Phosphine-free tetradentate salicylaldimine ligand complexed with palladium: First application in heck reactions

Article abstract of DOI:10.1080/00397911.2014.941501

Heck reactions were carried out using phosphine-free tetradentate salicylaldimine ligand complexed with PdCl₂ under mild reaction conditions, short reaction time, and low palladium loading. All aryl iodides underwent coupling reactions with olefins, giving corresponding trans-products, with good to excellent yields, whereas aryl bromides gave very poor yields and aryl chlorides failed to react.

Full text of DOI:10.1080/00397911.2014.941501

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Phosphine Free Tetradentate Salicylaldimine Ligand
Complexed with Palladium: First Application in Heck
Reactions
Rahul S. Kalhapure a , Thirumala Govender b & Krishnacharya G. Akamanchi a
a Department of Pharmaceutical Sciences and Technology , Institute of Chemical Technology,
Matunga , Mumbai , IndiaCurrent address: Discipline of Pharmaceutical Sciences, School
of Health Sciences, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South
Africa.
b Discipline of Pharmaceutical Sciences, School of Health Sciences, University of KwaZulu-
Natal , Durban , South Africa
Accepted author version posted online: 22 Jul 2014.
To cite this article: Rahul S. Kalhapure , Thirumala Govender & Krishnacharya G. Akamanchi (2014): Phosphine
Free Tetradentate Salicylaldimine Ligand Complexed with Palladium: First Application in Heck Reactions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, DOI:
10.1080/00397911.2014.941501
To link to this article: http://dx.doi.org/10.1080/00397911.2014.941501
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Phosphine Free Tetradentate Salicylaldimine Ligand Complexed With Palladium:
First Application In Heck Reactions
Rahul S. Kalhapure1, Thirumala Govender2, Krishnacharya G. Akamanchi1
1Department of Pharmaceutical Sciences and Technology, Institute of Chemical
Technology, Matunga, Mumbai, India 2Discipline of Pharmaceutical Sciences, School of
Health Sciences, University of KwaZulu-Natal, Durban, South Africa
Current address: Discipline of Pharmaceutical Sciences, School of Health Sciences,
University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa.
Correspondence author E-mail: kgap@rediffmail.com
Abstract
Heck reactions were carried out using phosphine free tetradentate salicylaldimine ligand
complexed with PdCl2 under mild reaction conditions, short reaction time and low
palladium loading. All aryl iodides underwent coupling reactions with olefins giving
corresponding trans-products with good to high yield whereas aryl bromides gave very
low yields and aryl chlorides failed to react.
KEYWORDS: Heck reactions; salicylaldimine; palladium chloride; aryl halides; olefins
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INTRODUCTION
With a high potential for exploitability of useful features such as enhanced performance
when compared to mononuclear complexes, ability to combine the best properties of
homogeneous and heterogeneous catalysts in one system, stable macromolecular
structures making them suitable for isolation via ultra-filtration thus providing the
potential for catalyst recycling[1] dendrimeric ligands based metal catalysts, the
metallodendrimers have recently attracted attention. Their exploitation is mainly aimed at
reducing the amount of Pd required and high performance of the catalysts. Reactions
which are catalyzed by using metallodendrimers include Heck reaction[2], Suzuki-
Miyaura reaction,[3a] oxidation,[3b,c] polymerization,[1a,b] Sonogashira reaction,[4]
hydroformylation, C-C coupling metathesis,[3d,5] and epoxide ring opening.[6]
Heck reaction, a widely used reaction in synthesis of various substituted olefins, dienes
and natural products, is one of the most important palladium catalyzed reactions in
organic chemistry for which several homogeneous and heterogeneous protocols have
been reported in literature.[7,8] Most of the reported protocols employ Pd, an expensive
metal, as catalyst in relatively large amounts and use of large amount also results in
presence of residual metal in the product creating environmental concerns.[8a,b] Ligands
play an important role in Heck reaction by increasing the solubility of metal, minimizing
the palladium loading, yielding catalyst with high turnover number and high reactivity[8c]
and in this context dendritic ligands have caught a good attention. Few examples are
Reetz’s dendritic phosphine, DAB-dendr-[N(CH2PPh2)2]16, bearing P-centres on the
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periphery co-ordinated to Pd catalysts,[9] DAB-G1(impyr-PdCl2)4 catalyst,[2] Pd(II)-
phosphine complexes modified poly(ether imine) PETIM dendrimers,[10] palladium
nanoparticle cored G3 dendrimer,[11] iminophosphine DAB-dendr-[1,2-
(NCHC6H4PPh2)]32, DAB-32-imiphos, and corresponding aminophosphine, DAB-dendr-
[1,2-(NHCH2-C6H4PPh2)]32, DAB-32-amiphos, i.e. two dendrimeric P, N-ligands,[12] G4
PAMAM dendrimer encapsulated Pd0 nanoparticles,[13] dendritic nanoreactor prepared by
incorporating Pd0 nanoparticles into poly (propylene imine) [PPI] dendrimer, and PPI
dendrimer covalently functionalized with per fluorinated polyether chains on their
periphery.[14] G1 and G2 dendrimeric salicylaldimine ligands based on PPI dendrimer
scaffolds have been synthesized for preparation of multinuclear nickel complexes and
evaluated in polymerization of norbornene.[1a] However these ligands have not been so
far used for preparation of Pd complexes and screened for Heck reaction. In the present
work we synthesized the same PPI tetradentate ligand [DAB-dendr-[1,2-N=CH-C6H4-
OH]4] and successfully prepared Pd(II) complex 1 (Fig. 1) and employed for Heck
coupling reactions.
