Cyclodextrin based palladium catalysts for Suzuki reaction: An overview

Carbohydrate Research 489 (2020) 107954

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Cyclodextrin based palladium catalysts for Suzuki reaction: An overview a

a

a,∗∗

U.S. Kanchana , Elizabeth J. Diana , Thomas V. Mathew a b

b,∗

, Gopinathan Anilkumar

T

Department of Chemistry, St. Thomas College Pala, Arunapuram P.O, Kottayam, Kerala, 686574, India School of Chemical Sciences, Mahatma Gandhi University, P D Hills PO, Kottayam, Kerala, 686560, India

ARTICLE INFO

ABSTRACT

Keywords: Suzuki reaction Cyclodextrin Pd complex Catalyst

The Suzuki reaction is one of the most effective methods for the formation of carbon-carbon bonds and is of great utility in organic synthesis. Recently, cyclodextrin based palladium catalysts were found to be very selective, convenient and efficient for Suzuki cross-coupling reactions. This review focuses on such cyclodextrin systems of palladium which act as efficient catalysts with high catalytic activity and recyclability in Suzuki reaction and covers literature up to 2019.

1. Introduction In modern organic synthesis one of the main aspects is to design simple, selective efficient, convenient and environment friendly catalysts in organometallic coupling reactions [1,2]. Formation of C–C bond is the most important coupling reaction and is the corner stone of organic chemistry. The Suzuki-Miyaura cross-coupling reaction is one of the most important protocols used for the formation of carbon–carbon bonds and has become the prominent approach in the synthesis of biaryls [3–13]. By making use of this coupling reaction, a large variety of complex organic molecules, drugs, heterocycles, agrochemicals, pharmaceutical intermediates and precursors were synthesized [14–17]. Biaryls from a wide range of reagents were synthesized using coupling reactions which include both cross- and homo-coupling of aryl halides, arylboronic acids and aryltriethoxysilanes [18]. Palladium and other transition metal catalysts are effectively used for achieving these C–C bond formations [19–23]. Even simple models of C–C coupling reactions can evaluate the activity, proficiency and stability of the catalyst. Cyclodextrins (CDs) are cyclic oligosaccharides composed of D-glucopyranoside units (glucose) linked by α-1,4-glycosidic bonds [24]. Cyclodextrin is nowadays a promising molecule in supramolecular chemistry. The presence of a hydrophobic cavity makes cyclodextrin an attractive topic for study. In particular, β-cyclodextrin with a hydrophilic outer surface and a hydrophobic inner cavity has an internal cavity shaped like a truncated cone of about 8 A0 deep and 6.0–6.4 A0 in diameter. Its exterior, bristling with hydroxyl groups, is polar, whereas the interior of the cavity is non-polar, with respect to the external environment. Due to the great potential of cyclodextrin as an

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environment friendly medium for catalytic process, much attention has been focused on its organic reactions. Recently cyclodextrin-supported Pd catalytic systems attracted much attention for green Suzuki–Miyaura coupling reactions [25–30]. Modified cyclodextrins offer great opportunities and challenges for chemists [31–33]. They are used for the heterogenization of metal nanoparticles in order to apply as reusable catalysts for organic transformations [34–37]. Developing a new and efficient method for the preparation of cyclodextrin-based catalyst with high catalytic activity, recyclability and simple work up is of prime interest. In this review we focus on such cyclodextrin-based palladium catalysts which are highly efficient for Suzuki reaction, and covers literature up to 2019. For simplicity and brevity, we have categorized these topics into ligand-free coupling and ligand -mediated coupling. 2. Cyclodextrin based catalysts for Suzuki coupling 2.1. Ligand - free Suzuki coupling 2.1.1. β-Cyclodextrin stabilised Pd nanoparticles as catalyst β-Cyclodextrin (CD) decorated with palladium nanoparticles (PdNPs@β-CD) without pre-treatment acts as a suitable catalyst for the biaryls synthesis in water [38]. The prepared catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements. It was found that PdNPs@β-CD exhibits high performance in Suzuki coupling reactions affording biaryls in very good yields (Scheme 1). The catalyst was recovered by simple filtration and reused for several cycles without a

Corresponding author. Corresponding author. E-mail addresses: [email protected] (T.V. Mathew), [email protected] (G. Anilkumar).

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https://doi.org/10.1016/j.carres.2020.107954 Received 29 November 2019; Received in revised form 14 February 2020; Accepted 14 February 2020 Available online 16 February 2020 0008-6215/ © 2020 Elsevier Ltd. All rights reserved.

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iodobenzene, 4-iodoanisole and bromobenzene with four boronic acids in the Suzuki reaction (Scheme 3). Mol ratio of Pd/CD = 1 and the reaction was carried in EtOH/water (1:1) at 25 °C. Malta and co-workers synthesized 2-hydroxypropyl-α-cyclodextrin (α-HPCD) capped Pd nanoparticles which are efficient in C–C bond formation in Suzuki cross-coupling in water [41]. Good yields and selectivities of coupling products were obtained under very low Pd loadings (0.5–0.01 mol%). Suzuki–Miyaura cross-coupling of aryl- or heteroaryl-boronicacids and halides gave the corresponding biaryls in good yields and chemoselectivities (Scheme 4). 2-aryl-substituted heterocycles could also be readily prepared in excellent yields using this system. Kaifer et al. disclosed the catalytic properties of Pd nanoparticles derivatized with surface attached perthiolated cyclodextrin receptors (Fig. 2) [42]. They used the catalyst in Suzuki cross-coupling reaction. Iodo substrates were found to be more active than the corresponding bromo substrates and electron - withdrawing substituent at the para position of the aryl halide favoured the coupling reaction. They also investigated the Suzuki cross-coupling reaction of iodoferrocene with phenylboronic acid to yield phenylferrocene (Scheme 5). The reactions were carried out with 5-fold excess phenylboronic acid in a mixed solvent (EtOH–H2O, 2:3 v/v), using Ba(OH)2 as the base and 1 mol% CD-capped Pd nanoparticles as the catalyst to afford the products in moderate yields.

Scheme 1. Suzuki coupling catalysed by PdNPs@ β-CD.

2.1.3. Functionalised β-CD/Pd (II) complex as catalyst Jin et al. reported the synthesis of picolinamide-modified β-cyclodextrin/Pd(II) complex (Pd(II)@PCA-β-CD) which acts as an efficient catalyst for Suzuki-Miyaura coupling of aryl, benzyl and allyl halides with arylboronic acids in aqueous medium (Scheme 6) [43]. The catalyst was reused without loss in its activity. The optimization studies were carried out using 4-bromo-acetophenone, cinnamyl bromide and benzyl bromide as well as 4-methylphenylboronic acid as model

Fig. 1. Structure of MNP-CD-Pd catalyst.

significant loss of activity. Hydrophilic Pd(II)-β-CD complex anchored on 3-aminopropyl triethoxysilane functionalised magnetite nanoparticles (MNP-CD-Pd) (Fig. 1) as a highly water dispersible heterogeneous catalyst with hydrophobic cavity for C–C coupling reaction in aqueous solutions have been designed and synthesized by Kaubdin et al. [39] The reusability of the nanocatalyst was examined successfully. In the optimal condition for the Suzuki coupling 4-bromoanisole and phenyl boronic acid were reacted with 0.15 mol% of the catalyst in presence of K2CO3 as base in water at 100 °C for 6 h. Using this catalyst, cross-coupling reaction of aryl halides with large number of aryl boronic acid were done (Scheme 2). Aryl halides with electron-withdrawing groups increased the reactivity while electron - donating groups decreased the reactivity. Biaryls in high yields (up to 99%) were obtained using 2-substituted aryl iodides and phenyl boronic acid but only 10% yields were obtained using iodobenzene and 2, 6-dimethylphenyl boronic acid. 2.1.2. Functionalised β-CD-Pd nanoparticles as catalyst The catalytic nature of mono, bis and tris(ferrocenyltriazolylmethyl) arene–β-cyclodextrin-stabilised Pd nanoparticles in Suzuki coupling reactions were investigated by Astruc et al. [40] They used

Scheme 3. Suzuki coupling reaction catalysed by the PdNP catalyst supported by the tris-ferrocenyl–CD material.

Scheme 2. Suzuki coupling reaction catalysed by MNP-CD-Pd.

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Scheme 4. Suzuki reaction catalysed by (α-HPCD) capped Pd nanoparticles.

multiple times to catalyse the coupling reaction. In the optimized condition, aryl halides reacted with phenyl boronic acids in presence of K2CO3 and 0.001 mol% catalyst loading affording the corresponding biaryls in excellent yields (Scheme 8). The presence of electron-withdrawing and electron-donating groups in the boronic acids gave excellent yields of the biaryls. Steric effects on the boronic acids also showed influence on the yield of the reaction. Synthesis of cyclodextrin functionalised with two diametrically opposed carbenes which acts as good platform for the complexation of Pd has been reported [45]. The catalytic activities of the synthesized complexes were studied in a Suzuki–Miyaura coupling reaction between 4-halogenoacetophenone and phenylboronic acid using Cs2CO3 in ethanol solvent (Scheme 9). Ponchal et al. reported the use of cyclodextrins as mass transfer promoters in catalytic reactions involving two phases [46]. Here, methylated cyclodextrins (RaMe-α-CD and RaMe-β-CD) act as efficient catalysts in the Suzuki cross-coupling reaction between phenyl iodide and phenylboronic acid (64% and 62% of biphenyl) rather than the permethylated Tri Me-β-CD and the partially methylated Crys Me-β-CD (38% and 30% of biphenyl). The mass transfer promoter contribution was enhanced in the case of water insoluble substrates. Substrates with an electron-withdrawing substituent (CN, CF3, or COCH3) underwent rapid conversion than the substrates having an electron-donor group (CH3, NH2, OCH3) (Scheme 10).

Fig. 2. Structure of CD capped Pd nanoparticles.

2.1.4. Heterogeneous catalysis A novel magnetically recyclable nano catalyst based on palladium supported by β-cyclodextrin@graphite oxide (M-GO/(AM-MBA-β-CD@ Pd) (M-GO, magnetic GO; AM, acrylamide; MBA, methylenebisacrylamide) nanocomposite was reported recently [47]. This finds application as an efficient and recyclable catalyst for the Suzuki–Miyaura crosscoupling reaction of aryl halides with boronic acids as well as for modified Suzuki-Miyaura cross-coupling reactions of N-acylsuccinimides and boronic acid in green media. This catalyst was easily separated using a magnetic field and was reused for six runs without significant loss of activity under optimum reaction conditions. The separated and reused catalyst needs no pre-activation for being used in a subsequent run. The catalyst also showed high TOFs (Turn Over Frequency) and TONs (Turn Over Number) as two important green chemistry metrics. In the optimized condition, high yields of the product were obtained using 0.01 mol% catalyst, which was then applied to various substituted aryl halides and boronic acids (Scheme 11). The presence of electron - donating and electron-withdrawing groups in the aryl bromides gave biaryls in excellent yields. As expected, aryl

Scheme 5. Suzuki cross-coupling reaction of iodoferrocene.

substrates using K2CO3 as the base and 5 × 10−6 mol% catalyst. A possible reaction mechanism was putforth involving the Pd(II)/Pd(IV) centers based on experimental methods and quantum chemical calculation as shown in (Scheme 7). The synthesis, characterization and the catalytic ability of a novel Pd (II)-β-cyclodextrin complex as an efficient nanocatalytic system was reported for the Suzuki–Miyaura carbon-carbon coupling reaction in water [44]. The novel Pd II)-β-cyclodextrin complex catalyst was characterized by TGA, FT-IR, XRD, NMR and atomic absorption analysis. By using the Pd(II)-β-cyclodextrin complex as a homogeneous catalyst, an efficient protocol for Suzuki-Miyaura coupling reaction was developed by the reaction of aryl boronic acids with aryl halides in water with high turn over frequencies (TOFs) for the synthesis of unsymmetrical biaryls. In addition, the catalyst was recycled and reused

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Scheme 6. Suzuki-Miyaura coupling of aryl, benzyl and allyl halides with arylboronic acids using (Pd(II)@PCA-β-CD).

Scheme 7. Possible mechanism for Suzuki coupling using (Pd (II) @ PCA-β-CD).

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Scheme 8. Suzuki reaction using novel Pd (II)-β-CD complex as catalyst.

Scheme 9. Suzuki coupling reaction using the functionalised CD–Pd catalyst. Fig. 3. Structure of Si-CD derivative.

