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An Ionic Liquid Immobilized Palladium Complex for an Effective Heck and Suzuki Coupling Reactions

  • Dileep Ramakrishna
Short Communication
  • 62 Downloads

Abstract

A highly efficient catalytic activity towards Heck and Suzuki cross-coupling reactions was observed for a palladium complex in ethyl-methyl imidazolium hexafluorophosphate, [EMIM] PF6 ionic liquid medium, at ambient temperature. The system provides a stable, reusable method for the reaction. The optimization for the suitable reactions conditions was explored, and the effect of substituents on boronic acid was investigated. Using a very modest amount of catalyst, good-to-excellent yields were achieved. The reaction conditions were mild and most importantly, the catalyst-ionic liquid mixture was easily recoverable and reused for six times without much loss in the catalytic activity, causing negligible impact on the environment.

Keywords

Palladium complexes Ionic liquid Immobilization Heck reaction Suzuki reaction 

1 Introduction

In the development of synthetic organic chemistry Palladium and Palladium based complexes have become very important. A numerous palladium catalysts in various significant reactions have been reported [1, 2, 3], for example, formation of C–H bond [4, 5, 6, 7, 8, 9, 10], C–N/C–O/C–S bond formation [11, 12, 13, 14], asymmetric symthesis [4, 7, 8, 11, 12, 13, 14, 15, 16], heterocycle functionalization [17, 18, 19], etc. The C–C bond formation simplifies organic synthesis and is the main basis for many pharmaceutical synthesis, advanced materials, etc. [20, 21, 22, 23, 24, 25, 26]. The C–C bond formation is one of the most important reactions and it is one among the foremost applications of palladium and palladium based catalysts.

Apart from their stability and ease of handling, Palladium complexes, as catalysts are well known for their adaptability to various reaction conditions. Under mild conditions the Schiff base complexes of palladium are found to be very efficient homogeneous catalysts. The one drawback of these homogeneous catalysts is that the same is nor recoverable and reusable. To overcome this, the complexes have been immobilized onto a base material like clay, polymer etc. The above mentioned methods are adopted to have a recyclable and reusable catalysts without compromising the activity of homogeneous catalysts. One important advancement in this field is the use of low molecular weight ionic liquids for the immobilization of catalysts [27, 28, 29]. When the reactions are carried out in polar solvents like water, a homogeneous system is formed between the immobilized catalyst and the solvent. After the reaction is complete, the reaction mixture is extracted with a solvent which is less polar, like diethyl ether, to separate the IL immobilized catalyst from the organic phase. In this way the IL immobilized catalyst can be recovered and reused.

The Pd-catalysed coupling reactions is increasing every now and then and hence demand the need of development of better reaction conditions [30, 31, 32, 33]. The development of a robust, efficient and cost effective catalyst that can give way to overcome the limitations of the existing processes is needed [34, 35, 36]. The consideration of the economic and the environmental factors should be done in order to develop recoverable and reusable catalytic systems. Hence there is a necessity for the development a catalytic system that is applicable in nearly neutral conditions without loss of activity at ambient temperature for the coupling reactions [37, 38, 39, 40, 41].

A series of palladium complexes with a triphenylphosphine and a ligand derived from 2-hydrazinopyridine and salicylaldehyde was reported earlier by the author used for the effective oxidation of a variety of alcohols [42]. Since most of the palladium complexes show catalytic activity for cross-coupling reactions, the above complex was also tested for the coupling reactions. The target was to minimise the reaction rime compared to the reported results in the literature and also to use milder reaction conditions. Hence a study was done to explore the catalytic activity of the above synthesized complexes in the Heck and Suzuki reactions in [EMIM]PF6 ionic liquid media under ambient conditions. The catalyst works well under these conditions without a phase transfer catalyst and importantly, without any organic solvents. The catalyst was found to give an excellent results which was comparable and/or better than other reported palladium-catalysed reactions [43].