RESULTS AND DISCUSSION
DAB-dendr-[1, 2-N=CH-C6H4-OH] 4 Pd (II) complex 1 having two catalytic sites was
prepared by using PdCl2(CH3CN)2 and refluxing in CH3CN for 24 h to ensure the
completion of reaction. The IR spectra of 1 showed shift in ν (C=N) and ν (C- O)
stretching frequencies from 1627 cm-1 and 1275 cm-1 to 1621 cm-1 and 1313 cm-1
respectively ensuring the formation of complex, and mass spectrometry and elemental
analysis confirmed the molecular formula.
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Previously dendrimers with two to sixty Pd atoms have been prepared either by
complexation or entrapment.[2,10,11,13] Two Pd atoms were complexed with G1 PETIM
dendrimer[10] but in the form of phosphine ligand. We for the first time complexed two Pd
atoms with tetradentate DAB-dendr-[1,2-N=CH-C6H4-OH]4 ligand without any
phosphine as phosphine free conditions for Pd catalyzed reactions are preferred because
of adverse environmental effects associated with phosphines.[12]
Heck coupling reactions between iodobenzene and methyl acrylate as model substrates
were attempted using 1 and the results are summarized in Table 1. Reactions were carried
out in different solvents such as toluene, CH3CN, acetone, acetone-water (1:1), water and
DMF with 1 mol% of 1 and K2CO3 as a base and at different temperatures. Reaction did
not proceed in acetone, acetone-water (1:1) mixture and water (entry 4-6, Table 1),
whereas in toluene and CH3CN reaction did proceed but gave low yields (10-17%) (entry
1-3, Table 1). Best results were obtained in DMF and at reaction temperature of 80 oC
giving very high yield in 4 h (entry 8, Table 1). Amount of 1 was reduced up to 0.1 mol%
keeping other reaction conditions same and noticed no significant change in yield (entry
10, Table 1). Therefore for further studies DMF was chosen as solvent with catalyst
amount of 0.1 mol% and reaction temperature of 80 oC.
To study the scope of the methodology, reactions were carried out using different
substrates and results are presented in Table 2. All of the olefins gave good to excellent
yields except acrylamide (entry 8, Table 2), which did not show any reaction. In all cases
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only trans isomer was obtained and none of the cis isomer or any other by-product could
be observed by TLC or 1H NMR spectra. In case of acrylic acid esters it was noticed that
going from methyl to t-butyl esters resulted in slight decrease in yield (entry 3-6, Table
2). Styrene also reacted well but yield was comparatively less (entry 7, Table 2). Failure
of acrylamide to react (entry 8, Table 2) was attributed to deactivation of catalyst by
complexation with amide group. This conclusion is supported by carrying out reactions
with methyl acrylate in presence of acrylamide and acetamide and in both the cases
reaction did not proceed. Reactions with p-chloroiodobenzene and p-methyliodobenzene
were equally facile but slightly lower yields were obtained with p-methyl- compared to p-
chloroiodobenzenes (entry 9-12, Table 2).
Attempts to carry out reaction on aryl chlorides were unsuccessful while reaction with
aryl bromides gave poor yield (entry 13 and 14, Table 2). A control reaction using only
equivalent amounts of PdCl2 as catalyst with no ligand gave < 3% yield.
