Microwave-assisted ligand-free, base-free Heck reactions in a task-specific imidazolium ionic liquid

1-(2-cyanoethyl)-3-(2-hydroxyethyl)-1 H -imidazol-3-ium tetrafluoroborate ( IL - 2 ) in presence of PdCl 2 was found to be an efficient and reusable, ligand-free, base-free catalytic system for Heck-coupling of activated and deactivated iodo-and bromoarenes with different olefins. Under microwave irradiation, it exhibited good efficiency in terms of activity, selectivity and recyclability for six consecutive runs without significant loss of activity.


Introduction
Heck reaction, a powerful method for synthesizing diverse olefinic compounds, generally involves carbon-carbon coupling of aryl halides with olefins. 1 In the last few years, considerable development involving coupling of various pseudo-halides such as aryl triflates, 2 acid chlorides, 3 aryl anhydrides, 4 aryl sulfonyl chlorides, 5 and aromatic diazonium salts 6 has been reported.Many drugs and intermediates such as naproxen (Anti-inflammatory agent), prosulfuron (Herbicide) and 2-ethylhexyl-p-methoxy cinnamate (sunscreen agent) are synthesized using the Heck reaction. 7The catalytic system for an efficient Heck reaction consists of a palladium source, a ligand, base and solvent.Usually, phosphine containing ligands are used in the Heck reaction, as they play an important role in stabilization and in-situ generation of Pd(0) species from Pd(II) complexes.Due to the high cost, toxicity and thermal instability of the phosphine containing ligands, 8 various phosphine free catalytic systems 9 have been developed.However, most of them showed poor efficiency and reusability in palladium catalyzed Heck reactions. 10ence, ligand free homogeneous catalytic systems 11 were of great interest for palladium catalyzed Heck reactions.However, in homogeneous systems, it is difficult to overcome the problems associated with catalyst/product separation and catalyst reusability.In this concern, various heterogeneous catalyst 12 systems have been developed by immobilization of palladium catalyst.However these systems are failed to show higher activity compared to homogeneous systems due to extensive palladium leaching. 13n the last few decades, ionic liquids have gained great interest in synthetic organic chemistry since they are eco-friendly, nonvolatile, recyclable, and possess good thermal properties. 14Ionic liquids can not only serve as a good alternative to volatile toxic, organic solvents as a reaction medium but also can provide mobile support to palladium catalysts.Many ionic liquids are reported to be used as reaction media for the Heck reaction, but most of them are used along with expensive phosphine-ligands. 15Hence, the synthesis of functionalized ionic liquids (FILs) through the additional incorporation of the specific pendent groups which could act as ligands and could enhance thermal stability, efficiency and recyclability of the catalyst, are areas of interest.Various reported functionalized imidazolium ionic liquids contain functional groups such as, amine, phosphonite, nitrile, hydroxyl and diols, phosphoryls.Some of the functionalized ionic liquids suffer from drawbacks like air and moisture sensitivity, high palladium loading, difficulties in product isolation, tedious recovery processes and low recyclability. 16Diol-functionalized ionic liquids have shown good catalytic efficiency in terms of activity and reusability, when used as a kind of ligand for the palladium catalyzed Heck reaction in DMF. 17 Hence it would be still enviable to develop simpler, cost effective, reusable, phosphine free catalytic system for palladium catalyzed Heck reaction.
Herein we report on 1-(2-cyanoethyl)-3-(2-hydroxyethyl)-1H-imidazol-3-ium tetrafluoroborate 18 (IL-2) (Scheme 1), as a simplest phosphine free catalytic system which acts not only as a solvent but also as a ligand and base and offers good activity and recyclability for six consecutive runs without significant loss of activity in the air.In this work, initially the Heck reaction was carried out using iodobenzene and ethyl acrylate with PdCl2 (2.0 mol %) in ionic liquid (IL-2) at 100°C for 24 h, which resulted in (E)-ethyl cinnamate in low isolated yield 25%.This was significantly improved on application of microwave heating at 100°C for 10 min (Scheme 2).Microwave heating is reported to accelerate the reaction rate in organic synthesis. 19 few microwave-assisted Heck reactions are reported in the ionic liquids; however, some of them used phoshine containing ligands. 20

Results and Discussion
The reaction conditions were further optimized in terms of palladium loading, reaction temperature and time.1.0 mol% PdCl2 loading resulted in identical yield 69% as compared to 2.0 mol% PdCl2.The rise in temperature from 100 °C to 120 °C showed significant increase in isolated yield 81% (Table 1), however, further increase in temperature to 140 °C, resulted in low yield 64%. Maximum yield 84% was achieved, when the reaction was carried out for 5 min.The optimal condition for Heck reaction in ionic liquid (IL-2) was obtained with the following conditions PdCl2 (0.01 mmol), iodobenzene (1 mmol), ethyl acrylate (2 mmol), microwave power 200 W, reaction temperature 120 °C, reaction time 5 min.The versatility of the optimized catalytic system for Heck reaction was evaluated with variety of substrates with different electronic and steric effects.The results are summarized in Table 2. Cross-coupling products of different iodoarenes (Table 2, runs 1-7) and bromoarenes (Table 2, runs 8-11) with ethyl acrylate and styrene were obtained in good yields under mild reaction condition (120 °C, 5-10 min); attributed to high reactivity of iodo-and bromoarenes.However, due to the steric hindrance, the coupling of 2-bromo methyl benzoate with different olefins (Table 2, runs 12-13) resulted in moderate yield.Good isolated yield were obtained when the effect of the alkyl chain length of terminal vinyl acrylates were studied (Table 2, runs 14-18).However, activated aryl chlorides were unsuccessful to give desired product, when tried to couple with ethyl acrylate under the same reaction condition (Table 2, runs 19-20).a The isolated (E)-products are confirmed by 1 H NMR Recyclability of ionic liquid (IL-2) was investigated using iodobenzene (1 mmol), ethyl acrylate (2 mmol) in ionic liquid (IL-2) and its results are listed in Table 3. Ionic liquid (IL-2) has showed good isolated yields in the range of 80% -84% for six consecutive runs without significant loss of activity.

Conclusions
In conclusion, we have successfully developed an efficient, air stable and phosphine free catalytic system for Heck reaction using ionic liquid (IL-2) as solvent, ligand and base and palladium chloride as a catalyst.The in-situ formed palladium complex proved to be an excellent catalyst for olefination of activated and deactivated iodo-and bromoarenes including sterically hindered, methyl-2-bromo-benzoate.It also exhibited significant efficiency for less reactive vinyl acrylates such as n-butyl acrylate and 2-ethylhexyl acrylate.Due to the advantageous features of the ionic liquid (IL-2) like multiple coordinating sites, the palladium catalyst showed good stability and could be recycled for six consecutive runs without loss of activity.The mechanism for the Heck reaction in ionic liquid (IL-2) in presence of PdCl2 is being experimentally validated.Further investigation of the developed catalytic system for various palladiumcatalyzed coupling reactions is under investigation.

Experimental Section
General.The ionic liquid was characterized by FTIR, 1 HNMR, Mass spectroscopy and CHN analysis.FTIR spectra were obtained on a Shimadzu FTIR-8400S spectrometer using neat samples and 1 H NMR (300 MHz) and 13 C NMR (75MHz) data were recorded on a Varian Mercury VXR-300 NMR spectrometer using DMSO and CDCl3 as a solvent with an internal standard.ESI-MS was obtained with micromass -Q -Tof (YA105) spectrometer and elemental analysis (C,H,N elements) were carried out using ThermoFinnigan EA112 Elemental analyser.All reactions were carried out in CEM Discover Benchmate TM microwave appratus (CEM Corp.).All chemicals were purchased from Sigma Aldrich, S.D. Fine chemicals and Spectrochem private Ltd, India and were used without further purification.

General procedure for palladium catalyzed Heck reaction. (E)-Acetyl stilbene (5)
In a typical example, 4-iodo-acetophenone (1 mmol), styrene (2 mmol) and PdCl2 (0.01 mmol) were added to 2 mL of ionic liquid (IL-2) in a microwave vessel.Vessel was then sealed and kept in the microwave cavity (CEM Discover Benchmate TM , CEM Corp).Initially microwave irradiation of 200 Watt was applied to reach the target temperature of 120°C (measured using built-in IR temperature device); the reaction mixture was stirred at this temperature for 5 min.During this time, the microwave power was varied automatically to retain the target temperature at 120 °C.The reaction mixture was allowed to cool to room temperature.The product was extracted with diethyl ether and ether layer was concentrated under reduced pressure.The residue was purified by column chromatography using hexane: ethyl acetate (9:1) as a solvent, to obtain (E)-acetyl stilbene as white solid (Yield = 82%).mp 149-150 °C (Lit. 23

General procedure for recyclability of catalytic system
After extracting the mixture from diethyl ether, Ionic liquid (IL-2) was dissolved in acetone (5 mL).Then, neutralization of the acid generated during the reaction was done by addition of the potassium carbonate.The salt precipitated out was filtered off and acetone layer was concentrated under reduced pressure at room temperature.Without any further purification; the resultant residue was used for the next run, after addition of iodobenzene (1 mmol), ethyl acrylate (2 mmol) again, for six consecutive runs.

Table 1 .
Effect of different temperature on the yield of Heck reaction

Table 2 .
Heck reaction of various aryl halides in ionic liquid (IL-2)