This work reports the coupling of selected aryl halides catalysed by palladium complex, 1, in ionic liquid, 2, in various reaction conditions (Scheme 1). To give comparatively clean environment the advantageous properties of the catalyst and [EMIM]PF6 have been combined. All additives including the catalyst are soluble in the ionic liquid, resulting in an oxidation solution that is completely homogeneous. This is one major advantage of this system. Based on the synthesized catalyst, an extremely effective and reusable system for the coupling reactions, performed in ionic liquid [EMIM]PF6 with an equal-proportioned water at ambient temperature is described.
Scheme 1

Structures of Palladium complex (1) and Ethyl methyl imidazolium Hexafluorophosphate ionic liquid (2)

2 Experimental

2.1 Procedure for Catalytic Activity

2.1.1 Heck Reaction

In a typical reaction, the reaction vessel was charged with bromobenzene (1 mmol), styrene (1.5 mmol), K2CO3 (2.0 mmol), Pd-catalyst (0.1 mol%) and [EMIM]PF6-H2O (1:1, 2 mL). The reaction temperature was maintained at 60 °C for 2 h. After the completion of the reaction, the reaction mixture was cooled to room temperature. The mixture was diluted with diethyl ether and the organic phase was dried over anhydrous Na2SO4. The solvent was evaporated under reduced pressure and the residue obtained was subjected to column chromatography on silica gel using a mixture of ethyl acetate and n-hexane. The purified product obtained were characterized by 1H NMR (400 MHz) and 13C NMR (100 MHz) in CDCl3 solution.

2.1.2 Suzuki Reaction

A mixture of aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), Pd-catalyst (0.1 mol%), K2CO3 (2.0 mmol) and [EMIM] PF6-water mixture (1:1, 2 mL) were added to a 50 mL round bottomed flask and continuously stirred at room temperature (25 °C). The reaction monitoring was done by using thin layer chromatography at every 5 min intervals. At the completion, the reaction mixture was diluted with water (20 mL) and extracted with a mixture of hexane and ethyl acetate (1:1 v/v, 3 × 10 mL). Diethyl ether was also used for extraction in the cases of few substrates. The organic layer was dried with anhydrous Na2SO4, and then the solvent was evaporated under reduced pressure using a roto evaporator. The residue was purified by column chromatography over a bed of silica gel (mesh 60–120), using ethyl acetate and n-hexane mixture as an eluent, to give the desired purified product. The purified product obtained were characterized by 1H NMR (400 MHz) and 13C NMR (100 MHz) in CDCl3 solution.

The completion of the reaction was also monitored by GC-FID, indicated by the disappearance of the aryl halide. The reaction mixture, after completion, was extracted with diethyl ether (3 × 3 mL). The organic layer was then treated with a known amount of toluene for GC analysis as an internal standard. The ionic liquid-catalyst mixture was washed with diethyl ether, dried prior to being reused.

3 Results and Discussion

3.1 Heck Reaction

First, the reaction conditions for the Heck reaction were optimized by modelling a reaction between styrene and bromobenzene. Table 1 shows the summary of the results of the different reaction conditions. It was found that, at 60 °C the reaction with 1 mol% of 1 in the presence of K2CO3 gave 1,2-diphenylethene in 96% yield (Table 1, entry 6). In the absence of Pd-complex, the maximum yield was only 12% under reaction conditions which was altered slightly at 60–120 °C, 24 h. With 0.5 mol% of the catalyst, the isolated yield was not obtained (Table 1, entry 5).
Table 1
Optimization of reaction conditions for the Heck reaction catalysed by Pd complex
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Entry

Catalyst concentration (mol%)

Base

Yielda (%)

1

2.0

Et3N

20

2

2.0

NaOH

62

3

3.0

K3PO4

70

4

3.0

Cs2CO3

72

5

0.5

K2CO3

72

6

0.1

K2CO3

96

7

1.0

K2CO3

96

8

2.0

K2CO3

96

9

0

K2CO3

12

All reactions were carried out with 1 mmol of bromobenzene, 1 mmol of styrene, x mmol of catalyst, 2 mmol K2CO3, 2 ml [EMIM] PF6-water (1:1) mixture

