A highly regioselective Cu-exchanged tungstophosphoric acid catalyst for hydroarylation and hydroamination of alkynes Nayeem Pasha, N. Seshu Babu, K. T. Venkateswara Rao, P. S. Sai Prasad, N. Lingaiah * Catalysis Laboratory, Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India article info Article history: Received 17 September 2008 Revised 23 October 2008 Accepted 28 October 2008 Available online 31 October 2008 Keywords: Hydroarylation Hydroamination Friedal–Craft’s alkenylation Cu-exchanged tungstophosphoric acid abstract An efficient and reusable Cu-exchanged tungstophosphoric acid catalyst is demonstrated for the solvent free hydroarylation and hydroamination reactions of alkynes with numerous arene and amine deriva- tives, respectively. The catalyst exhibited exceptionally high activity and regioselectivity in both the reactions. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The formation of new C–C and C–N bonds is of great importance in both academia and synthetic organic chemistry. The alkenyla- tion of arenes or hydroarylation of alkynes leading to the formation of C–C bonds is an important transformation in the synthesis of pharmaceuticals, agrochemicals, and fine chemicals. Friedel– Craft’s alkylation of various arenes, especially electron-rich arenes with alkyl halides, alcohols, or alkenes, involves the formation of new C–C bonds via the alkylation/alkenylation of arenes. 1 On the other hand, C–N bond-forming processes are highly interesting due to the presence of nitrogen-containing molecules as building blocks in industrial applications. Hydroarylation has received much attention due to its simple approach when compared to existing methods such as Heck reac- tions, 2 cross-coupling reactions, 3 and olefin cross-metathesis reac- tions. 4 Alkenylation of arenes with alkynes proceeds via alkenyl carbocation intermediates, which is followed by an electrophilic attack to the arene that can be promoted by Lewis acids. However, due to the instability of vinyl cationic intermediates, the oligomer- ization of alkynes takes place during the alkenylation 5 of arenes. Some Lewis acid metal chlorides such as ZrCl 4 and AlCl 3 are known to produce the desired alkenylated products with lower yields. 6 La- ter, Tsuchimoto et al. reported several metal triflates, Sc(OTf) 3 , In- (OTf) 3 , and Zr(OTf) 4 as catalysts for the alkenylation of arenes with both internal and terminal alkynes through the formation of alke- nyl cationic intermediates. 6 Several other transition metal catalysts such as rhodium, 7 ruthenium, 8–10 palladium, platinum, 11–13 gold 14–16 or rare earth-metals, 17 FeCl 3 , 18 and transitional metal tri- flates 19–23 in the presence of ionic liquid media also promote the alkenylation of arenes. On the other hand, the synthesis of nitrogen-containing build- ing blocks has gained significant importance in the pharmaceutical industry. One of the potential approaches for the synthesis of nitro- gen-containing molecules is hydroamination, which is an atom- economic process for the formation of amines, enamines, and imi- nes. The hydroamination process converts inexpensive and readily available starting materials into the desired products in a single step reaction without the formation of any other side products. Based on this concern, an efficient hydroamination reaction is al- ways considered economically and environmentally benign. How- ever, the hydroaminations of alkynes can only be achieved in the presence of certain catalysts, as the amines generally do not react spontaneously with alkynes due to the electron-rich nature of both the species. The alkynes are converted into reactive species (enam- ines and imines), which are reactive intermediates and can be used for various sorts of subsequent transformations. The hydroamina- tion of alkynes has been accomplished with different cationic met- als (e.g., Ag 1+ , Cu 2+ , Zn 2+ , and Pd 2+ ) supported on acidic materials such as montmorillonite clay K-10, phosphotungstic acid, and zeo- lite HBEA. 24–28 Despite the advantages of the above catalysts in both reactions, a few drawbacks are associated with them such as moisture sensi- tivity, high cost of metal, difficult catalyst preparation, and low recyclability. Therefore, the use of new, mild, and efficient catalysts remains to be explored for both hydroarylation and hydroamina- tion protocols. As part of our ongoing programs on the use of 0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2008.10.131 * Corresponding author. Tel: +91 40 27193163; fax: +91 40 27160923. E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah). Tetrahedron Letters 50 (2009) 239–242 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet