Palladium Acetate/[CPy][Br]: An Efficient Catalytic System towards the Synthesis of Biologically Relevant Stilbene Derivatives via Heck Cross‐Coupling Reaction

The main objective of this study was to replace the traditionally applicable volatile organic solvents and imidazolium‐based ionic liquid solvents with thermally stable surfactant, cetylpyridinium bromide as surfactant ionic liquid (SIL) and examine the potential of the same as viable reaction medium in the synthesis of stilbenes, quinoxaline and pyridone decorated substrates via Heck cross‐coupling reaction. This work thoroughly investigates the thermal, spectral and morphological properties of cetylpyridinium bromide as surfactant ionic liquid. Cetylpyridinium bromide is a well‐known germicide with well‐established degradation mechanism and displays remarkable solvent properties for the homogeneously catalysed Heck cross‐coupling reactions, which is of great significance to enlarge the applications window of cetylpyridinium bromide from biologically significant surfactant material to the choice of solvent in synthetic organic chemistry.

Keywords: Cetylpyridinium bromide; Heck coupling reaction; Heteroarylsubstituted stilbenes; Homogeneous catalysis; Surfactant ionic liquids

Pd/[CPy][Br] catalytic system has been very aptly tuned for the activation of the C−X (X=Cl, Br, I) bond and which affords biologically and synthetically relevant hetero‐stilbene derivatives with exceptionally high yield. This catalytic system effective up to the sixth cycle.

Introduction

In the past few years, significant attention has been focussed on the topic; ionic liquids (ILs), as an outstanding, environmentally benign substitute to the volatile organic compounds from the perspective of "sustainable development and green chemistry". This has been achieved for a variety of areas including the modern disciplines such as electrolytic and electrochemical devices, metal processing and metallurgy synthetic organic chemistry and catalysis, materials science, nanotechnology, photochemistry, energy and fuels, separation science and polymer science. These "neoteric materials" are usually salts having a cationic part having an organic framework offering "hydrogen bonding, van der waals forces of interaction and π‐π stacking interactions possibility" in combination with a wide variety of anions that are responsible for the stabilization of liquid configurations of these supramolecular skeletons. The proper tuning and selection of an appropriate combination of cations and anions allows the advancement of a unique set of structurally induced physico‐chemical properties to the resulting composition as molten salt or Ils including very low to negligible vapour pressure, high thermal and chemical stability, solubility and solute‐solvent interactions in solutions and their potential can be explored as an environmentally benign alternative to volatile organic solvents for industrial scale‐up applications.

Accompanied with these solvating properties, ILs have allowed their potential exploration in the area of organometallic chemistry, thereby developing a new bridge between homogeneous and heterogeneous catalysis. Remarkable and ground‐breaking advances have also been recently made in the field of IL‐mediated Heck reaction with several optimal protocols having been reported in the past for the cross‐coupling reactions catalysed by palladium precursors (including palladacycles,[77] nanoparticles[78] and N‐heterocyclic carbenes) in ILs. However, drawbacks associated with the use of ILs are also obvious. In addition to obtaining a pure IL‐free product, the concomitant generation of side products including the formation of N‐heterocyclic carbene coordinated intermediates is unavoidable while using imidazolium‐based ILs, employment of high catalyst loading because of strong coordination with the metal centre, low atom efficiency, and further purification of the product.

To circumvent the problems associated with the use of imidazolium based ILs as solvent for Heck cross‐coupling reactions, improvement in the constituents of the ILs were later suggested. For example, Fortea‐Perez et al.[83] reported Heck reactions of iodobenzene and styrenes using catalytic amounts of bis(oxamato)palladate(II) complexes in molten tetrabutylammonium bromide as a reaction medium. Further exploration of the mixture of tetrabutylammonium acetate and tetrabutylammonium bromide salts as reaction medium for pseudo‐domino protocol for Heck reaction followed by an intramolecular Buchwald–Hartwig reaction was reported by Battistuzzi et al.[84] More recently, Taskin et al.[85] reported the use of surface active ILs for palladium‐catalyzed cross‐coupling reactions in water. However, these modified methods still have limitations. For example, the former protocol associated with the use of non‐commercially available bis(oxamato)palladate(II) complexes and the substrate scope is limited to only aryl halides. Similarly, the work reported by the Battistuzzi having limited scope and found applicable towards only aryl halides, additionally longer reaction time was required for the completion of the reaction, thus reducing the significance of the protocol. In the Taskin's protocol, the feasibility of the surface active imidazolium ILs for Heck cross‐coupling of acrylate and iodobenzene is screened but no further exploration of the methodology has been advanced, and the limited substrate scope restricts the generalization of these imidazolium surface active agents. It is envisaged that ILs are considered as highly structural solvents after water, and several ILs are considered as amphiphilic substances, which exhibit exceptional surface activity and having an ability to self‐organize individually as well as in the solutions. In particular, ILs with imidazolium, pyridinium and ammonium cations are the most widely spread ILs. To the best of our knowledge, however, the use of cetylpyridinium bromide ([CPy][Br]); as an excellent biologically significant surfactant material, used in nanoscience,[101] electrochemistry, metal processing,[104] material science,[105] separation science, and as germicide[108] has not been explored to date as reaction medium for organic transformations.[109]

Herein we disclose an efficient and environmentally benign protocol to address the problems associated with the use of dialkylimidazolium ILs as reaction medium for Heck cross‐coupling reaction. During our studies aimed at developing an elegant strategy for the synthesis of wide variety of stilbenes, we accidently found that the Heck cross‐coupling reaction of iodoanisole with styrene as coupling partners can be accomplished very smoothly in the melt of [CPy][Br] in the presence of palladium acetate as catalyst affording excellent yield of the corresponding stilbene. This unprecedented exploration of surfactant [CPy][Br] is found to be a model reaction for C−C bond formation applicable to the reaction between a wide range of styrenes and halides (iodobenzene, 2‐chloroquinoxaline and 3‐bromopyridone) to accomplish the synthesis of heteroaryl substituted and highly fluorescent stilbenes. Notably, we hypothesized that the present protocol having a sound potential to be categorized as a efficient protocol because it comprehends the use of biologically important and easily degradable surfactant material [CPy][Br] as surfactant ionic liquid (SIL), which is presently a topic of interest in IL research.

Results and Discussion

Our investigations began with the characterization of SIL, palladium acetate and the catalytic material by utilizing modern analytical methods of characterization such as thermal analysis, scanning electron microscopy, FT‐IR and 1H‐NMR. The results obtained for the characterization of the catalytic system and its exploration as a medium and catalysts for Heck cross‐coupling reaction discussed briefly in the following sections.

Thermal analysis of SIL, palladium acetate and the catalytic system

Thermal stability is a very significant factor to study the performance of the catalysts and the catalytic materials.36 The thermal properties (measured using Thermal Gravimetric Analysis‐TGA and Differential Scanning Calorimetry‐DSC) of SIL and palladium acetate which are the precursors of the catalytic system or as catalytic material, were examined. It was observed that both the catalytic materials are thermally stable at room temperature and can be sustained without any disintegration over a longer period of time. The TGA (Figure ) profile of pure [CPy][Br] reveals the salt to be having high thermal stability, however, a measurable weight loss at 100 °C indicates the removal of an adventitious amount of water. Thereafter the salt is quite stable up to 190 °C. TGA analysis of the palladium acetate salt revealed that the palladium precursor used was free from moisture and stable up to 195 °C, while over 195 °C, a constant weight loss pattern was observed. The thermogram of the palladium acetate‐SIL catalytic system(details of the synthesis given in supporting information) shows an initial weight loss at around 100 °C suggesting the removal of water molecule, thereafter the complex was found to be thermally stable up to 200 °C.

The differential scanning calorimetric studies (Figure ) provide sound information about the mesophasic transitions due to the presence of longer alkyl chain, and the melting behaviour of [CPy][Br]. A closer scrutiny of the DSC data reveals that the salt is having a liquidous range from 67–190 °C i. e. from its melting temperature (67 °C) to its decomposition temperature (190 °C). In the case of palladium acetate, heat flow was increased with increase in temperature and reaches maxima at 275 °C, and from there it starts to decompose. The heat flow data of the palladium acetate‐SIL catalytic system signifies that the melting temperature of the complex was slightly different than pure SIL i. e. 71 °C, in addition to that the DSC profile also visualizes a noticeable improvement in the liquidous range up to 215 °C of the catalytic system, which is quite higher than that of pure SIL. The preliminary melting point analysis discloses that SIL as well as the catalytic system have had the melting transitions <100 °C therefore, both can be classified as ILs.

Furthermore, the specific heat capacity (Cp) data of the SIL, palladium acetate and the catalyst system as a function of temperature was elucidated from the corresponding DSC profiles of the salt, and tabulated in Table .

Specific heat capacity (C p ) data in J/K.g for the SIL, palladium acetate and the catalyst system as a function of temperature.

Temperature (°C)	SIL	Pd(OAc)2	Catalyst System
75	1.92	0.87	0.86
85	1.82	1.07	0.86
95	1.78	1.15	0.83
105	1.77	1.17	0.79
115	1.76	1.17	0.75
125	1.76	1.17	0.71
135	1.77	1.17	0.69
145	1.77	1.16	0.67
155	1.78	1.15	0.66
165	1.78	1.14	0.64

A closer look at Figure  suggests that the specific heat capacity (Cp) of the SIL shows significant decrease over 90 °C and this has been attributed to the release of water molecule over 90 °C, afterwards quite a steady pattern of the specific heat capacity was observed, the specific heat capacity pattern for palladium acetate shows measurable thermal kinks from 85 to 95 °C and from 145 to 155 °C, while in case of the catalyst system a continual decrease in the trend of specific heat capacity was observed.

With high liquidous range and comparable specific heat capacity values with those of imidazolium and other pyridinium cored ILs, surfactant [CPy][Br] over its melting transition provides us with an opportunity to employ SIL for carrying out organic transformations as a reaction medium. In addition to this data, the NMR absorption pattern (Figure S1 in supporting data) for pure SIL and the catalyst system was studied, and the experimental data reveals that the presence of palladium has no measurable effect on the chemical shift values of both the heteroaryl as well as alkyl regions of the SIL, which led us to discard the possibility of the formation of N‐heterocyclic carbene type complexation with palladium acetate. It is also noted that, tremendous data related to the formation of structural voids due to the stacking interactions present in the structure of ILs already exists in the literature. In the present investigation, the possibility of palladium acetate trapping in such a stacked cage is very high and cannot be refused. Such an assumption is based on the above experimental observations and we therefore advocate that the present study could help open up an avenue for the chemists to explain the interactions present in SIL and palladium acetate or any other metal precursor experimentally as well as theoretically.

SEM and FT‐IR analysis of SIL, Pd(OAc) 2 and catalytic system

After the catalytic material and catalyst system were thoroughly examined by means of their spectral and thermal response, the characterization of the surface morphology of SIL, Pd(OAc)2 and catalytic system (combination of Pd(OAc)2 in SIL) was carried out by performing scanning electron microscopy and FT‐IR analysis. Figures a–c, are the low and high magnification images of the SIL, Pd(OAc)2 and catalytic system, respectively. On closer inspection of Figure c reveals that a layer of SIL covered the whole surface of palladium acetate very efficiently. Such kind of encapsulation of the metal catalyst should in theory have a positive impact on the catalytic activity of the catalyst system.

