Recyclable catalyst with cationic phase tags for the Sonogashira coupling of aryl bromides and aryl chlorides

The new phase-tagged phosphines 1-(CH2NEt3+), 4-(CH2P(1-Ad)(2))C6H4.Br- and 1-(CH2PPh3+), 4-(CH2PH+(1-Ad)(2))C6H4.2Br(-) were prepared in the reaction of 1-(CH2Br), 4-(CH2PH+(1-Ad)(2))C6H4.Br- with Et3N or PPh3, respectively. The Pd(0)-phosphine catalysts formed on reaction of Na2PdCl4, CuI, phosphine, and acetylene are able to Sonogashira couple aryl bromides and aryl chlorides in high yields in DMSO or in DMSO/heptane mixtures. The products of the coupling reactions can be isolated from the DMSO solution by extraction with heptane or by separation of the product containing heptane solvent. The phase-tagged catalysts remain in the DMSO solution and can be reused for at least five consecutive coupling reactions. The catalysts retain their high activity throughout all cycles, as evidenced by the high reaction yields and, more importantly, by the almost constant turn-over frequency. Leaching of the catalyst into the product containing heptane solvent is negligible (>99.95% retention in the DMSO catalyst phase) as evidenced by TXRF and by photometric palladium determination.