Homogeneous Catalysts Supported on Soluble Polymers: Biphasic Sonogashira Coupling of Aryl Halides and Acetylenes Using MeOPEG‐Bound Phosphine–Palladium Catalysts for Efficient Catalyst Recycling

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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: PR2=CH2C6H4CH2P(1‐Ad)2, C6H4‐P(1‐Ad)2, 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.

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