Highly efficient palladium(II) hydrazone based catalysts for the Suzuki coupling reaction in aqueous medium

Article abstract of DOI:10.1039/c6ra06734d

Synthesis of a new family of air stable palladium(ii)benzhydrazone complexes of the general formula [PdCl(PPh₃)(L)] (where HL = thiophene aldehyde benzhydrazones) incorporating PPh₃ and chloride as co-ligands has been described through a single and convenient step with good yields. All the new complexes have been fully characterized by means of elemental analysis, IR, UV-vis, and NMR spectral methods. The molecular structures of three of the complexes were determined by single crystal X-ray crystallography, which confirm the coordination mode of benzhydrazone and reveal the presence of a distorted square planar geometry around the Pd ion. The complexes 1-5 (0.05 mol%) have been found to be a highly active catalytic system in the mono and double Suzuki-Miyaura cross coupling reaction of deactivated aryl and heteroaryl bromides with different types of aryl boronic acids in neat water and the maximum yield was up to >99%. Notably, these catalysts work well with ultra-low loading of the catalysts and show high turnover numbers in a short time towards different substrates. Moreover, the catalysts could be simply recovered and reused five times without significant loss of efficiency.

Full text of DOI:10.1039/c6ra06734d

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Highly efficient palladium(II) hydrazone based
catalysts for the Suzuki coupling reaction in
Cite this: RSC Adv., 2016, 6, 52101
aqueous medium†
*
Subramanian Muthumari and Rengan Ramesh
Synthesis of a new family of air stable palladium(II)benzhydrazone complexes of the general formula
[PdCl(PPh3)(L)] (where HL ¼ thiophene aldehyde benzhydrazones) incorporating PPh3 and chloride as
co-ligands has been described through a single and convenient step with good yields. All the new
complexes have been fully characterized by means of elemental analysis, IR, UV-vis, and NMR spectral
methods. The molecular structures of three of the complexes were determined by single crystal X-ray
crystallography, which confirm the coordination mode of benzhydrazone and reveal the presence of
a distorted square planar geometry around the Pd ion. The complexes 1–5 (0.05 mol%) have been found
to be a highly active catalytic system in the mono and double Suzuki–Miyaura cross coupling reaction of
deactivated aryl and heteroaryl bromides with different types of aryl boronic acids in neat water and the
maximum yield was up to >99%. Notably, these catalysts work well with ultra-low loading of the
catalysts and show high turnover numbers in a short time towards different substrates. Moreover, the
catalysts could be simply recovered and reused five times without significant loss of efficiency.
Received 14th March 2016
Accepted 21st May 2016
DOI: 10.1039/c6ra06734d
www.rsc.org/advances
Introduction
Suzuki–Miyaura cross coupling reaction1 is a powerful method
for carbon–carbon bond formation that has been applied in
a variety of settings such as natural product synthesis, poly-
mers, many pharmaceutically active compounds, herbicides,
new materials, liquid crystals and ne chemical synthesis.2
Further, the attractive features of the Suzuki coupling reac-
tions are the wide availability, stability to air and moisture,
and low toxicity of boronic acid as well as the facile removal of
boron containing side products of the coupling process. In the
past few years, great advances have been made in developing
active and efficient catalysts by modifying traditional ligands
and discovering new systems.3–5 Sterically demanding, elec-
tron rich phosphines, such as tri-tert-butylphosphine, have
shown high catalytic activity for a variety of substrates.6
Further, new types of ligands, such as heterocyclic carbenes,7
imine and amine palladacycles (1a),8 oximepalladacycles (1b),9
diazabutadiene (1c),10 and simple amines (1d)11 have also
emerged for the use in the Suzuki–Miyaura cross coupling
reaction.
Though palladium complexes exhibit excellent results in
Suzuki cross coupling reactions, most of them were conducted
in some organic solvents. Because, reaction solvents are one of
the most important constituents in any chemical process and
play a key role in deciding its environmental impact as well its
cost, safety and health issues. The ideal solution is to use green
solvent which does not pollute the environment. Thus, naturally
abundant water appears to be a superior choice as a ‘green
solvent’ because of its non-toxic, non-corrosive and non-
ammable nature. In particular case of Suzuki–Miyaura cross
coupling reaction, the choice of water as the medium is
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu,
India. E-mail: rrameshbdu@gmail.com
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
for all the complexes and Suzuki–Miyaura cross coupling products are described
in the text. CCDC 1403396, 1434719 and 1435108 for complexes 1, 4 and 5. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c6ra06734d
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new palladium catalyst precursors of the type [Pd(Cl)(PPh3)(L)]
were synthesized by reacting one equivalent of benzhydrazone
ligands HL1–L5 with one equivalent of PdCl2(PPh3)2 in DMF/
ethanol under reuxed for 2 h (Scheme 2). All the complexes
are obtained as orange solids in high yields 80–83%. The
addition of triethylamine to the reaction mixture was used to
abstract a proton from the imidol oxygen and to facilitate the
coordination of the imidolate oxygen to the palladium. All the
complexes are soluble in basic water, CH3OH, DMF, DMSO, and
CH3CN. The analytical data of all the palladium(II) complexes
are in good agreement with the molecular structures proposed.
Scheme 1 The Suzuki biaryls coupling reaction.
particularly suitable due to the excellent stability of boronic
acids in aqueous media. Though the Suzuki coupling reaction
has strongly beneted from aqueous chemistry,12 most of
halogenated substrates are insoluble in water, resulting in poor
reactivity. To solve this problem, numerous efforts have been
made, such as synthesizing water-soluble ligands,13 adding
surfactants and/or phase-transfer agents, using co-solvents14 or
using microwave or ultrasound15 even utilizing an inorganic salt
as a promoter.16 Yet, most aqueous protocols for cross coupling
reactions have some drawbacks such as high catalyst loading,
elevated temperature, complex workup procedure including
column chromatography or require the addition of organic co-
solvents.17
Generally, the coordination chemistry of hydrazone based
ligands proved to be very interesting, because of their excellent
complexation ability towards transition metals and the possi-
bility of analytical applications. The chemical interest arises
from the ability of benzoylhydrazone to adopt various coordi-
nation modes, leading to enormous structural diversity of their
complexes. These chelating ligands can exhibit amido–imidol
tautomerism and coordinate in either neutral, monoanionic,
dianionic or tetra anionic form bearing unusual coordination
numbers. However, coordination modes are depended on the
reaction conditions, such as metal ion, its concentration, the
pH of the medium and the nature of the hydrazone ligand.18
The most predominant coordination mode is bidentate, ach-
ieved through hydrazine nitrogen and carbonyl oxygen atoms.19
Though several hydrazone metal complexes are known for their
biological activities,20 catalytic applications of these complexes
in carbon–carbon coupling reactions are not much explored.
In continuation of our research on synthesis and catalytic
applications of Ru and Pd complexes,21 herein, we describe the
synthesis and characterization of new air stable palladium(II)
benzhydrazone complexes incorporated with chloride and tri-
phenylphosphine as ancillary ligands. Further, we have devel-
oped the complexes as excellent catalysts for the Suzuki cross-
coupling reactions. The assessment of palladium catalysts and
optimization of the reaction for the coupling of a number of
substrates including deactivated and sterically hindered aryl
and hetero aryl bromides with different boronic acids in water
have been carried out (Scheme 1).
Characterization of the complexes
The FT-IR spectra of the free ligands showed a medium to
strong band in the region 3191–3280 cmꢀ1 which is character-
istic of the N–H functional group. The ligands also display nC]N
and nC]O absorptions in the region 1590–1649 cmꢀ1. The
disappearance of the C]O band observed in the free ligand
correlates with the loss of double-bond character upon depro-
tonation of the N–H group. This supports the lengthening of the
C–O bond which we observe in representative molecular struc-
tures. These observations may be attributed to the enolisation
of –NH–C]O and subsequent coordination through the
deprotonated imidolate oxygen n(C–O) is appeared around 1290–
1254 cmꢀ1. A strong sharp absorption band around 1648–1660
cmꢀ1 in the spectrum of the ligand may be assigned for the
imine stretching frequency. This band is shied to the lower
wave numbers upon complexation with the metal by 15–20
cmꢀ1. In addition, other characteristic bands due to triphenyl-
phosphine are also present around 1436–1486 cmꢀ1 in the
spectra of all the complexes. The absorption spectrum of the
complexes in chloroform at room temperature showed three
bands in the region 240–397 nm. The high intensity bands in
the region 240–339 nm were assignable to ligand-centered (LC)
transitions and have been designated as p–p* and n–p* tran-
sitions. In the complexes the lowest energy bands observed in
the region 389–397 nm were attributed to the Pd(dp) / L(p*)
metal to ligand charge transfer (MLCT) transitions.
1
The H NMR spectra of all the complexes were recorded in
CDCl3 to conrm the bonding of the benzoylhydrazone ligand
to the palladium(II) ion. Multiplets observed in the region
d 8.40–6.45 ppm in the complexes have been assigned to the
Result and discussions
The benzhydrazone ligands HL1–HL5 were prepared according
to the literature procedure,22 in which to a stirred ethanolic
solution of p-substituted benzhydrazide,
a solution of
thiophene-2-aldehyde or 3-methylthiophene-2-aldehyde in
equimolar ratio in ethanol was added drop wise and the reac-
tion mixture was reuxed for 3 h. The cream or pale brown solid
Scheme 2 The formation of palladium catalyst precursors of the type
was obtained in 83–88% yield aer ltered and dried in air. The [PdCl(PPh3)(L)].
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aromatic protons of PPh3 and benzhydrazone ligands. The
signal due to the azomethine proton appears in the region
d 8.93–8.31 ppm. The position of the azomethine signal in the
complexes is slightly downeld in comparison with that of the
free ligand, suggesting deshielding of the azomethine proton
due to its coordination to palladium. The singlet due to the –NH
proton of the free ligand in the region d 11.61–11.92 ppm is
absent in the complex, further supporting enolisation and
coordination of the imidolate oxygen to the Pd(II) ion. There-
1
fore, the H NMR spectra of the complexes conrm the biden-
tate coordination mode of the benzhydrazone ligands to Pd(II)
ion and also indicates the presence of PPh3 group. For the
complexes 4 and 5, additional methyl signals of the thiophene
ring are observed as a singlet at d 2.49–2.50 ppm, whereas for
complexes 2 and 4, the methoxy signals of the benzhydrazone
ring are observed as a singlet at d 3.82 ppm. The 1H NMR spectra
of all the complexes further support the coordination mode of
the benzhydrazone ligand to the palladium(II) ion via the azo-
methine nitrogen and the imidolate oxygen along with the
presence of triphenylphosphine group (Fig. S2–S7, ESI†).
Fig. 2 Molecular structure of 4; thermal ellipsoids are drawn at the
30% probability level. All hydrogen atoms were omitted for clarity.
94.52(8)ꢁ and P(1)–Pd(1)–Cl(1) ¼ 92.03(4)ꢁ, and bond lengths of
˚
˚
1.992(3) A for Pd(1)–O(1), Pd(1)–P(1) 1.992(3) A for Pd(1)–P(1),
˚
˚
2.054(3) A for Pd(1)–N(1) and 2.2913(10) A for Pd(1)–C(1). The
bond lengths and bond angles of the present complexes are in
good agreement with the reported data on related square planar
palladium benzhydrazone complexes.23 Further, the Pd(1)–P(1)
˚
bond length of 2.2631(9) A is in agreement with other struc-
X-ray crystallographic studies
turally characterized palladium–phosphine complexes.24
Further, it was observed that the complexes 4 and 5 adopts
a similar geometry as in the complex 1, with slight changes in
bond angles and bond distances.
The solid state structures of the complexes 1, 4 and 5 were
determined by single crystal X-ray diffraction to conrm the
coordination mode of the benzhydrazone ligand in the
complexes and the geometry of the palladium complexes.
Crystals of 1, 4 and 5 grew from slow evaporation of DMF/
ethanol solvent at room temperature and the complexes crys-
tallized in P21/n space group. The ORTEP view of complexes 1, 4
and 5 has shown in Fig. 1–3. The selected bond lengths and
bond angles are given in Table 1 and the crystallographic data
and structural renement parameters are given in Table 8. The
ligand coordinates with Pd(II) ion via the azomethine nitrogen
and the imidolate oxygen forming ve membered chelate ring.
One triphenylphosphine group (trans to the azomethine
nitrogen) and one chloride ion (trans to imidolate oxygen) also
coordinate to the palladium(II) ion to form a ONClP square
plane. The complex is having a distorted square planar geom-
etry as reected in all the bond parameters around Pd(II) ion.
The bond angles around palladium(II) ion are O(1)–Pd(1)–N(1) ¼
79.30(11)ꢁ, O(1)–Pd(1)–P(1) ¼ 94.02(8)ꢁ, N(1)–Pd(1)–Cl(1) ¼
Coupling reaction of aryl halides
The Suzuki coupling reaction of deactivated aryl halides with
aryl boronic acids catalyzed by palladium complexes are
enlarged due to their unique properties and variable oxidation
states. Our study commenced with the coupling of a wide range
of deactivated aryl or hetero aryls bromides and different aryl
boronic acids using all ve palladium(II) hydrazone complexes
as catalysts for the synthesis of biaryls. Literature studies reveal
that there is not a set of rule that a specic solvent and a certain
base are used to attain the highest efficiency of catalysis in
Suzuki–Miyaura cross coupling reactions. Aryl bromides are
cheaper and readily available substrates than the corresponding
iodides but more reluctant to undergo the catalytic Suzuki
Fig. 1 Molecular structure of 1; thermal ellipsoids are drawn at the Fig. 3 Molecular structure of 5; thermal ellipsoids are drawn at the
50% probability level. All hydrogen atoms were omitted for clarity.
50% probability level. All hydrogen atoms were omitted for clarity.
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Table 1 Selected bond lengths (A˚) and bond angles (ꢁ) of complexes 1, Table 3 Effect of catalyst loadinga
4 and 5
Complex
1
4
5
Pd(1)–O(1)
Pd(1)–Cl(1)
Pd(1)–P(1)
Pd(1)–N(1)
C(1)–S(1)
C(4)–S(1)
C(6)–O(1)
C(5)–N(1)
1.992
2.2913
2.2631
2.054
1.702
1.726
1.313
1.279
1.304
1.387
79.30
94.02
172.79
173.31
94.52
92.03
130.3
113.1
110.5
116.6
91.1
2.005
2.2749
2.2565
2.057
1.702
1.738
1.302
1.289
1.303
1.396
78.88
93.22
169.89
174.42
95.65
92.33
130.1
113.1
110.9
116.8
90.7
2.0153
2.2667
2.2773
2.0566
1.7125
1.7320
1.3024
1.3063
1.2824
1.3921
78.66
Entry
Catalyst (mol%)
Yieldb (%)
TONc
1
2
3
4
5
6
7
8
1.0
0.5
0.2
0.1
0.05
0.01
0.001
0.0001
99
99
99
95
93
71
60
Traces
99
198
495
950
1860
7100
60 000
—
C(6)–N(2)
N(1)–N(2)
O(1)–Pd(1)–N(1)
O(1)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
O(1)–Pd(1)–Cl(1)
N(1)–Pd(1)–Cl(1)
P(1)–Pd(1)–Cl(1)
C(5)–N(1)–Pd(1)
N(2)–N(1)–Pd(1)
C(6)–O(1)–Pd(1)
C(5)–N(1)–N(2)
C(1)–S(1)–C(4)
96.17
174.64
172.07
94.44
a
Reaction condition: 4-bromoacetophenone (1 mmol), phenylboronic
acid (1.5 mmol), base (2 mmol) and solvent (2 mL) at 100 ꢁC for 3 h.
b
Isolated yield aer column chromatography based on 4-
c
90.62
bromoacetophenone (average of two runs). TON mol of product/mol
129.24
113.19
110.77
111.23
91.61
of catalyst.
feasibility of our study and to optimize the reaction condition
including solvents, bases (Table 2) and catalyst loading (Table
3). The initial study was begun in water as a solvent with 2.0
equiv. of K2CO3 as base at 100 ꢁC in the reaction and the desired
coupling product was obtained in 99% isolated yield (Table 2,
entry 1) and yield was decreased to 63% at r.t. (entry 2).
Although cross-coupling reactions were carried out in an open
vessel under an atmosphere of air, less than 1% of homo
coupling was observed. Correspondingly, washing the residue
twice with desalinated water was sufficient to obtain pure
products. However, we have also examined to increase the yield
of the reaction by optimize the different solvents and co-solvent.
The dioxane–H2O solvent system with K2CO3, however, gave
lesser yield of 87% than water as a solvent (entry 3). The yield
was gradually increase to 94% and 97% with traces of by-
product when DMF–H2O and toluene–H2O were the solvent of
reaction. Therefore, we have checked the catalytic efficacy of
palladium benzhydrazone catalysts in Suzuki reaction using
aryl bromide as the substrate. We initially investigated the
reaction of 4-bromoacetophenone with phenylboronic acid
using complex 1 (1 mol%) as a test catalyst to establish the
Table 2 Optimization of reaction conditionsa
Entry
Solvent
Base
Yieldb (%)
TONc choice (entry 4 and 5). The reaction was more efficient in
toluene (89%) but still signicant amounts of biphenyl as a by-
1
2
3
4
5
6
7
8
H2O
H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
—
Na2CO3
KOH
NaOH
K3PO4
Et3N
99
63d
87
95
97
89
86
54
94
—
96
93
92
90
73
51
99
product prompted us to study in various solvents (entry 6).
When, the etheral solvent dioxane was used in the reaction the
63
89
95
97
Dioxane–H2O
DMF–H2O
Toluene–H2O
Toluene
Dioxane
THF
DMF
H2O
H2O
H2O
89
86
54
94
—
96
93
92
90
73
51
Table 4 Effect of the catalyst on Suzuki couplinga
9
10
11
12
13
14
15
16
Entry
Complex
Yieldb (%)
H2O
H2O
H2O
H2O
1
2
3
4
5
(1) [Pd(Cl)(PPh3)(L1)]
(2) [Pd(Cl)(PPh3)(L2)]
(3) [Pd(Cl)(PPh3)(L3)]
(4) [Pd(Cl)(PPh3)(L4)]
(5) [Pd(Cl)(PPh3)(L5)]
93
91
88
92
90
Pyridine
a
Reaction condition: 4-bromoaceophenone (1 mmol), phenylboronic
acid (1.5 mmol), base (2 mmol) catalyst 1 (1 mol%) and solvent (2 mL)
b
at 100 ꢁC for
3
h
under air.
Isolated yield aer column
a
chromatography based on 4-bromoacetophenone (average of two
Conditions: 4-bromoacetophene (1.0 mmol), phenylboronic acid (1.5
c
d
runs). TON mol of product/mol of catalyst. Reaction in open air at mmol), catalyst 0.05 mol%, K2CO3 (2.0 mmol), water (3 mL), 100 ꢁC, 3
r.t. for 32 h.
h. b Isolated yields.
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Table 5 Suzuki coupling of aryl bromides with arylboronicacidsa
Entry
Aryl bromide (R)
Aryl boronic acid (R1)
Product
Yieldb (%)
1
2
3
–H
93
95
98
–CH3
–OCH3
4
5
6
–Cl
–H
–H
90
58d
91
7
–CH3
95
Entry
Aryl bromide (R)
Aryl boronic acid (R1)
Product
Yieldb (%)
8
–OCH3
–Cl
98
87
93
9
10
–H
11
12
13
–CH3
–OCH3
–Cl
94
96
86
14
15
–H
81c
82c
–OCH3
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Table 5 (Contd.)
Entry
Aryl bromide (R)
Aryl boronic acid (R1)
Product
Yieldb (%)
16
–Cl
74
a
Reaction condition: aryl bromide (1 mmol), substituted phenylboronic acid (1.5 mmol), base (2 mmol), solvent (2 mL) at 100 ꢁC for 3 h. b Isolated
c
yield aer column chromatography based on aryl bromide (average of two runs). Double Suzuki cross coupling (aryl bromide (1.0 mmol),
d
arylboronic acid (3 mmol), 0.05 mol% complex 1, K2CO3 (4.0 mmol) were used). Aryl chloride (1 mmol), substituted phenylboronic acid (1.5
mmol), base (2 mmol), solvent (2 mL) at 100 ꢁC for 8 h.
yield was decreased to 86% (entry 7) while in THF the yield was the benzhydrazone ligand coordinated to palladium (Table 4).
drastically decreased to 54% (entry 8). The reaction rate and To this purpose, we have studied the catalytic activities of all
yield was signicantly enhanced when polar solvent like DMF complexes 1–5 in the coupling reaction using C : S ratio of
used in the reaction (entry 9) but still with traces of biphenyl 1 : 2000 (0.05 mol%) and it was observed that all the complexes
was obtained as a by-product. Hence, water is used as a choice displayed more or less similar catalytic activity, suggesting that
of solvent for further studies due to its high yield, non-toxic, there is no signicant effect on the catalysis despite the change
environmental benets and safety. In the absence of base no in the substituent on the aromatic fragment of the benzhy-
reaction was occurred, indicates base is necessary for the reac- drazone ligand in the complexes. Substitution of methyl and
tion (entry 10). Since base plays a major role in Suzuki cross methoxy group of the coordinated ligand in complex 4 and 5
coupling reaction, a series of bases such as K2CO3, Na2CO3, does not make any changes in yield. So we have chosen complex
KOH, NaOH, K3PO4, NEt3, and pyridine were tested and highest 1 for further studies for its excellent catalytic activity over the
yield was obtained with K2CO3 base (entries 11–16). Among all other four complexes.
the bases, inorganic bases gave better result than organic bases.