Scheme 10. Suzuki coupling catalysed by methylated CD-Pd complex.

Scheme 12. Hybrid CD silica derivative supported PdNP catalysed Suzuki coupling.

chlorides were found less reactive than aryl bromides and iodides in this reaction. Cravotto et al. reported the preparation of a new hybrid cyclodextrin silica derivative supported Pd nanoparticle catalyst (Fig. 3) [48]. The catalytic performances of this catalyst in ligand-free C–C Suzuki coupling with a large number of aryl iodides and bromides has been reported. The reaction was repeated with the same catalyst five times with no significant loss in its catalytic activity.

Scheme 11. Suzuki coupling catalysed by (M-GO/(AM-MBA-β-CD@Pd).

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Scheme 13. Pd@β-CD–GNS catalysed Suzuki coupling.

Scheme 14. β-cyclodextrin capped Pd NPs catalysed Suzuki coupling.

Scheme 16. Suzuki coupling reaction catalysed by (PdLn@Et-β-CD).

loading (0.2 mol%) at 90 °C (Scheme 13). The presence of hydrophilic substituents in aryl chlorides and bromides enhanced the yields of the product. Zhao et al. successfully reported that β-cyclodextrin capped Pd nanoparticles work as an efficient catalytic system without ligand in Suzuki-Miyaura coupling in low melting β-cyclodextrin/NMU (N-methylurea) mixture (Scheme 14) [50]. Good yields were obtained even with very low catalytic loading and the catalyst was recycled and reused without significant loss in its catalytic activity. The ternary system LDH-Pd-CD (LDH-layered double hydroxides) was used as catalyst in Suzuki reaction in water at room temperature by Malta et al. [51] Good yields were obtained for bromo and iodoarenes after a period of 8 h at room temperature using a 1:20 ratio of Pd:HP-βCD and 0.5 mol% of homogeneous catalyst (Scheme 15). As expected, bromo and chloro arenes gave lower yields compared to iodoarenes. 2.2. Ligand mediated Suzuki coupling Scheme 15. Suzuki coupling reaction catalysed using LDH-Pd-CD.

2.2.1. Functionalised β-CD-Pd complex as catalyst Jung et al. reported an aminoethylamino-β-cyclodextrin-supported palladium complex (PdLn@Et-β-CD) which catalysed the Suzuki–Miyaura coupling reaction in water demonstrating excellent catalytic activity [52]. In the optimized reaction condition, bromoarene (1 mmol) reacted with boronic acid (1.2 mmol) and catalytic PdLn@Etβ-CD (2 mol%) in presence of K2CO3 (1.5 mmol) and water (4 mL) at 60 °C giving the coupling products in good to excellent yields (Scheme 16). Boronic acids with electron donating groups, such as –CH3 and -OMe, afforded the coupling products in good yields, while aryl halides containing electron withdrawing groups, such as –NO2, –CHO, and –OH, gave high yields. Moderate yields were obtained when aryl chlorides were used. A plausible mechanism was also suggested for the

The catalytic activity was tested in Suzuki reaction using halobenzenes and phenyl boronic acid as model substrates in H2O/dioxane 9:1 mixture (Scheme 12). Palladium nanoparticle–β-cyclodextrin–graphene nanosheet (Pd@ β-CD–GNS) catalysed C–C coupling reactions have been investigated under green reaction conditions [49]. Characterization of the catalyst was done using FT-IR, XRD, RAMAN, UV–Vis spectroscopy, TEM, SAED, XPS and ICP-AES. With low Pd loadings (0.2–0.05 mol%) excellent yields were obtained, while ensuring the recovery and reusability of the catalyst. The scope of the catalyst was investigated using several aryl chlorides and bromides with boronic acids in water under low catalyst

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Scheme 17. Plausible mechanism for the Suzuki coupling reaction using (PdLn@Et-β-CD) as catalyst.

phenyl from phenylboronic acid through the transmetallation reaction which then underwent reductive elimination, giving the final desired product. Zhou and co-workers described the synthesis and crystal structure of the Ln@β-CD ligand (Fig. 4) [53]. Furthermore, they have developed a catalyst PdCl2 Ln@β-CD) in situ (ligand + PdCl2 in water) which showed excellent catalytic properties for aqueous Suzuki cross-coupling reactions. The PdCl2 (Ln@β-CD) catalyst showed good performance parameters, such as high efficiency, low toxicity, low palladium loading (only 0.00084 wt%), and good recyclability, which makes this catalyst suitable for future industrial applications. The effect of the catalyst PdCl2 Ln@β-CD) in the Suzuki coupling reaction was studied using aryl boronic acids and aryl halides in water (Scheme 18). When the reaction was carried out using aryl bromides and boronic acids in presence of the catalyst PdCl2 (Ln@β-CD) (0.01 mol%) and K3PO4 in water for 4 h, the biaryl products were obtained in 80–100% isolated yields. The synthesis and physico-chemical characterization of an air stable, water soluble Pd (II)-complex of a β-CD based N,N/,O-tridentate ligand and its catalytic efficacy in Suzuki reaction in aqueous media has been investigated [54]. This catalyst worked effectively for water insoluble aryl halides in the Suzuki reaction with lesser reaction time, lower temperature (ambient temperature) and optimum catalyst loading without any phase transfer catalyst and organic solvent. Finally the catalyst was easily recovered and recycled several times. In the optimized condition, boronic acids reacted with aryl halides in presence of 3 mol% catalyst and 2 equiv. of K2CO3 in water at room temperature to afford good yields of the biaryl products (Scheme 19). A water soluble, cyclodextrin-supported palladium catalyst (DACHPd-β-CD) (DACH-is 1, 2-cyclohexanediamine) was efficiently used for Suzuki–Miyaura cross-coupling reaction between aryl halides and arylboronic acid in water under mild conditions (Scheme 20) [55]. The catalyst was successfully characterized by TEM, EDXS, XRD, FT-IR, NMR and TGA. Recovery of the catalyst from the reaction was found to be easy and the recovered catalyst maintained high catalytic activity even after 10 cycles. The use of green solvent (water), short reaction time (2–6 h), low catalyst loading (0.001 mol%), excellent yields (up to 99%) and reusability of the catalyst are the main advantages of this protocol. A possible mechanism suggested by the authors is discussed in (Scheme 21).

Fig. 4. Structure of the ligand Ln@β-CD.

Scheme 18. Suzuki coupling reaction catalysed by PdCl2(Ln@β-CD).

reaction using this particular catalyst (Scheme 17). The various steps involved in the mechanism are oxidative addition, transmetalation, and reductive elimination. At first, PdLn@Et‐β‐CD was initiated to Pd(0) followed by oxidative addition of aryl halides with active PdLn@ Et‐β‐CD forming an intermediate. The next step is the insertion of the 7

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Scheme 19. Suzuki coupling reaction catalysed by Pd(II) complex of β-CD based N,N/,O-tridentate ligand.

Scheme 20. Suzuki coupling reaction using (DACH-Pd-β-CD) as catalyst.

Pitchumani et al. developed a novel, ionic Pd(II) pyridinium modified β-cyclodextrin (Pd(II)@Pyr-β-CD) (Fig. 5) homogenous catalyst for Suzuki-Miyaura cross-coupling reaction in aqueous media [56]. The prepared catalyst was characterized by NMR, Mass, FT-IR, UV–Visible spectroscopy and Dynamic Light Scattering (DLS) techniques. It catalysed (0.2 mol%) a wide range of aryl halides (X = I, Cl, Br) and phenylboronic acids/styrene in water in presence of K3PO4 affording good yields of the products showing reusability till six runs without significant loss in its activity (Scheme 22). A putative mechanism for the coupling reaction was also illustrated (Scheme 23). Schmitzer et al. designed a novel dodecyl imidazolium functionalised β-cyclodextrin (Fig. 6) as a catalytic system along with Pd in water for Suzuki-Miyaura reaction [57]. In the optimized reaction condition, boronic acids (1.2 equiv.) reacted with aryl bromides (1 equiv.) in presence of 1 mol% of ligand and 0.5 mol% of Pd(OAc)2 and 2 equiv. of Cs2CO3 as base at 100 °C (Scheme 24). Hetero coupling products in moderate to high yields were observed,

despite the nature of the halide or the presence of electron-donating or electron-withdrawing groups. Even naphthalene substrates which have increased hydrophobicity and potential steric hindrance also gave good yields. Suzuki–Miyaura cross-coupling reaction involving sterically hindered and strong hydrophobic character substrates were first reported by this group, without the use of degassed water or nitrogen. Ding et al. designed a water-soluble triazolyl β-cyclodextrin-supported palladium complex (PdLn@β-CD) with excellent catalytic activity in Suzuki-Miyaura coupling reactions [58]. The catalyst exhibited high TONs (Turn Over Number) and TOFs (Turn Over Frequency) for Suzuki–Miyaura coupling reactions in water. The reaction was optimized using p-bromobenzaldehyde and phenyl boronic acid as model substrates. Using Na2CO3 as base and PdLn@β-CD (10−7 mol%) as catalyst, boronic acids were reacted with halobenzenes affording very high yields of the biaryl products (Scheme 25). Monflier et al. synthesized an N-heterocyclic carbene (NHC) ligand based on a β-cyclodextrin–imidazolium salt, ie. (PM-β-CD-MIM,Cl) permethylated β-cyclodextrin-methylimidazole (Fig. 7) and its catalytic

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Scheme 21. Plausible mechanism for the Suzuki coupling reaction using (DACH-Pd-β-CD) as catalyst.

Fig. 5. Structure of Pd (II)@Pyr-β-CD complex.

activity was evaluated using Suzuki reaction [59]. The catalytic activity of the system was examined using phenyl boronic acid and bromobenzene using Pd(OAc)2 as the Pd source and Cs2CO3 as the base and 1,4-dioxane as the solvent affording 99% yield (Scheme 26). 2.2.2. Heterogeneous catalysis Rui et al. reported the possibility of using cyclodextrin-modified h-

Scheme 22. Suzuki coupling reaction catalysed by Pd (II)@Pyr.β-CD complex.

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Scheme 23. Proposed mechanism for the coupling reaction using Pd (II)@Pyr-β-CD complex.

Fig. 6. Imidazolium functionalised β-CD ligand (L).

Scheme 24. Imidazolium functionalised β-CD-Pd catalysed Suzuki coupling reaction.

BN (hexagonal boron nitride) as an efficient support for the hydrophilic heterogeneous catalysts in Suzuki coupling [60]. This green methodology represents a cost-effective and operationally convenient process for the synthesis of biaryls, stilbenes and acrylates. Wide scope of substrates, good to excellent yields, low reaction time, water as solvent, ligand-free condition, non-toxicity and recyclability of the catalyst were found to be the main merits of this work. The h-BN@γ-CD@Pd (II) nanomaterial has been fully characterized by TG, SEM, IR, XRD, XPS

and ICP-AES analysis. The catalyst could be easily recovered and reused for at least nine times without any considerable loss of catalytic activity. A great number of pharmacologically relevant products has been successfully synthesized using this catalyst. Due to its hydrophilic nature, sufficient binding sites, stability and recyclability, h-BN@γ-CD could be a very promising catalyst for organic synthesis. The ideal 10

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Scheme 25. Suzuki coupling reaction catalysed using triazolyl β-CD supported Pd.

Scheme 27. Suzuki coupling catalysed by h-BN@γ-CD@Pd(II) nanomaterial.

Fig. 7. Structure of permethylated β cyclodextrin-methylimidazole ligand.

reaction conditions were found to be aryl halides (1 mmol), catalyst hBN@γ-CD@Pd(OAc)2 (0.05 mmol) and K2CO3 (1.5 mmol) with PhB (OH)2 (1.5 mmol) in water at 70 °C. When the catalytic activity was examined using various aryl halides and phenyl boronic acids, it was revealed that good yields were obtained for substrates with electron donating and electron withdrawing groups. A variety of functional groups were tolerated using this procedure (Scheme 27). Under the same reaction conditions, iodobenzene afforded better yield than the chloro- and bromo-benzenes. Sollogoub et al. synthesized and assessed a new dideoxytetraphosphine α-cyclodextrin α-Cytep which displayed exceptionally high TONs (Turn Over Number) and TOFs (Turn Over Frequency) in the Suzuki-Miyaura coupling [61]. In Suzuki reaction they used phenylboronic acid and 4-bromoacetophenone and PdCl(η3-C3H5)2 as a palladium(0) precursor with α-Cytep, in a 1: 2 ratio. The reaction was

carried out using K2CO3 as base, refluxing xylene as solvent for 7 days (Scheme 28). 3. Conclusion The cyclodextrin-catalysed Suzuki cross-coupling reaction is a highly useful and versatile method for the formation of C–C bonds, as is evident from the papers discussed in this review. This type of reaction finds wide applications in the synthesis of natural products of pharmacological importance. This review focused mainly on functionalised cyclodextrin-Pd catalysts with and without ligands, with high catalytic efficiency and recyclability in Suzuki coupling reactions. Therefore, there is an urge to focus on more and more modified cyclodextrin systems which can catalyse effectively such coupling reactions with structurally different substrates and this will be the subject of future Scheme 26. Suzuki coupling catalysed by (PM-βCD-MIM, Cl)–Pd system.

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Scheme 28. Suzuki coupling reaction catalysed by α-Cytep.

research in this area.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements USK and EJD thank the Mahatma Gandhi University and the Council of Scientific and Industrial Research (CSIR-New Delhi) for the award of junior research fellowships respectively. GA thanks the Kerala State Council for Science, Technology & Environment (KSCSTE), Trivandrum, India for a research grant (No. 341/2013/KSCSTE dated 15.03.2013) and EVONIK Industries, Germany for a financial support (ECRP 2016 dated 6.10.2016). References [1] M. Sollogoub, Synlett 24 (2013) 2629. [2] S. Noel, B. Leger, A. Ponchel, K. Philippot, A. Denicourt Nowicki, A. Roucoux, E. Monfiler, Catal. Today 235 (2014) 20. [3] M.M. Heravi, E. Hashemi, Tetrahedron 68 (2012) 9145. [4] L. Yizhu, L. Guosong, Z. Dajian, W. Renshu, Z. Xiao‐Yu, L. Guangxing, Appl. Organomet. Chem. 32 (2018) e4421. [5] N. Mohsen, G. Farshid, G.C. Arash, E. Zahra, Appl. Organomet. Chem. 32 (2018) e4282. [6] H. Veisi, M. Pirhayati, A. Kakanejadifard, Tetrahedron Lett. 58 (2017) 4269. [7] H. Veisi, N. Mirzaee, Appl. Organomet. Chem. 32 (2018) e4067. [8] H. Veisi, S. Najafi, S. Hemmati, Int. J. Biol. Macromol. 113 (2018) 186. [9] H. Veisi, M. Pirhayati, A. Kakanejadifard, P. Mohammadi, M.R. Abdi, J. Gholami, S. Hemmati, Chem. Sel. 3 (2018) 1820. [10] H. Veisi, P.M. Biabri, H. Falahi, Tetrahedron Lett. 58 (2017) 3482. [11] H. Veisi, S.A. Mirshokraie, H. Ahmadian, Int. J. Biol. Macromol. 108 (2018) 419. [12] F. Heidari, M. Hekmati, H. Veisi, J. Colloid Interface Sci. 501 (2017) 175. [13] E. Farzad, H. Veisi, J. Ind. Eng. Chem. 60 (2018) 114. [14] R. Chinchilla, C. Nájera, Chem. Rev. 107 (2007) 874. [15] H.U. Blaser, A. Indolese, F. Naud, U. Nettekoven, A. Schnyder, Adv. Synth. Catal. 346 (2004) 1583. [16] T.K.K. Afsaneh, H.M.M. Tayebeh, Appl. Organomet. Chem. 32 (2018) e4210. [17] M. König, L.M. Reith, U. Monkowius, G. Knör, K. Bretterbauer, W. Schoefberger, Tetrahedron 67 (2011) 4243. [18] L. Yin, J. Liebscher, Chem. Rev. 107 (2007) 133. [19] I. Ojima, et al., I. Ojima (Ed.), Catalytic Asymmetric Synthesis, third ed., John Wiley & Sons, Hoboken, New Jersey, 2010, p. 437. [20] A. Molnár, A. Molnar (Ed.), Á Palladium-Catalyzed Coupling Reactions: Practical Aspects and Future Developments, Wiley-VCH, Weinheim, 2013. [21] S. Enthaler, X.-F. Wu, S. Enthaler, X.-F. Wu (Eds.), Zinc Catalysis: Applications in Organic Synthesis, Wiley-VCH, Weinheim, 2015.

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