The Fourier‐transform infrared spectroscopy (Figure ) was used to assess the structural changes in the skeletons of SIL, palladium acetate in comparison with the catalyst system. The slight deformation in stretching frequencies in the catalyst system as compared to the parent materials was observed.

Optimization studies for Heck reaction

The effect of palladium precursors, solvents, bases and reaction temperature

Recently, numerous state‐of‐the‐art and selective C−C bond formation strategies for un‐functionalized arenes via Heck cross‐coupling reactions have already been disclosed in the literature. Notably, high order of regio‐ and chemo‐selectivities were observed in some of those reaction strategies with wide range of functional group tolerance. Meanwhile, considering prior reports of cross‐coupling reactions by our group,[125] we questioned whether the direct solvent‐free Heck cross‐coupling of arenes with easily available halide source can be established as a general procedure. At the outset of our studies, a SIL mediated palladium‐acetate catalysed Heck cross‐coupling of iodoanisole 1 with styrene 2 was selected as the benchmark system (Scheme ).

To ensure the efficacy of SIL, several model reactions were performed in the melt of SIL as well as in organic solvents like acetonitrile (ACN), dimethylformamide (DMF) and tetrahydrofuran (THF), where SIL and commercially available N‐heterocyclic carbene (iPr‐NHC) were utilized as a co‐catalysts. By comparing the experimental data obtained from all the transformations, the yield and selectivity was found to be exceptional in the case of SIL as solvent (Figure a). In order to assess the feasibility of SILs, we studied the solvating ability of [CPy][Br] and cetylpyridinium tetrafluoroborate [CPy][BF4] for this transformation. For Heck cross‐coupling reactions it is well‐known that the dielectric medium preferentially leads to the formation of cross‐coupled products from the coupling partner used.[131] It has also been evidenced that the electrochemical dispersion of palladium metal in the presence of stabilizers such as quaternary ammonium salts or surfactants favours the formation of colloidal palladium very efficiently. Additionally the rate of conversion of reactants into desired product can be considerably improved by the presence of stabilizers for Heck cross‐coupling reactions.– To our delight, SIL was identified as the most effective additive as well as reaction medium in the melt state to afford the product 1 a in excellent yield (isolated through column) of 93 % via Heck cross‐coupling reaction. Notably, the Heck coupling in SIL provides trans‐stilbene 1 a as a sole product.

The present protocol has been substantially influenced by the reaction temperature. Figure b demonstrates that the reaction temperature have had a significant impact on the formation of corresponding stilbene 1 a. It is noted that a continuous increase in the yield as a function of temperature was observed during the course of the reaction, and finally higher yield of the target product 1 a was furnished at 110 °C and elevated temperature. It is quite clear from the above observations that the higher temperature makes water free melt of SIL, which is already explained in the former section (Figure ). Similar to the cyclodextrins,[135] removal of water molecule could help enhance the activity of the molten SIL for the present transformation.

Encouraged by these results, we continued to further optimize the reaction conditions. We then turned our attention to investigate the effect of palladium source on the magnitude of the Heck cross‐coupling in molten SIL. Pd sources such as PdCl2, Pd2(dba)3, Pd(Cl)2(PPh3)2 and Pd(PPh3)4 were almost equally effective towards this transformation (Figure c). Although, Pd(PPh3)4 also achieved comparable yield (91 %) of 1 a, palladium acetate was superior to other Pd sources and is preferred due to its ready availability.

The observations reported in the earlier reports, where imidzolium based ILs were utilized as a reaction medium for a wide range of transition‐metal catalyzed reactions highlighted non‐innocent nature of these ILs. To our delight, the present cross‐coupling reaction was smoothly performed with variety of organic and inorganic bases (Figure d). The efficiency of the transformation is highly dependent on the amount of base used. For example, the use of triethylamine (1 eq.) as a base in the reaction of 1 with 2 under optimized reaction conditions afforded 52 % yield of the desired product, while 2 equivalent of triethylamine offers excellent yield 93 % of the corresponding product 1 a. On the other hand, when potassium phosphate or potassium carbonate was used as a base, desired stilbene product, 1 a was formed, in low to acceptable yields of 30 and 82 %, respectively. When DBU and DIPEA were used as bases, the yields of 1 a dramatically increased to 85 and 91 %, respectively.

The effect of concentration of SIL and palladium acetate

Noteworthy, the conditions for this Heck cross coupling reaction are fundamentally green, since they are based on ILs; the best solvent from environmental point of view for any organic transformation.[136] In addition to that no excess catalysts is required and avoid the contamination based on the presence of the catalytic auxiliary in the final product. In other words ILs minimizes the steps necessitate for the purification of the final product and offering high order of recycling of the catalyst system. In order to challenge the robustness of our present strategy for the Heck cross coupling reaction in SIL−Pd(OAc)2, synthesis of 1 a was scaled up by varying the concentration 1.5 g, 1.0 g, 0.5 g and 0.1 g of the SIL and 0.5 mol %, 0.1 mol %, 0.05 mol % and 0.01 mol % of the Pd(OAc)2. A remarkable feature of this strategy is that it allows very low loading of SIL 0.1 g (Figure a) as well as palladium acetate 0.1 mol % (Figure b) to drive the reaction towards the formation of desired cross‐coupling product formation electronic nature of differently substituted styrenes as well as aryl halides on the catalytic activity (Scheme ). Various substituents with electron‐donating and electron‐withdrawing character at para, meta and ortho positions of the aryl ring of the styrene as well as halide were well tolerated. The corresponding products were obtained in exceptionally high yields. Gratifyingly, naphthylstyrene was found to be a suitable substrate under optimized reaction conditions to afford the desired cross‐coupling product 1 i in respectable yield 84 %. Finally, when vinylimidazole was used as a coupling partner, the possibility of the formation of NHC was quite high, which have had an adverse effect on the overall progress of the reaction.

Success achieved with the Heck coupling of aryl iodides with styrene further allowed us to examine, 2‐chloroquinaxaline as a coupling partner for the formation of stilbene analogues of quinoxalines (Scheme ), which are invaluable targets in organic synthesis due to the wide distribution of such compounds in functionalized bioactive molecules e. g. Montelukast IV, VUF5017 III, Chimanine B I, L‐660, CGS23113 II, 711 V and VI.44 Although various strategies has been developed for the formation of such biologically significant organic molecules, expensive and toxic noble metals and high temperatures are always necessary. Therefore, a straightforward, highly efficient and comparatively simple protocol is highly desirable for the synthesis of such bioactive motifs. The outcome (2a–2e) of the reaction between 2‐chloroquinoxaline and variety of substituted styrenes (Scheme ) unambiguously demonstrated that the Pd(OAc)2‐SIL catalysis was uniquely accountable for the Heck coupling. Very surprisingly, the effect exerted by the substituents attached to the phenyl ring of the styrene both electronic as well as steric having negligible effect on the outcome of the corresponding products. As an additional result, high order of selectivity for halosubstituted heteroaryl was observed when a reaction between 2,6‐dichloroquinoxaline with styrene was performed under standard optimized reaction in the synthesis of 2 f.

With the standard reaction conditions to access styryl substituted quinoxalines in hand, we focused our attention on exploring the potential of the developed protocol towards the synthesis of highly fluorescent stilbenes decorated with pyridone ring system (Scheme ). To our great surprise, quite a few reports were available for the synthesis of such type of highly fluorescent stilbenes[146] with a common disadvantage of the yield of the final product. In contrast, we propose here a highly versatile, practical protocol for the synthesis of pyridone decorated stilbenes, affording excellent yields of the corresponding products. With the optimal reaction conditions in hand (Scheme ), we then set out to explore the scope of the reaction. As shown in Scheme , styrene reacted smoothly with 3‐bromopyridone to afford the corresponding pyridone substituted stilbene 3a in excellent yields. The reaction also proceeds in good to excellent yields (90–94 %) with a wide range of styrenes bearing alkyl (4‐Me, 3‐Me), electron‐donating (4‐OEt) as well as halo (4‐Cl, 4‐F, 3‐F) substituents on the phenyl ring of the styrene (3b–3 g).