All the reactions were carried out at 60 °C and for 2 h

aIsolated yield

Usually in the coupling reactions the selection of an ideal solvent will have a crucial role for the successful progress of the reaction. The evaluation of the influence of solvent in the present system was done (Table 2), with reference to the green solvent selection guide [44]. Ethyl-methyl imidazolium hexafluorophosphate [EMIM] PF6 ionic liquid was found to be the best solvent in our screening experiments (Table 2, entries 5 and 6). One major advantage of this is, both base and the catalyst are soluble in the ionic liquid. This gives a solution that is completely homogeneous with water. With ethanol and THF, only low to moderate results were obtained (Table 2, entries 2 and 4). Good yield was observed when methanol was used as solvent. But the time and temperature conditions during this reaction were more drastic compared to that of ionic liquid as solvent. By monitoring the results obtained using different solvents, it was resolved to use a mixture of [EMIM] PF6-water (1:1) as the solvent for the Heck reactions. With ethanol and THF, only low to moderate results were obtained (Table 2, entries 2 and 4). Good yield was observed when methanol was used as a solvent. But time and temperature conditions during this reaction were more drastic compared to that in ionic liquid.
Table 2
Effect of solvents on the Heck reaction catalysed by Pd complex
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Entry

Solvent

Temperature (°C)

Time (min)

Yieldb (%)

1

Watera

60

120

36

2

Ethanola

60

120

75

3

Methanola

60

120

83

4

THFa

80

120

12

5

EMIM [PF6]

27

30

94

6

EMIM [PF6]-Water [1:1]

27

30

93

All reactions were carried out with 1 mmol of bromobenzene, 1 mmol of styrene, 0.1 mol% of catalyst, 2 mmol K2CO3, 2 ml of the solvent

aThe initial reaction conditions for the reactions were at room temperature for 30 min, but slowly increased to 60 ̊C for 120 min

bIsolated yield

The scope of the above catalytic reaction, other terminal olefins were subjected to the cross-coupling reaction of aryl halides with K2CO3 as base in [EMIM] PF6-water (1:1) at 60 °C, was examined. The results are presented in Table 3. In the Heck reaction of bromobenzene and terminal olefins such as methyl acrylate and t-butyl acrylate, the reaction shows no activity (Table 3, entries 8–9). Methyl acrylate and t-butyl acrylate are less reactive than styrene, resulting in no activity. However, for n-butyl acrylate, the reaction gives a moderate yield (Table 3, entry 7), because n-butyl acrylate is more active than methyl acrylate and t-butyl acrylate. Besides aryl bromides, the reaction between aryl iodides and olefins (e.g. Ph, CO2nBu, CO2tBu, CO2Me) shows high activity under the same reaction conditions (Table 3, entries 10–13). This result is attributed to the nature of iodobenzene which is more active than bromobenzene.
Table 3
Heck reactions between aryl halides and acrylates catalysed by Pd complex in [EMIM] PF6
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Entry

R1, X

R2

Yielda (%)

TON

1

H, Br

Ph

94

9.4

2

4-CN, Br

Ph

90

9.0

3

4-COMe, Br

Ph

84

8.4

4

4-CF3, Br

Ph

80

8.0

5

4-MeO, Br

Ph

60

6.0

6

4-NO2, Br

Ph

70

7.0

7

H, Br

CO2nBu

58

5.8

8

H, Br

CO2tBu

0

9

H, Br

CO2Me

0

10

H, I

Ph

96

9.6

11

H, I

CO2nBu

90

9.0

12

H, I

CO2tBu

85

8.5

13

H, I

CO2Me

60

6.0

All reactions were carried out with 1 mmol of R1, X, 1 mmol of R2, 0.1 mol% of catalyst, 2 mmol K2CO3, 2 ml [EMIM] PF6-water (1:1) mixture at 60 ̊C

aIsolated yield

3.2 Suzuki Reaction

The next focus was on exploration of the catalytic activity of the above complex for Suzuki coupling reactions in ionic liquid media. The type and the amount of catalyst used can affect the Suzuki reaction generally. A model reaction between iodobenzene and phenylboronic acid to give biphenyl was used for the optimization study of the reaction conditions. The solvent effects were studied for different solvents under same conditions (Table 4). The same solvent mixture, [EMIM] PF6-water (1:1), as in case of Heck reaction, was used for the Suzuki reaction.
Table 4
Effect of solvents on the Suzuki reaction catalysed by Pd complex
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Entry

Solvent

Temperature (°C)

Time (min)

Yieldb (%)

1

Watera

60

120

12

2

Ethanola

60

120

42

3

Methanola

60

120

51

4

THFa

80

120

8

5

EMIM [PF6]

27

30

89

6

EMIM [PF6]-Water [1:1]

27

30

91

All reactions were carried out with 1 mmol of iodobenzene, 1 mmol of phenylboronic acid, 0.1 mol% of catalyst, 2 mmol K2CO3, 2 ml of solvent

aThe initial reaction conditions for the reactions were at room temperature for 30 min, but slowly increased to 60 ̊C for 120 min

bIsolated yield

The results of the catalyst optimization, summarized in Table 5 promoted the choice of 0.1 mol% palladium complex and K2CO3 (2.0 eq) at room temperature as the most feasible condition for high yield coupling reaction (Table 5, entry 3). The yield of the product was not affected much by increasing the concentration of the catalyst up to 2.0 mol% (Table 5, entries 5–6).
Table 5
Optimization of reaction conditions for the reaction catalysed by Pd complex
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Entry

Catalyst concentration (mol%)

Yield (%)a

1

0.01

64

2

0.05

80

3

0.1

92

4

0.2

92

5

1.5

91

6

2.0

91

All reactions were carried out with 1 mmol of iodobenzene, 1 mmol of phenylboronic acid, x mmol of catalyst, 2 mmol K2CO3, 2 ml [EMIM] PF6-water (1:1) mixture, 30 min, room temperature

aIsolated yield

A detailed study was carried out to expand the scope of the Suzuki reaction catalyzed by the palladium complex in ionic liquid medium for different aryl halides and arylboronic acids. The results are summarized in Table 6. Good to excellent yields (70–94%) to the corresponding coupled products were obtained by the reaction of substituted halobenzenes with arylboronic acids. However, it was observed that the reaction time taken for bromobenzene and chlorobenzene was more than that for iodobenzene. The same method was applied for other aryl iodides substituted with either electron-withdrawing or electron-donating groups. For example, 4-nitro, 4-methyl, 4-methoxy and 2-aminoiodobenzenes were converted easily to the corresponding coupled products. The above findings were comparable to the previously reported results of widely used procedures [45, 46, 47, 48, 49, 50, 51]. The advantage for the present work lies in the less reaction time compared to previously reported results and also gave a high yield for over a range of products. It is a known fact that the presence of mercury will decrease the catalytic activity of a heterogeneous catalyst. Hence a mercury poisoning test was performed for a model reaction with 1 mmol of iodobenzene, 1 mmol of phenylboronic acid, 0.1 mol% of catalyst, 2 mmol K2CO3, 2 ml [EMIM] PF6-water (1:1) mixture, 30 min, room temperature in presence of excess mercury (Table 5, entry 19). The result showed that the catalytic activity did alter and hence can be concluded that the reaction follows a heterogeneous pathway [52].
Table 6
Suzuki reactions between aryl halides and arylboronic acids catalysed by Pd complex in [EMIM] PF6
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Entry

Aryl halides

Arylboronic acid

Time (min)

