Short communicationSynthesis of 3-ethynyl-3-hydroxy-2-oxindoles and 3-hydroxy-3-(indol-3-yl) indolin-2-ones using CuWO4 nanoparticles as recyclable heterogeneous catalyst in aqueous medium
Graphical abstract
Introduction
3-Hydroxyindolin-2-one moiety is present in many natural products (convolutamydines, donaxaridine, dioxibrassinine, welwitindolinone C and maremycine etc.) with diverse biological activity [1]. The synthetic derivatives of 3-hydroxyindolin-2-ones and 3,3-bis(indol-3-yl)indolinones are known to show anticancer [2], spermicidal activity [3], microbial and anticonvulsant [4] properties. Similarly, 3-alkynyl-3-hydroxyindolin-2-ones have been evaluated against cyclophyllid tapeworm [5] and for the inhibition of reverse transcriptase (HIV) [6] (Fig. 1). The 3-hydroxyindolin-2-ones and bis-indolyl-2-oxindoles are prepared using Friedel-Crafts alkylation reaction of isatin in presence of chiral organo and organometallic catalysts [7], Bronsted and Lewis acids [8], ionic liquids [9], cyclodextrins [10] and under catalyst-free conditions [11]. Along with these, the heterogeneous catalysts like the use of NiO nanoparticles [12], amberlyst15 [13], ZnO nano rods [14], TiO2/SiO2 [15], Kaolin/KOH [16] and triton B [17] are known for this purpose. However, only ZnO, Kaolin and triton B gives mono substituted 3-hydroxy-3-indolyl-2-oxindoles selectively. The other methods give bis-indolyl-2-oxindoles.
Propargylic alcohols are important intermediates for the synthesis of heterocyclic molecules of biological interest. Functionalized propargylic alcohols are usually prepared by the nucleophilic addition of the metal acetylides generated by metalation of terminal acetylenes [18].
Alkynylation of isatins have gained attention in recent times because of the structural and biological properties. Alkynylation of isatins have been achieved either by a combination of halogenated acetylenes and isatin [19] or direct alkynylation of isatins and phenyl acetylenes in presence of Zn(Et)2 [20], Zn(OTf)2/chiral oxazolin [21], CuI/DBU [22], CuI/chiral guanidine [23], CuI/chiral diamine-phosphine [24], NHC–Cu(I) halide [25] and NHC-Ag/DIPEA [26], NHC-Ag complexes hybrid catalysts [27], PTC [28] and metal-free (KOtBu) conditions [29] in racemic or stereoselective form. Though these methods generalize the substrate scope, the use of pre-catalyst and anhydrous condition could limit some of these methods for the large scale synthesis.
The use of copper-based nanomaterials for organic transformations is well established because of natural abundance and economic viability of copper. Many variations of the copper based nano catalysts are known in the literature [30]. In the quest of finding new materials, the synthesis and application of CuWO4 nanoparticles is reported for the developing super capacitor, photo degradation of dyes (methyl orange) [31], methylene blue [32], ozone gas sensing [33] etc. However, the application of CuWO4 nanoparticles is not yet reported for organic transformation.
Heterogeneous catalysis is preferred over homogeneous catalysis because of catalyst recovery and easy separation of the products from the reaction mixture. The use of water for the organic transformations is also preferred because of the green aspects. In this direction we have developed synthetic methods using bismuth tungstate oxide [34] and iron tungstate oxide [35] nanomaterials for the multicomponent and one-pot Hantzsch ester formation and sp3-C-H functionalization reactions in water. Further to explore the reactivity of CuWO4 nanoparticles, herein we report the first example and straight forward method for the alkynylation of isatins and synthesis of 3-hydroxy-3-(indol-3-yl) indolin-2-ones and 3,3′-bis-indolyl-2-oxindoles using CuWO4 nanoparticles (Fig. 2).
Section snippets
Experimental
- a)
Preparation and characterization of CuWO4 nanoparticle by powder XRD and SEM-EDAX
CuWO4 nanoparticle were prepared by using reported method and characterized by powder XRD and SEM-EDAX data (see supporting information). After confirmation of the CuWO4 nanoparticle to test the catalyst activity we selected CH activation of aryl acetylenes and Friedel-Crafts reactions with isatin as a model reaction as described in below sections. The catalyst was recovered and reused up to six cycles for the same
Results and discussion
In continuation with our interest in the development of greener synthetic methods, herein, we report the first application CuWO4 nanoparticles as recyclable heterogeneous catalyst for the synthesis of 3-hydroxy-3-(arylethynyl)indolin-2-ones, 3-hydroxy-3-(1H-indol-3-yl)indolin-2-ones and bis(indolyl)indolin-2-ones via spC-H activation of aryl acetylenes and Friedel-Crafts alkylation of isatins in aqueous medium. Thus, the reaction of isatin 1 with phenyl acetylene 2 in presence of CuI (10 mol%)
Conclusions
In conclusion, we have developed a simple and efficient method for the synthesis of 3-hydroxy-3-(arylethynyl) indolin-2-ones via CH activation of arylacetylenes and 3-hydroxy-3-(1H-indol-3-yl)indolin-2-ones, bis-indolyl indoles via Friedel-Crafts alkylation of isatins using CuWO4 nanoparticles as recyclable heterogeneous catalyst in aqueous medium. All three types of reactions gave good to excellent yields. This is the first example where CuWO4 nanoparticles have been used for the CH activation
Declaration of Competing Interest
None.
Acknowledgements
BP thanks MHRD for fellowship. KS thanks DST for the INSPIRE fellowship. SN thanks, UGC-New Delhi for the fellowship. DK thanks, DST (SERB), New Delhi for the financial support (SB/FT/CS-136/2012 and SB/EMEQ-103/2014).
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These authors have equal contribution.