Ligand Effect in Recycled CNT-Pd Heterogeneous Catalyst Bull. Korean Chem. Soc. 2013, Vol. 34, No. 7 2099 http://dx.doi.org/10.5012/bkcs.2013.34.7.2099 Ligand Effect in Recycled CNT-Pd Heterogeneous Catalyst for Decarboxylative Coupling Reactions Ji Dang Kim, Ayoung Pyo, Kyungho Park, Gwui Cheol Kim, † Sunwoo Lee, * and Hyun Chul Choi * Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Korea * E-mail: sunwoo@chonnam.ac.kr (S. Lee); chc12@chonnam.ac.kr (H. C. Choi) † Jeonnam Nano Bio Research Center, Jeollanam-do 515-806, Korea Received March 15, 2013, Accepted April 20, 2013 We present here an efficient and simple method for preparation of highly active Pd heterogeneous catalyst (CNT-Pd), specifically by reaction of dichlorobis(triphenylphosphine)palladium (Pd(PPh 3 ) 2 Cl 2 ) with thiolated carbon nanotubes (CNTs). The as-prepared CNT-Pd catalysts demonstrated an excellent catalytic activity for the carbon-carbon (C-C) cross-coupling reactions (i.e. Suzuki, Stille, and decarboxylative coupling reactions) under mild conditions. The CNT-Pd catalyst could easily be removed from the reaction mixture; additionally, in the decarboxylative coupling of iodobenzene and phenylpropiolic acid, it showed a six-times recyclability, with no loss of activity. Moreover, once its activity had decreased by repeated recycling, it could easily be reactivated by the addition of phosphine ligands. The remarkable recyclability of the decarboxylative coupling reaction is attributable to the high degree of dispersion of Pd catalysts in CNTs. Aggregation of the Pd catalysts is inhibited by their strong adhesion to the thiolated CNTs during the chemical reactions, thereby permitting their recycling. Key Words : CNT-Pd catalyst, Decarboxylative reaction, Recycling, Reactivation, Ligand effect Introduction One of the most important issues in current organic synthesis is the development of new, efficient carbon-carbon (C-C) and carbon-heteroatom bond-forming reactions. In this regard, transition metal-based catalysts generally have been used for these reactions. Among them, palladium (Pd) probably is the most versatile catalyst for C-C bond-forming reactions, and there is a great deal of literature on its pro- perties in many reactions. 1 Although soluble Pd complexes traditionally have been used as homogeneous catalysts for these coupling reactions, there remain problems that need to be addressed concerning catalyst-product separation and the non-recyclability of homogeneous catalysts. Such problems are of great significance to industrial applications, due to economical and environmental concerns. As a potential solution, the development of heterogeneous catalysts where- in the catalyst is immobilized on solid supports including mesoporous materials, 2 dendrimers, 3 ionic liquids, 4 carbon nanotubes (CNTs), 5 polymers, 6 and magnetic particles, 7 has attracted much attention in recent years. However, hetero- geneous systems exhibit generally lower activity than is observed with homogeneous catalysts, and that activity decreases, gradually, further in recycled systems, due to Pd leaching from the support. 8 Therefore, the development of Pd catalysts offering high activity, stability, and recyclability currently is a very popular research focus. A variety of recyclable catalytic systems based on Pd nanoparticles have been developed and applied in cross- couplings such as Suzuki-Miyaura, Mizoroki-Heck and Sono- gashira reactions. 9 CNT-supported Pd nanoparticles and their catalytic activity in cross-coupling reactions have been report- ed on by the present research group and others. 10 Although various heterogeneous Pd catalysts have shown recyclability in cross-coupling reactions, decreases in their activity, and relatively low recycle numbers, frequently have been observed. Recently, we were the first to report decarboxylative coupling reactions of alkynylcarboxylic acids and aryl halides pro- ductive of aryl alkynes in the presence of a homogenous Pd catalyst. 11 This, notably, is a very useful tool for the employ- ment of low-molecular-weight alkynes. 12 Positively too, this method does not result in the homocoupled by-products often formed in the Sonogashira coupling reaction, and shows high functional-group tolerance as well. In light of these ad- vantages, a variety alkynyl carboxylic acid coupling reactions have been reported. 13 However, there is no report on the re- cyclable catalytic system as it pertains to decarboxylative coupling reactions. Neither is there any report on catalytic reactivation when activity is decreased by recycling. Develop- ment of a more efficient recyclable catalytic system, certain- ly, is required. Herein we report an efficient CNT-supported Pd catalyst (denoted CNT-Pd) formed by deposition of di- chlorobis(triphenylphosphine)palladium [Pd(PPh 3 ) 2 Cl 2 ] on thiolated multiwall CNT surfaces. The prepared catalyst was easily recovered, and exhibited high activity and recyclability in the decarboxylative coupling reactions. Moreover, this catalyst could be reactivated by phosphine ligands once its activity had decreased after repeated recycling. Experimental Chemicals. Both multiwall carbon nanotubes (MWNTs)