To ensure the homogeneity of this transformation and remove any ambiguity about the formation of Pd nanoparticles and heterogeneous catalysis, the reaction between 4‐iodoanisole and stryrene under the developed reaction conditions was performed in the presence of mercury; a distinguishing test between homogeneous and heterogeneous catalysis (Scheme ). The experimental results advocates that no substantial effect has been exerted by the presence of mercury on the progress of reaction, and most importantly it is obvious that no colloidal palladium species is formed during this transformation.

To our delight, no adverse effect of mercury poisoning was observed and the reaction progresses smoothly with high yield of the corresponding product 1 a. Next, carbon disulphide in excess has also been utilized as a catalyst poison (Scheme ). Very surprisingly, total inhibition of the cross‐coupling transformation was observed, this has been attributed to the interactions of carbon disulphide with non‐polar as well as polar part of the corresponding ILs, such interactions alters static as well as dynamic properties of ILs. Gratifyingly, the inhibitory action of carbon disulphide evidenced that in the present transformation SIL not only plays the role of solvent, but also performs very crucial role of co‐catalyst.

Having established the scope and the nature of our SIL mediated Heck reaction protocol, we turned our attention to evaluate the role of SIL in the reaction. We initially investigated the reaction of 4‐iodoanisole (1) with styrene (2) without SIL under optimized reaction conditions for 12 h, which gave corresponding stilbene (1 a) in 30 % yield (Scheme a). When 2‐chloroquinoxaline (3) was treated with styrene (2) without SIL in the presence of Pd(OAc)2 at 110 °C for 18 h, negligible progress was observed in the reaction (Scheme b). Moreover, the reaction of 3‐bromopyridone (4) and styrene (2) was conducted under optimal reaction conditions, no product formation was observed even after 24 h (Scheme c).

This results suggests that IL plays a dual role i. e. a solvent as well as a co‐promoter.On the basis of the above observations, a plausible mechanism for this Heck cross‐coupling reaction was represented in the following Scheme . Initially, intermediate a was generated through oxidative addition of an alkyl halide 1 to palladium acetate in SIL. Subsequently insertion of a styrene 2 across intermediate a affords intermediate c via intermediate b. Eventually, the corresponding stilbene 1 a as a target product and the Pd(OAc)2‐SIL catalyst system were separated out after final reductive elimination process.

In order to evaluate the catalytic potential of Pd(OAc)2‐SIL from the perspectives of industrial production of the stilbenes, a gram‐scale reaction was accomplished (Scheme a and 9b), and the results demonstrates that the aryl iodides were completely consumed into corresponding stilbene suggesting that the Pd(OAc)2‐SIL catalytic system could also prove to be useful for large‐scale synthesis of stilbenes. Furthermore, the exploration of this methodology for Suzuki and Sonogashira cross‐coupling reactions is in progress and will be reported shortly.

Recycling ability of catalyst system

'Green" has become a catch‐phrase in modern times for sustainability. The contribution of sustainable practices is triggering the advancement of clean processes and technologies in chemical syntheses. It comprises of, reduction in waste, atom economy and energy, renewability and tuning processes with environmentally benign reagents. In particular, the reuse of catalysts, is very significant if one considers the problems associated with the availability and dwindling resource of an expensive metal precursor. Taking this into consideration and to make the present methodology all the more inexpensive, the reusability of the Pd catalyst in SIL for the Heck reactions of 4‐iodoanisole, 2‐chloroquinoxaline and 3‐bromopyridone with styrene were investigated. After the completion of the reactions, 3 %, 5 % and 10 % hexane:ethyl acetate solvent system was added respectively into the corresponding reaction masses, followed by sonication and filtration to obtain the recovered Pd(OAc)2‐SIL system. The subsequent catalytic reactions could be performed using the same procedure for six times for 4‐iodoanisole without a considerable change in the catalytic performance. It was observed that in the case of 2‐chloroquinoxaline, the catalyst system could be recycled only three times, while in the case of 3‐bromopyridone reaction the catalyst system could be without much loss in catalytic activity recycled up to two runs (Figure ). The minimmal reuse of catalytic system in case of 2‐chloroquinoxaline and 3‐bromopyridone has been attributed to the use of higher concentration of ethyl acetate, which might be responsible for the leaching of Pd(OAc)2. Therefore, the addition of Pd(OAc)2 is essential for further accomplishments in case of 2‐chloroquinoxaline and 3‐bromopyridone.

After the catalyst was recycled six times, the Pd content of the spent catalyst during 1st, 2nd 4th and 6th cycles was determined using ICP‐AES, and no appreciable change was observed (the data has been given in supporting information) because of very low amount of palladium acetate was used initially. While SEM‐EDX images clears that after 4th cycle the significant alteration was observed in the catalyst system.

Recovery of SIL

Although, the treatment of commercially available oxidant ferrate (K2FeO4) ruptures the nitrogen moiety with opening of the pyridinium ring subsequently after the complete oxidation of the alkyl chain in to non‐toxic alkanes is well‐known degradation mechanism for the biologically viable surfactant [CPy][Br]. We next set out to recover an ionic liquid from the reaction mass, after accomplishing the reaction, isolating product from the reaction mixture, washing with appropriate solvents, and finally recovering Pd(OAc)2‐SIL system. We treat the recovered Pd(OAc)2‐SIL mixture with diethyl ether (5×5 ml), which leads pure SIL as a solid free from palladium acetate and other organic impurities. The resulting solid was analysed by means of NMR (1H and 13C) and the chemical shift values having good agreement with the chemical shift values of pure SIL. All of these results are encouraging, indicating that the Pd(OAc)2‐SIL catalyst is very promising homogeneous catalytic pathway for the Heck cross‐coupling reaction.

Conclusion

A novel biologically important surfactant [CPy][Br], has been characterised as surfactant ionic liquid in this study. To the best of our knowledge this is the first report in the literature, which deals with the utilization of surfactant [CPy][Br] in metal mediated transformation. Heck cross‐coupling reaction of styrene with 2‐chloroquinoxaline and 3‐bromopyridone is found to be effective. Despite the positives, the Pd(OAc)2‐SIL system is not suitable to the Heck reaction of vinylimidazole because the possibility of the formation of N‐heterocyclic intermediate is very high. To achieve a high yield of corresponding stilbene, a balanced catalytic activity is required, which is highly related to Pd and SIL loading, concentration of the base and reaction temperature. Indeed, Pd(OAc)2‐SIL system offers an fascinating avenue for the gram‐scale preparation of stilbenes, which comprises with maximum atomic economy and the use of biologically relevant easy to degrade, highly significant surfactant material, which advocates high feasibility of the process. This study suggests that a synthesis of chemicals of high commercial and biological relevance via Heck cross‐coupling reaction with homogeneous palladium catalysts in surfactant ionic liquid is achievable. Stimulated by these results we are further exploring the use of the palladium catalysts in other cross‐coupling reactions in SIL.

Supporting Information Summary

All experimental details and characterization data of all synthesized compound and catalyst are given in the supporting information.

Acknowledgements

University Grants Commission India for a UGC‐SAP fellowship for TG

Conflict of interest

The authors declare no conflict of interest.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

GRAPH: Supplementary

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By Tejpalsingh Ramsingh Girase; Kesharsingh J. Patil; Anant R. Kapdi and Gaurav R. Gupta

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