Yield (%)b

TON

1

C6H5I

C6H5B(OH)2

30

94

9.4

2

C6H5Br

C6H5B(OH)2

90

88

8.8

3

C6H5Cl

C6H5B(OH)2

90

84

8.4

4

C6H5I

3,4,5-(OCH3)3C6H2B(OH)2

30

85

8.5

5

C6H5Br

3,4,5-(OCH3)3C6H2B(OH)2

90

82

8.2

6

C6H5Cl

3,4,5-(OCH3)3C6H2B(OH)2

90

79

7.9

7

C6H5I

4-CH3C6H4B(OH)2

30

87

8.7

8

C6H5I

2-C4H3S B(OH)2

30

70

7.0

9

C6H5I

2-NH2C6H4 B(OH)2

30

89

8.9

10

C6H5I

4-NO2C6H4 B(OH)2

30

89

8.9

11

C6H5I

4-OCH3C6H4 B(OH)2

30

82

8.2

12

C6H5Br

4-OCH3C6H4 B(OH)2

90

80

8.0

13

OCH3C6H5I

2-CH3C6H4 B(OH)2

30

85

8.5

14

CH3C6H5I

2-CH3C6H4 B(OH)2

30

82

8.2

15

NO2C6H5I

2-CH3C6H4 B(OH)2

30

87

8.7

16

NH2C6H5I

2-CH3C6H4 B(OH)2

30

86

8.6

17

OCH3C6H5Br

2-CH3C6H4 B(OH)2

90

80

8.0

18

OCH3C6H5Cl

2-CH3C6H4 B(OH)2

90

78

7.8

19a

C6H5I

C6H5B(OH)2

30

86

8.6

All reactions were carried out with 1 mmol of aryl halides, 1 mmol of arylboronic acid, 0.1 mol% of catalyst, 2 mmol K2CO3, 2 ml [EMIM] PF6-water (1:1) mixture, room temperature

aIn the presence of excess Hg (Hg:Pd = 300:1)

bIsolated yield

It is indeed very noteworthy that the palladium catalyst in [EMIM] PF6, the catalytic activity, after recycling, even after 6–7 runs was observed to be almost the same (Fig. 1). The numerical data to study the effect of catalyst reuse is given in Table 7. However, the degradation of catalyst was observed with the addition of excess base to the reaction mixture. Also ILs are not intrinsically “green” as most of them exhibit toxicity just like other organic solvents. The biodegradability and toxicity levels of most ILs are being studied by many researchers and is of a prime interest [53]. But in case of imidazolium based ILs the toxicity increases with increase in the size of the alkyl group as the ionic nature decreases as the bulkiness of the organic cation increases. In the case of the Ionic liquids like [EMIM] PF6 which have a simple alkyl group, helps in retaining good ionic character and hence their toxicity levels are very low. Moreover the complete ionic liquid-catalyst mixture was recovered and reused.
Fig. 1

Effect of recycling on yield

Table 7
Effect of catalyst reuse on yield
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Reaction run no.

Time (min)

Yield (%)a

1

30

94

2

30

91

3

30

90

4

30

89

5

30

88

6

30

84

7

30

79

All reactions were carried out with 1 mmol of iodobenzene, 1 mmol of phenylboronic acid, 0.1 mol% of catalyst, 2 mmol K2CO3, 2 ml EMIM [PF6]-water mixture, room temperature (30 °C)

aIsolated yield

4 Conclusion

In summary, a Schiff base palladium complex is explored for its catalytic activity in Ethyl–methyl imidazolium hexafluorophosphate [EMIM] PF6 ionic liquid, which provides a robust air stable system for Heck and Suzuki reactions. The palladium catalysed coupling reaction was found to be highly efficient in [EMIM] PF6-Water (1:1 ratio) system in the screening experiment and the reusability of the catalyst was efficient in the above system. The time required for the completion of the reactions for iodobenzene was shorter than that for bromobenzene and chlorobenzene, irrespective to the different substituents with either electron withdrawing or electron-donating groups on iodobenzene.

Notes

Acknowledgements

DR would like to thank the IISc Bangalore for all the spectroscopic analyses.

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Copyright information

© The Tunisian Chemical Society and Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of Chemistry, School of EngineeringPresidency UniversityBangaloreIndia

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