Under the above optimal condition for a cross-coupling
The bases such as carbonates or phosphate allow us to obtain reaction (0.05 mol% of catalyst 1, 2 equiv. of K2CO3, water,
much higher reaction rates. Based on this we chose K2CO3 as 100 ꢁC) different aryl and heteroaryl bromides and various
base and H2O as solvent for further studies.
functional boronic acids were studied. It should be pointed out
Further to optimize the catalyst loading, it is preferred to use that, in contrast with other catalytic systems that require
small amount of catalyst and still achieve high TON in the specic bases/additives or relative amounts of starting mate-
Suzuki–Miyaura cross coupling reactions. Recently, it has been rials, the use of complex 1 in water proved to be quite insensible
reported high catalyst loading leading to palladium inactive to such variables/variations, a clear advantage from a practical
black formation and also inhibit the catalytic cycles.25 To our point of view.26
pleasure the reaction worked well with different catalyst:
Among all possible combinations 2,6-biphenylic coupling
substrate (C/S) ratio and the results are summarized in Table 3. products were synthesized in good to excellent isolated yields
The reaction of 4-bromoacetophenone with phenylboronic acid (Tables 5 and 6, entries 1–30). A variety of functionalities are
furnished excellent yields (99%) when 1.0, 0.5 or 0.2 mol% of well tolerated for the aryl bromide containing ketone, methyl
catalysts were loaded (entries 1 to 3). Even when 0.05 mol% of the and methoxy groups. Furthermore, a diverse choice of aryl
catalyst used, excellent isolated yield of the cross coupling boronic acids bearing methyl, chloride and methoxy substitu-
product is obtained (entry 5). Further, it was observed that even ents were successfully coupled in the cross coupling reactions.
under very low catalyst loading of 0.01 mol% the reaction The coupling of 4-bromoacetophenone with phenylboronic
proceeds smoothly accompanied by a drop in isolated yields with acid, 4-methyl phenylboronic acid, 4-methoxy phenylboronic
high turnover number (entry 6). Notably, the reaction can also be acid and 4-chlorophenylboronic acid gave the desired products
conducted with an ultra-low catalyst loading of 0.001 mol% in high yields from 90–98% showing high efficiency and good
which resulting the highest turnover value of 60 000 (entry 7). selectivity (Table 5, entries 1–4). Among these, coupling of 4-
Since the isolated yield are good with appreciable TON when 0.05 methoxy phenylboronic acid gave excellent conversion of biaryls
mol% of catalyst is used, it was concluded that 0.05 mol% cata- (entry 3), which takes place at a faster rate than that of phe-
lyst loading is the best condition for further coupling reactions. nylboronic acid. The presence of electron withdrawing substit-
Since the importance of catalyst stability and longevity has uent (chloro) on boronic acids reduces the conversion
major applications for industrial processes, next we examined considerably. Additionally, 4-bromotoluene could also couple
the efficiency of performance of changing the R substituents on with various arylboronic acids in high yields (entries 6–9). 4-
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Table 6 Suzuki coupling of heteroaryl bromides with aryl boronic acidsa
Entry
17
Heteroaryl bromide (R)
Aryl boronic acid (R1)
Product
Yieldb (%)
80
–H
18
19
20
–CH3
–OCH3
–Cl
81
82
78
21
22
23
–H
80c
81c
78c
–OCH3
–Cl
24
25
26
–H
72
76
70
–OCH3
–Cl
27
28
29
–H
80
81
82
–CH3
–OCH3
30
–Cl
78
a
Reaction condition: heteroaryl bromides (1 mmol), substituted phenylboronic acids (1.5 mmol), base (2 mmol), solvent (2 mL) at 100 ꢁC for 3 h.
Isolated yield aer column chromatography based on heteroaryl bromides (average of two runs). Double Suzuki cross coupling (heteroaryl
b
c
bromide (1.0 mmol), arylboronic acid (3 mmol), 0.05 mol% complex 1, K2CO3 (4.0 mmol) were used).
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Table 7 Recycling of catalysta
have been proved to be a particularly difficult class of substrates
for Suzuki reactions. The Suzuki–Miyaura cross coupling reac-
tion of hetero aryl bromides with boronic acids using the same
complex 1 was performed and the results are summarized in
Table 6. Interestingly, the reactions could also be conducted
with catalyst loading of 0.05 mol%. The reactions of 2-bromo-
pyridine with boronic acids bearing electron withdrawing and
electron donating substituents were efficient and afforded the
desired products in high yields (entries 17–20). Further, double
Suzuki coupling of 2,6-dibromo pyridine with various aryl
boronic acid have been performed and gave the expected triaryls
product in 78–81% isolated yield (entries 21–23). Electronically
activating group in heteroaryl ring provided the corresponding
coupling product in 70–76% yield (entry 24–26). The reaction of
2-bromothiophene with different aryl boronic acids proceeded
well and corresponding products were obtained in good yields
(Table 6, entries 27–30) and the electronically activated boronic
acid gave best yield then electronically deactivated and non-
activated boronic acids. All these indicated that the electron
donating substituents on the boronic acids had great inuence
on the cross coupling reactions. The loss of yields in these
reactions compared to aryl halides indicate that the hetero
element in the a-substituted hetero aryl bromides would have
a possible interaction with palladium which has a deactivation
effect on the rate of the reaction.28
At this point, a comparison in terms of catalytic efficiency
and scope of application can be established between our
complex 1 and other structurally related palladium catalysts.
Zeng and his co workers have reported a pyridyl-supported
pyrazolyl-N-heterocyclic carbene complex for Suzuki–Miyaura
reaction of aryl halides in air and water with 2 mol% catalyst
loading.29 A palladium chelating complex with nitrogen ligands
have proven to be efficient catalyst for Suzuki reaction of aryl
bromides in the presence of TBAB–water mixture at higher
temperature.30 Moreover, an efficient palladium-catalyzed
ligand-free Suzuki–Miyaura coupling in water has been
Catalyst recycle
Yieldb (%)
TONc
1
2
3
4
5
6
7
93
90
82
68
59
44
Traces
1860
1800
1640
1500
1280
880
—
a
Conditions: 4-bromoacetophene (1.0 mmol), phenylboronic acid (1.5
mmol), catalyst 0.05 mol%, K2CO3 (2.0 mmol), water (3 mL), 100 ꢁC, 3
h. b Isolated yields. c TON mol of product/mol of catalyst.
Methoxybromobenzene underwent smooth reaction with
various arylboronic acids and gave the coupled products in high
yields (entries 10–13). Among all p-bromoacetophenone gave
high yield compared to other aryl bromides. An increase in the
amount of phenyl boronic acid and the prolongation of the
reaction time have a slight impact upon the selectivity toward
terphenyl (entries 14–16). Under present optimal condition, we
have attempted for coupling of aryl chloride with arylboronic
acid and the catalyst was found to stable and active enough to
handle deactivated aryl chloride. However the coupled product
was obtained albeit with low yield even prolonging reaction
time (entry 5).
The Suzuki–Miyaura cross coupling reactions of hetero-
arylhalides is of more interest in the pharmaceutical industry
for the preparation of biologically active compounds. Practi-
cally, pyridine and thiophene based compounds are the most
common heterocyclic pharmaceutically active compounds.27
Despite efforts by numerous groups, pyridine based bromides
Table 8 Crystal data and structure refinement for complexes
Complex 1
Complex 4
Complex 5
Empirical formula
Formula weight
Wavelength (A)
C
30H24CN2OPPdS
C31H26ClN2OPPdS
647.42
0.71073
C32H28ClN2O2PPdS
677.44
0.71073
633.39
0.71073
˚
Temperature (K)
Crystal system
Space group
296(2)
Monoclinic
P2(1)/c
14.4356(4)
11.5647(3)
17.7528(4)
90
107.8100(10)
90
2821.68(12)
293
Monoclinic
P2(1)/n
9.4677(5)
31.1867(15)
10.2754(5)
90
107.706(2)
90
2890.3(3)
296(2)
Triclinic
ꢀ
P1
9.605(2)
18.386(5)
19.197(5)
112.769(12)
101.774(12)
97.141(13)
2981.6(13)
˚
a (A)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
g (ꢁ)
3
˚
Volume (A )
F(000)
1280
1312.0
1376
Crystal size (mm3)
Goodness-of-t on F2
Final R indices [I > 2s(I)]
R indices (all data)
0.35 ꢂ 0.30 ꢂ 0.30
0.30 ꢂ 0.20 ꢂ 0.20
0.16 ꢂ 0.14 ꢂ 0.12
1.143
1.313
0.846
R1 ¼ 0.0347, wR2 ¼ 0.0959
R1 ¼ 0.0605, wR2 ¼ 0.1292
R1 ¼ 0.0339, wR2 ¼ 0.0774
R1 ¼ 0.0774, wR2 ¼ 1.313
R1 ¼ 0.0380, wR2 ¼ 0.0693
R1 ¼ 0.0693, wR2 ¼ 0.1168
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developed with good yields using 2 mol% of catalyst and 8 mol%
of additives.31 Suzuki–Miyaura coupling of heterocyclic substrates
using palladium complexes containing in expensive phosphine
ligands with lower substrate scope has been described by Aziz
Fihri et al.32 Recently, Yuhong Zhang and co-workers have devel-
oped a phosphine free catalyst system for the Suzuki reaction of
aryl halides with high catalyst loading and aid of poly(ethylene
glycol) (PEG).33 Our complex 1, bearing the key hydrazone ligand
and phosphine moiety, has proven to be an excellent, much more
efficient, and general catalyst in Suzuki coupling reactions, with
the crucial advantage of the use of aqueous environments. It can
be tentatively proposed that the relatively distorted square planar
geometry due to a higher ring strain and the phosphine function
have a benecial effect not only in the activity of palladium
complex 1 but also in its solubility in aqueous media and stability
under the reaction conditions.
Further, to show the recycling ability of our new catalyst, the
reaction of phenylboronic acid with 4-bromoacetophenone was
conducted under optimal reaction condition (Table 7). A ask
was charged with 4-bromoacetophenone, phenylboronic acid,
catalyst 1, K2CO3, and water (3 mL). The mixture was stirred at
100 ꢁC for 3 h in open air. Aer the mixture was cooled, the
aqueous layer was extracted with diethyl ether (3 ꢂ 3 mL), and
the ask was charged again with 4-bromoacetophenone (1
mmol) and phenylboronic acid and K2CO3. Every time aer
cooling and extraction with diethyl ether, the reagents and base
were added and the reaction was repeated. The combined
Experimental section
General
All reactions were carried out under an atmosphere of air. C, H,
N and S analyses were carried out with a Vario EL III CHNS
elemental analyzer. IR spectra were recorded on a Perkin-Elmer
597 spectrophotometer, using KBr pellets. 1H NMR and 13C
NMR were conducted on a high resolution Bruker Avance 400
spectrometer, in CDCl3 as solvent. All chemical shis are re-
ported in parts per million (ppm) downeld from tetrame-
thylsilane. Abbreviations used in the NMR follow-up
experiments: br, broad; s, singlet; d, doublet; t, triplet; m,
multiplet. Electronic spectra of the complexes in chloroform
solution were recorded with a Cary 300 Bio UV-vis Varian
spectrophotometer in the range 800–200 nm using cuvettes of 1
cm path length. Melting points were performed with an elec-
trical instrument and are uncorrected. The ligands HL1–HL5,
were prepared according to literature methods. All other
chemicals were used as received. Solvents were dried and
freshly distilled prior to use. Ethanol was dried over Na/
diethylphthalate, distilled under argon and deoxygenated
prior to use. NEt3 was purchased from Merck and dried over
CaH2, distilled under argon and deoxygenated prior to use.
