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Keywords:

  • homogeneous catalysis;
  • biphasic catalysis;
  • C–C coupling;
  • palladium;
  • polymers

Abstract

The Sonogashira coupling of various aryl bromides and iodides with different acetylenes was studied under biphasic conditions with soluble, polymer-modified catalysts to allow the efficient recycling of the homogeneous catalyst. For this purpose, several sterically demanding and electron-rich phosphines of the type RPPR2 were synthesised. They are covalently linked to a monomethyl polyethylene glycol ether with a mass of 2000 Dalton (RP=MeOPEG2000) RPPR2: [BOND]PR2=[BOND]CH2C6H4CH2P(1-Ad)2, [BOND]C6H4-P(1-Ad)2, [BOND]C6H4-PPh2. To couple aryl iodides and acetylenes, the catalyst [(MeCN)2PdCl2]/2 RP-C6H4-PPh2 was used in CH3CN/Et3N/n-heptane (5/2/5). The combined yields of coupling product over five reaction cycles are between 80–95 %. There is no apparent leaching of the catalyst into n-heptane, as evidenced by 1H NMR spectroscopy. The new catalyst [(MeCN)2PdCl2]/2 (1-Ad)2PBn can be used for room-temperature coupling of various aryl bromides and acetylenes in THF with HNiPr2 as a base. A closely related catalyst Na2[PdCl4]/2 RP-CH2C6H4CH2P(1-Ad)2 linked to the polymer was used to couple aryl bromides and acetylenes in DMSO or DMSO/n-heptane at 60 °C with 0.5 mol % Na2[PdCl4], 1 mol % RPPR2 and 0.33 mol % CuI. The combined yield of coupling products over five cycles is always greater than 90 %, except for sterically hindered aryl bromides. The determination of the turnover frequency (TOF) of the catalyst indicates only a small decrease in activity over five cycles. Leaching of the catalyst into the product containing n-heptane solution could not be detected by means of 1H NMR and TXRF; this is indicative of >99.995 % catalyst retention in the DMSO solvent.