To assess the standing of 1 with respect to other metallodendrimers, literature data is
compiled in Table 3. Metallodendrimers with palladium loading in the range of 0.2 to 8
mol % have been used for 5 mmol of iodobenzene in evaluating Heck reactions whereas
in our case 0.2 mol % of Pd loading was sufficient to give almost the same yield as that
of the best catalyst but in short time (entry 1 and 5, Table 3) showing superiority of 1.
CONCLUSION
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In conclusion we have developed a new phosphine free tetradentate palladium complex-
[DAB-dendr-[1,2-N=CH-C6H4-OH]4] [PdCl2]2 for efficient Heck reaction. The catalyst
is superior with respect to less amount of Pd requirement to achieve the same yield under
similar reaction conditions in comparison with known methods utilizing
metallodendrimers.
EXPERIMENTAL
Bis(acetonitrile)dichloropalladium (II), 1,4-diaminobutane, iodobenzene were obtained
from Aldrich and used as received. All other chemicals were purchased from local
chemical suppliers and used without any purification. IR spectra were recorded on FT-IR
RX1 PerkinElmer instrument. 1H NMR spectra were recorded on JEOL MY-60 operating
at 60 MHz instrument. Chemical shifts are given as parts per million (ppm) downfield
from tetramethylsilane (TMS) in δ units. Electrospray ionization (ESI)- mass spectra
(MS) was recorded by using Agilent 6524 Q-TOF accurate Mass LC MS/MS system,
Agilent (USA). Melting points were determined with Veego melting point apparatus
having stirred paraffin bath. Silica gel # 60-120 was used for column chromatography
and Thin Layer Chromatography (TLC) was performed using Merck Silica gel 60 F254
Plates.
15
Synthesis Of DAB-Dendr-(NH2)4
DAB-dendr-(NH2)4 was prepared by some modifications in the reported method.[15]
DAB-dendr-(CN)4: To a solution of diaminobutane (100 mmol, 8.81 g) in 100 ml of
water was added acrylonitrile (800 mmol, 42.45 g) drop wise with stirring. The reaction
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mixture was heated at 80 oC for 1 h. Excess of acrylonitrile was removed as a water
azeotrope under vacuum using rotary evaporator to get the product (29.7 g, 99%) as
residue. This product was characterized by FT-IR and NMR. DAB-dendr-(NH2)4: To a
hydrogenation vessel, filled with Raney Nickel catalyst (4 g), NaOH (65 mmol, 2.6 g)
and methanol (100 ml) was added DAB-dendr-(CN)4 (66.6 mmol, 20 g) dissolved in
methanol (100 ml) and the mixture was hydrogenated at 100 psi at ambient temperature
for 8 h. Reaction mixture was filtered through a bed of celite to remove Raney Nickel and
filtrate was concentrated under vacuum. Crude product obtained was dissolved in dry
acetone and filtered to remove NaOH. Acetone layer was concentrated in vacuo to get the
colorless oil (20.44 g, 97%). This G1 PPI was used without further purification.
Preparation Of Ligand DAB-Dendr-[1,2-N=CH-C6H4-OH]4:1a
Salicylaldehyde (12.6 mmol, 1.54 mL) was added to a mixture of DAB-(NH2)4 (3.2
mmol, 1g) and dry toluene (25 mL) under N2 atmosphere. Mixture was stirred at room
temperature for 72 h. Solvent was evaporated in vacuo leaving a yellow oil.
Dichloromethane (20 mL) followed by petroleum ether (40 mL) were added and the
mixture was kept at -4 oC for 72 h. Yellow precipitate obtained was recovered by
filtration, washed with cold petroleum ether and air dried. Yield 2 g (85%). mp 65-67 oC
(Lit 66-68 oC). IR (KBr): ν = 3082, 1627, 1394, 1275 cm-1. 1H NMR (60 MHz, CDCl3): δ
= 1.41 (4 H, m), 1.99 (8 H, m), 2.40-2.62 (12 H, m), 3.69 (8 H, t), 6.83 (4 H, m), 7.16-
7.26 (12 H, m), 8.32 (4 H, s). MS (ESI): m/z = 733.45.
Preparation Of [DAB-Dendr-[1,2-N=CH-C6H4-OH]4][Pdcl2]2 1
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A mixture of ligand (1 mmol, 0.73 g) and PdCl2.(CH3CN)2 (2 mmol, 1.037 g) in CH3CN
(15 mL) was refluxed under N2 atmosphere for 24 h. The solvent was removed under
vacuum using rotary evaporator. Orange colored solid residue obtained was washed with
cold methanol and dried in oven at 100 oC for 1 h, Yield 1.20 g (83%). mp 183 oC (dec).
IR (KBr): ν = 2926, 1621, 1313 cm-1. MS (ESI): m/z = 1083.8 [M+]. Anal Calcd for
C44H54Cl4N6O5Pd2: C, 47.97; H, 4.94; N, 7.63. Found: C, 48.04; H, 5.06; N, 7.26.
General Procedure For Heck Reaction
Aryl halide (5 mmol), olefin (10 mmol), K2CO3 (5 mmol) and 1 (0.1 mol%) were taken
in DMF (10 mL) in a sealed tube (25 mL) fitted with a Teflon cap under N2 atmosphere
and mixture was heated with stirring at 80 oC for 4 h, cooled to room temperature and
solvent evaporated under reduced pressure on rotary evaporator. The residue was
extracted using EtOAc (3 x 10 mL), dried over Na2SO4 and the solvent was removed on a
rotary evaporator. The crude residue obtained was purified by column chromatography
on silica gel, 60-120 mesh (petroleum ether–EtOAc, 98:2).
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
ACKNOWLEDGMENT
Rahul S. Kalhapure is thankful to the University Grants Commission (UGC) Government
of India and National Research Foundation of South Africa for financial support.
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Table 1 Optimization of reaction conditions for Heck
reactiona
Entry Amount of 1 in mol% Solvent
Temperature oC Yieldb(%)
1
2
3
4
5
6
7
8
9
10
1
PhMe
110
80
17
12
10
n.r.
n.r.
n.r.
92
94
89
90
1
PhMe
1
CH3CN
Acetone
80
1
60
1
Acetone-H2O (1:1) 80
1
H2O
80
1
DMF
DMF
DMF
DMF
120
80
1
0.1
0.1
120
80
aReaction conditions: Iodobenzene (5 mmol), methyl acrylate (10 mmol), K2CO3 (5
mmol), 1, solvent (10 mL), reaction time 4 h, in a sealed tube under N2 atmosphere.
bIsolated yield.
n.r. = no reaction
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Table 2: Heck reactions using [DAB-dendr-[1,2-N=CH-C6H4-OH]4] [PdCl2]2] complex
1a
Entry Ar-X
1
R
Product
Yieldb(%)
88
I
O
COOH
OH
4a
4b
I
I
CN
2
3
CN
84
94
O
COOCH3
O
4c
I
I
I
I
O
4d
O
4
5
6
7
COOC2H5
COOn-C4H9
COOt-Bu
C6H5
86
82
75
78
O
O
O
4e
O
4f
4g
I
O
8
CONH2
n.r.
NH2
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I
CN
9
CN
88
4h
Cl
Cl
I
CN
10
11
CN
82
93
4i
I
O
COOCH3
O
Cl
4j
Cl
I
O
12
COOCH3
85
O
4k
Br
Cl
13
14
COOCH3
COOCH3
4c
4c
04
n.r.
aReaction conditions: Aryl halide (5 mmol), olefin (10 mmol), K2CO3 (5 mmol), 1 (0.1
mol%), DMF (10 mL), temperature 80 oC, reaction time 4 h, in a sealed tube under N2
atmosphere.
b Isolated yield.
n.r. = no reaction
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Table 3 Comparison of different metallodendrimer catalysts with catalyst 1 for Heck
reaction between iodobenzene and methyl acrylate
Entry Catalyst
Pd
Temperature Time Yield Reference
loading
(mol%)
0.96
(oC)
(h)
(%)
96
69
56
54
94
1
2
3
4
5
DAB-G1 (impyr)-
82
8
2
(PdCl2)4
G1 PETIM
2-0.2
4-0.4
8-0.8
0.2
140
140
140
80
4
10
10
10
metallodendrimer
G2 PETIM
4
metallodendrimer
G3 PETIM
4
metallodendrimer
[DAB-dendr-[1,2-
N=CH-C6H4-OH]4]
[PdCl2]21
4
This
work
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Figure 1. Structure of [DAB-dendr-[1,2-N=CH-C6H4-OH]4] [PdCl2]2] complex 1
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