an irreversible
p
R1
reduction peak (C ) at
(
E
= −0.53 V versus SHE
Figure 1a). When TEA (4 mM) is added to the above reac-
tion mixture, strong peaks (A and A ) and a weak peak
1
1
2
(
A ) newly appear on the oxidation curve. It is very interest-
3
ing to note that peaks C1 and A1 are reversible giving
ΔE = 0.200 V and E1/2 = −0.549 V (with respect to ferrocene
internal standard) for this bielectronic reduction process. This
clearly reflects the role of base in the reduction of Pd complex
B from Pd(II) into Pd(0) complex (Figure 1b). When
iodobenzene (3 mM) is added to the above reaction mixture,
4
| POSSIBLE MECHANISM OF C─C
CROSS‐COUPLING
A proposed catalytic cycle for the cross‐coupling reaction of
triarylbismuthine with aryl halide under palladium catalysis,
which involves oxidative addition–transmetallation–reductive
elimination sequences, is depicted in Scheme 6.
peak C disappears with significant reduction in the intensity
1
of peaks A and A thereby signifying the role of Pd(0)
1
2
Mechanistic investigations on a long time scale where t1/2
is greater than a few minutes are accessible using analytical
techniques such as UV–visible or NMR spectroscopy. How-
ever, these techniques are not adapted to the short times
required for investigating short‐lived species. Electrochemi-
cal techniques like CV or chronoamperometry fill in this
gap. In these techniques, short‐lived species can be generated
by reduction/oxidation of known compounds at steady disc
electrodes and their reactivity monitored by evolution of their
reduction/oxidation current versus scan rate or duration of
complex in the oxidative addition (Figure 1c). Here the
noteworthy feature of the CV curve is that the peak on the
oxidation curve at +0.95 V intensifies with time which
might be due to the oxidation of PPh3 ligand from Pd
complex B to triphenylphosphine oxide. Moreover, when
triphenylbismuthine (1 mM) is added to the above reaction
mixture, there is no peak on the reduction curve but peak
A shows a minor shift in its potential with an increase in
2
intensity. This clearly indicates the role of bismuthine in the
catalytic cycle during the transmetallation step (Figure 1d).
−1
−8
potential steps. Generally time scales from 10 to 10
s
A similar experiment was performed starting from palla-
are amenable to experimentation. Reactivity of stable com-
pounds can be monitored electrochemically since currents
are proportional to concentrations.
dium catalyst D (PdCl (py) ). Ayellowsolution is formed after
2
2
addition of PPh (2 mM) to a brown solution of D (1 mM) in
3
DMF containing nBu NClO (0.1 M). An irreversible reduc-
4
4
As the CV technique is useful for generating, by succes-
sive electron transfers and chemical steps, stable or short‐
lived organometallic species, by reduction/oxidation of metal
complexes, we used it here to detect and characterize the tran-
sient palladium species generated from the synthesized palla-
dium precursors during controlled experiments. Catalyst
activation and conversion of Pd(II) into Pd(0) from the syn-
thesized catalyst were followed using CV. For this purpose
p
R1
tion peak (C ) at E = −1.05 V and oxidation peak (A ) at
1
3
p
O1
E
= +0.85 V versus SHE are observed in the first recorded
CV curve (2 min after mixing) assigned to [Pd(py) (PPh ) ]
2
3 2
(Figure 2a). This reduction peak slowly disappears with time
and new peaks (A and A ) on the oxidation curve appear at
1
2
p
O1
E
= −0.84 and −0.14 Vof which A is reversible to C dem-
1
1
onstratingtheinitialformationofaPd(0)complexfromaPd(II)
complex (Figure 2b). WhenTEA (4 mM) is added to the above
reaction mixture, a clear equilibrium between C and A is
1
1
highlighted with ΔE = 0.211 V and E1/2 = −0.949 V (with
respect to ferrocene reference standard) for the bielectronic
reduction process (Figure 2c). This control experiment clearly
demonstrates the role of base in the catalyst activation during
cross‐coupling reaction under palladium catalysis. There is a
similar trend observed for Pd complex D towards further steps
such as oxidative addition of PhI followed by transmetallation
using triphenylbismuthine as that for catalyst B (Figure 2d).
The present study demonstrates the reduction of a stable
Pd(II) precursor to an active Pd(0) catalyst which occurs prior
to entering the catalytic cycle.
SCHEME 6 Proposed catalytic cycle for cross‐coupling reaction

JADHAV AND PARDESHI
9
FIGURE 1 CV scans of Pd precursor B (PdCl
reaction conditions with a scan rate of 0.1 V s at 25°C: (a) B only; (b) B
2
(PPh
3
)
2
) under different
FIGURE 2 CV scans of Pd precursor D (PdCl
tion conditions with a scan rate of 0.1 V s at 25°C: (a) D only; (b) D in
2
(py)
2
) under different reac-
−
1
−1
in the presence of TEA (4 mM); (c) after addition of PhI (3 mM); (d) sfter
3
the presence of PPh (2 mM); (c) D in the presence of TEA (4 mM); (d) after
3
addition of Ph Bi (1 mM)
addition of PhI (3 mM)

1
0
JADHAV AND PARDESHI
A detailed explanation of the experimental conditions,
ACKNOWLEDGMENTS
procedure and full CV scans for the same are provided in
the supporting information.
B.D.J. is grateful to the University Grant Commission, New
Delhi for awarding a Teacher Fellowship under FIP.
The Pd catalyst precursor added to the reaction mixture
first undergoes an activation step in which it is converted to
the active catalytic species Pd(0). Considering the formation
of both homo‐ and cross‐coupled products, here one can
consider a number of pathways for the reaction between
triarylbismuth with aryl halides catalysed by Pd catalyst A
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How to cite this article: Jadhav, B. D., and Pardeshi,
S. K. (2016), Synthesis and catalytic application of Pd
complex catalysts: Atom‐efficient cross‐coupling of
triarylbismuthines with haloarenes and acid chlorides
under mild conditions, Appl. Organometal. Chem.,
doi: 10.1002/aoc.3591
[
SUPPORTING INFORMATION
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version of this article at the publisher's website.