Column chromatography was performed on neutral silica gel.
Synthesis of the palladium complexes
organic layer was dried over sodium sulfate and evacuated in All the new palladium complexes were prepared by the
1
vacuo and the residue was analyzed by H and 13C NMR. The following general procedure: ethanolic solution (20 mL) of
recovered catalyst was successfully reused in the subsequent [PdCl2(PPh3)2] (50 mg, 1 equiv.) was slowly added the appro-
three cycles with consistent activity with 93%, 90%, and 82% priate hydrazone ligand (16.4–19.5 mg, 1 equiv.) and triethyl-
isolated yields (entries 1–3). The coupled product was signi- amine (0.5 equiv.) in DMF (5 mL). The reaction mixture was
cantly dropped in fourth and h cycles (entries 4 and 5). A allowed to reux for 2 hours. Orange color solid was ltered,
drastic decrease was observed aer h run (entries 6 and 7), washed with diethyl ether (5 mL) and recrystallized from
probably due to a gradual decomposition of catalyst 1 under the ethanol–dimethylformamide. The purity of the complexes was
reaction conditions.34
checked by TLC. All the complexes were highly soluble in basic
water, MeOH, CH2Cl2, CHCl3 and acetone.
[Pd(Cl)(PPh3)(L1)] (1). A solution of [PdCl2(PPh3)] (50 mg,
0.071 mmol) in ethanol (20 mL) was added drop wise to the
Conclusion
In conclusion, we have successfully demonstrated the synthesis DMF solution (5 mL) of ligand HL1 (16.4 mg, 0.071 mmol) and
and characterization of a new series of simple and efficient air triethylamine (0.69 mL, 0.5 equiv.) were added. The reaction
stable Pd(II) complexes bearing bidentate benzhydrazone mixture was reuxed for 2 h and thereaer kept for crystalli-
ligands. Analytical, spectral and single crystal X-ray diffraction zation. Brown crystals of the product were obtained on slow
studies of the complexes evidenced azomethine nitrogen and evaporation of the reaction mixture over the period of 2 weeks.
the imidolate oxygen co-ordination mode of the ligand to The crystals obtained were suitable for X-ray diffraction. Yield:
palladium and reveals the presence of a distorted square planar 82% (45.4 mg), mp 220 ꢁC (with decomposition). Found: C,
geometry around Pd(II) ion. The catalysts were found to be active 56.88; H, 3.82; N, 4.42; S, 5.06. Calc. for C30H24ClN2OPPdS: C,
for the Suzuki–Miyaura coupling reaction of challenging 56.81; H, 3.99; N, 4.44; S, 5.07. IR (KBr, cmꢀ1): 1597 s n(C]N–N]C)
substrates like deactivated aryl and heteroaryl bromides in good 1281 (m) n(C–O), 425 (s) n(M–N)
1H NMR (400 MHz, CDCl3) (d
,
.
to excellent yields in water. The catalysts work well with 0.05 (ppm)): 8.34 (s, 1H, CH]N), 7.13–7.84 (m, 23H, aromatic). dC
mol% and exhibit appreciable TON in relatively short time. The (100 MHz) 144.97, 136.20, 137.41, 131.47, 129.36, 126.86. UV-vis
catalyst can be recyclable for at least ve times with excellent (CHCl3, lmax (nm)); 3 (dm3 molꢀ1 cmꢀ1): 389(2614), 368(4219)
yields. The coupling reaction can be readily carried out under in and 282(4004).
open atmosphere and the reaction conditions were optimized
[Pd(Cl)(PPh3)(L2)] (2). The complex 2 was prepared by the
for a simple, green and support free procedure, without using same procedure as that described for complex 1 with HL2 (18.5
any phase transfer agents/additives. Detailed studies of the mg, 0.071 mmol) and [PdCl2(PPh3)] (50 mg, 0.071 mmol). Yield:
reaction mechanism will be undertaken in our future work.
81% (42.9 mg), mp 218 ꢁC (with decomposition). Found: C,
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56.12; H, 3.95; N, 4.22; S, 4.83. Calc. for C31H26ClN2O2PPdS: C, added catalyst (0.05 mol%) in water (3 mL) at 100 ꢁC in the
56.11; H, 3.96; N, 4.21; S, 4.82. IR (KBr, cmꢀ1): 1557 s n(C]N–N]C) presence of K2CO3 (2.0 mmol). The mixture was stirred at 100 ꢁC
1255 (m) n(C–O), 434 (s) n(M–N)
1H NMR (400 MHz, CDCl3) (d for 3 h in air. Aer the mixture was cooled, the aqueous layer
,
.
(ppm)): 8.31 (s, 1H, CH]N), 6.79–8.30 (m, 24H, aromatic), 3.82 was extracted with diethyl ether (3 ꢂ 3 mL) and the ask was
(s, 3H, methoxy). dC (100 MHz) 135.07, 135.00, 132.06, 130.53, charged again with 4-bromoacetophenone (1.0 mmol), phenyl-
128.54, 128.02, 113.22. UV-vis (CHCl3, lmax (nm)); 3 (dm3 molꢀ1 boronic acid (1.5 mmol) and K2CO3 (2.0 mmol) was added and
cmꢀ1): 390(1081), 369(7689) and 288(5534).
the reaction was repeated. The identity of the recovered catalyst
[Pd(Cl)(PPh3)(L3)] (3). The complex 3 was prepared by the is further conrmed by TLC and 1H NMR spectrum before next
same procedure as that described for complex 1 with HL3 (18.8 cycle.
mg, 0.071 mmol) and [PdCl2(PPh3)] (50 mg, 0.071 mmol). Yield:
83% (43.7 mg), mp 226 ꢁC (with decomposition). Found: C,
X-ray crystallography
53.95; H, 3.47; N, 4.19; S, 4.80. Calc. for C30H24Cl2N2OPPdS: C,
Single crystals of all the complexes were grown by slow evap-
53.94; H, 3.48; N, 4.18; S, 4.79. IR (KBr, cmꢀ1): 1595 s n(C]N–N]C)
,
oration of DMF–ethanol mixture at room temperature. A single
crystal of suitable size was covered with Paratone oil, mounted
on the top of a glass ber, and transferred to a Stoe IPDS
diffractometer using monochromated Mo-Ka radiation (kI ¼
0.71073). Data were collected at 293 K. Corrections were made
for Lorentz and polarization effects as well as for absorption
(none). The structure was solved with direct method using SIR-
97 (ref. 1) and was rened by full matrix least-squares method2
on F2 with SHELXL-97. Non-hydrogen atoms were rened with
anisotropy thermal parameters. All hydrogen atoms were
geometrically xed and allowed to rene using a riding model.
Frame integration and data reduction were performed using
the Bruker SAINT-Plus (Version 7.06a) soware. The multi-
scan absorption correction was applied to the data using
SADABS soware. Crystal data for the structure are given in ESI
(Table 1†). CCDC reference numbers 1403396, 1434719 and
1435108 contain the supplementary crystallographic data for
1, 4 & 5.†
1290 (m) n(C–O), 423 (s) n(M–N).
1H NMR (400 MHz, CDCl3) (d
(ppm)): 8.82 (s, 1H, CH]N), 6.45–7.79 (m, 24H, aromatic).
dC(100 MHz) 135.11, 135.05, 134.51, 134.46, 130.21, 128.81,
128.20, 126.37, 125.64. UV-vis (CHCl3, lmax (nm)); 3 (dm3 molꢀ1
cmꢀ1): 394(0948), 339(97 416) and 242(8926).
[Pd(Cl)(PPh3)(L4)] (4). The complex 4 was prepared by the
same procedure as that described for complex 1 with HL4 (17.3
mg, 0.071 mmol) and [PdCl2(PPh3)] (50 mg, 0.071 mmol). Yield:
80% (43.4 mg), mp 223 ꢁC (with decomposition). Found: C,
57.51; H, 4.05; N, 4.33; S, 4.95. Calc. for C31H26ClN2OPPdS: C,
57.50; H, 4.07; N, 4.30; S, 4.96. IR (KBr, cmꢀ1): 1588 s n(C]N–N]C)
1288 (m) n(C–O), 430 (s) n(M–N)
1H NMR (400 MHz, CDCl3) (d
,
.
(ppm)): 8.44 (s, 1H, CH]N), 6.94–7.84 (m, 27H, aromatic), 2.49
(s, 3H, methyl). dC (100 MHz) 135.11, 135.05, 134.99, 134.70,
134.62, 132.02, 131.24, 130.55, 129.86, 128.44, 127.93, 127.35,
14.08. UV-vis (CHCl3, lmax (nm)); 3 (dm3 molꢀ1 cmꢀ1): 396(1186),
348(6205) and 242(1599).
[Pd(Cl)(PPh3)(L4)] (5). The complex 5 was prepared by the
same procedure as that described for complex 1 with HL5 (19.5
mg, 0.071 mmol) and [PdCl2(PPh3)] (50 mg, 0.071 mmol). Yield:
82% (42.4 mg), mp 224 ꢁC (with decomposition). Found: C,
56.73; H, 4.31; N, 4.17; S, 4.73. Calc. for C32H28ClN2O2PPdS: C,
Acknowledgements
S. M. thanks University Grants Commission, New Delhi, for the
award of UGC-BSR RFSMS fellowship. We also thank DST-India
(FIST programme) for the use of Bruker Avance DPX 400 MHz
NMR at the School of Chemistry, Bharathidasan University. Also
we thank SAIF, CUSAT, Cochin and Gauhati University, Guwa-
hati for X-ray diffraction studies.
56.72; H, 4.18; N, 4.15; S, 4.75. IR (KBr, cmꢀ1): 1599 s n(C]N–N]C)
1284 (m) n(C–O), 428 (s) n(M–N)
1H NMR (400 MHz, CDCl3) (d
,
.
(ppm)): 8.41 (s, 1H, CH]N), 6.79–8.40 m, 25H, aromatic). 2.50
(s, 3H, methyl). 3.81 (s, 3H, methoxy). dC (100 MHz) 135.12,
135.06, 134.82, 134.63, 131.05, 130.54, 129.38, 127.92, 113.22,
55.42, 14.08. UV-vis (CHCl3, lmax (nm); 3 (dm3 molꢀ1 cmꢀ1):
396(2686), 348(6205) and 240(7573).
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