Synthesis of symmetrical diarylalkyne from palladium-catalyzed decarboxylative couplings of propiolic acid and aryl bromides under water Kyungho Park a , Goun Bae a , Ahbyeol Park a , Yong Kim a , Jaehoon Choe c , Kwang Ho Song b,⇑ , Sunwoo Lee a,⇑ a Department of Chemistry, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea b LG Chem Research Park, Daejon 305-380, Republic of Korea c Department of Chemical & Biological Engineering, Korea University, Seoul 136-713, Republic of Korea article info Article history: Received 28 October 2010 Revised 17 November 2010 Accepted 19 November 2010 Available online 1 December 2010 Keywords: Propiolic acid Palladium Decarboxylative coupling Water Microwave abstract Symmetric diarylalkynes were obtained from the decarboxylative coupling reactions of aryl bromides and propiolic acid in water solvent condition. In the presence of phase transfer surfactant C 18 H 37 N(CH 3 ) 3 Cl, the catalytic system of both Pd(PPh 3 ) 2 Cl 2 /dppb and Pd(TPPMS) 2 Cl 2 /TPPMS afforded the desired coupled products in good yields. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The Sonogashira reaction, which is the coupling of terminal alkynes with aryl or alkenyl halides, is the most straightforward and powerful method for constructing the C(sp)–C(sp 2 ) bond. Since first proposed by Sonogashira in 1975, many new developments have been reported, including improved catalytic activity and expended scope of substrates. 1 This reaction has been widely used in the synthesis of bioactive compounds 2 and conjugated func- tional materials. 3 For the synthesis of symmetric diarylalkynes via the Sonogashira coupling reaction, a variety of alkyne compounds such as acetylene, 4 trimethylsilylacetylene, 5 2- methyl-but-2-yn-2-ol, 6 bis(trimethylsilyl)acetylene, 7 and bis(trib- utylstannyl)acetylene 8 have been employed. Recently, propiolic acid was employed as an alkyne source by our group and showed good activity in the coupling reaction. 9 With the increasing concern about environmentally friendly reaction conditions, numerous water-based solvent systems have been reported in the organic transformations including Sonogash- ira coupling reaction. 10 This reaction is more important in the pharmaceutical industry than the petrochemical industry, because 80% of waste comes from solvents in the former. 11 As a solvent, water has many advantages over common organic solvents. 12 For example, it is the most abundant, inexpensive, non-toxic and non-flammable solvent. On the other hand, microwave-assisted reaction systems have also been employed in metal-catalyzed cou- pling reactions including the Sonogashira coupling reaction to speed up the reaction rate. 13 Microwave heating technology has been often employed in the water reaction condition. 14 Since Myers first reported palladium-catalyzed decarboxylative Heck-type reactions using benzoic acid, 15 a variety of decarboxyla- tive coupling reactions using carboxylic acids have been developed in metal-catalyzed cross-couplings. 16 The coupling substrate bear- ing a carboxylic acid group offers unique advantages. It is stable and gives the desired coupled products without producing stoichi- ometric amounts of toxic metal halides. Very recently, we reported the efficient synthesis of symmetric diarylalkyne from propiolic acid using the Pd-catalyzed decarboxylative coupling reaction. 17 Considering propiolic acid as an alkyne source, it is suitable reagent to be applied in a water-based solvent system in the coupling reac- tion of alkyne due to its good solubility in water. However, to the best of our knowledge, there is no example of metal-catalyzed decarboxylative coupling reaction in water solvent. This stimu- lated us to develop a catalytic system of decarboxylative coupling which works in water condition. Here, we report the palladium- catalyzed decarboxylative coupling reactions of propiolic acid for the synthesis of symmetric diarylalkynes in water under micro- wave condition. As a starting point for the coupling reaction in water, we chose as model reactions the coupling between propiolic acid and phenyl bromide. The results are summarized in Table 1. For fast investiga- tion of the optimized condition, the microwave condition was 0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2010.11.110 ⇑ Corresponding authors. Tel.: +82 2 3290 3307 (K.H.S.), +82 62 530 3385 (S.L.). E-mail addresses: khsong@korea.ac.kr (K.H. Song), sunwoo@chonnam.ac.kr (S. Lee). Tetrahedron Letters 52 (2011) 576–580 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet