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Gram-Scale Preparation of Pd@PANI: A Practical Catalyst Reagent for Copper-Free and Ligand-Free Sonogashira Couplings Lei Yu, Zhe Han, and Yuanhua Ding Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00322 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Organic Process Research & Development

Gram-Scale Preparation of Pd@PANI: A Practical Catalyst Reagent for Copper-Free and Ligand-Free Sonogashira Couplings Lei Yu,* Zhe Han and Yuanhua Ding Jiangsu Co-innovation Center for Prevention and Control of Important Animal Infectious Diseases and Zoonoses, Jiangsu Key Laboratory of Zoonosis, School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, People’s Republic of China. KEYWORDS. Palladium, polyaniline, nanoparticles, Sonogashira coupling, copper-free.

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ABSTRACT. Palladium nanoparticles on polyaniline (Pd@PANI) catalyst is now easily prepared in gram-scale through the oxidative polymerization of aniline in the presence of PdCl2 by using air as clean oxidant. The material is found to be very stable and can be stored for more than one year without deactivation. Thus, it may become a commercial reagent in organic synthesis, depending on its application scopes. This article reported the first example of Pd@PANI-catalyzed Sonogashira couplings free of copper and ligand. Keywords: Polyaniline, Sonogashira coupling, palladium nanoparticle, ethisterone, phosphorusfree

INTRODUCTION Polymer-supported catalysts are important materials for their great application potential in industrial production.1-3 Different from inorganic supporters, polymers could be well modified through the introduction of specialized functional groups into their monomers, which provided sufficiently variable factors for catalyst activity tunings. Among reported works, polyaniline2c,3-4 (PANI) are promising supporters because of their low-toxicity5 and availability. PANI-supported metal catalysts (M@PANIs)2c,3 are readily accessible through the oxidative polymerization of anilines in the presence of metal chlorides, which, in comparison with organometallic complexes, are extremely cheap and available. M@PANIs may serve as an important class of catalysts in not far future. But the field belongs to materials chemistry and attracts less attention from organic chemists. Investigations on M@PANI catalysts mainly focused on the design of new materials, while the applications in organic reactions are just a method to test their catalytic activities by using very simple reactions with narrow application scopes.3 Currently, M@PANI catalysts have



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been applied in Suzuki cross-couplings, Heck reactions, the hydroxylation of aryl halides and some redox reactions and cyclo-additions.2c,3 On the other hand, Sonogashira cross-coupling is able to form a C-C bond between the terminal sp hybridized carbon from an alkyne with a sp2 carbon of an aryl or vinyl halide.6 It is one of the most important reactions in organic synthesis to prepare aryl alkynes and conjugated enynes, which are important intermediates for the synthesis of many natural products, pharmaceuticals and organic functional materials.7 A typical Sonogashira cross-coupling employs ligand and copper salt as co-catalyst, which introduces in reaction another environmentally unfriendly and difficult to recover reagent and leads to homo-coupling product of the terminal alkyne.8 In line with the calls for greater environmental protection, the copperfree and ligand-free Sonogashira cross-couplings using simplified catalyst are the trend of development in the field.9 Our group aims to develop green technologies with industrial potential.2,10 In 2015, we developed a novel method to fabricate the ultrasmall and uniform Pd@PANI catalyst by using air as oxidant instead of the conventionally employed chemical oxidant.2c The catalyst was then successfully employed in Heck reactions. Recently, gram-scale preparations of Pd@PANI have been achieved. The light-yellow powder was obtained in good yield (85%) and was found to be very stable (Figure 1). IPC analysis indicated that the weight ratio of Pd in the material was 0.85%. It could be stored under normal conditions for more than one year without deactivation, indicating great potential for reagent development. After a series of conditional optimizations, the Pd@PANI catalyst was successfully applied in copper-free and ligand-free Sonogashira couplings for the first time. Herein, we wish to report our findings.


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Figure 1. Preparation of Pd@PANI in gram-scale. RESULTS AND DISCUSSION The reaction of iodobenzene 1a with phenylacetylene 2a was chosen as the model reaction for conditional optimizations. For each reaction performed in 1 mmol scale, 0.6 mg of the catalyst was employed. The catalyst loading was 0.005 mol % on the basis of the substrates, as calculated according to the IPC analysis result. The reaction was first performed in DMF with excess Et3N, but gave the product 3a in low yield (42 %, Table 1, entry 1). Reactions in other polar solvents, such as NMP, DMSO, EtOH, EtOAc and MeCN were also tested and MeCN was screened out to be the best one among them, giving 3a in 72 % yield (Table 1, entries 6 vs 2-5). The coordination ability of the solvent might benefit the reaction in MeCN. Low-polar solvent, such as THF, c-C6H12 and toluene, led to very poor results (Table 1, entries 7-9). The product yield was further elevated to 86 % with extended reaction time (Table 1, entry 10). Table 1. Optimization of the reaction conditiona



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Entry Solvent t (h) 3a yield / %b 1 DMF 24 42 2 NMP 24 23 3 DMSO 24 62 4 EtOH 24 45 5 EtOAc 24 9 6 MeCN 24 72 7 THF 24 29 8 c-C6H12 24

Gram-Scale Preparation of Pd@PANI: A Practical ... - ACS Publications

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