Homogeneous catalysts supported on soluble polymers:: Biphasic sonogashira coupling of aryl halides and acetylenes using MeOPEG-bound phosphine-palladium catalysts for efficient catalyst recycling

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 (R-p = MeOPEG(2000)) RpPR2: -PR2 = -CH2C6H4CH2P(1-Ad)(2), -C6H4-P- (1-Ad)(2), -C6H4-PPh2. To couple aryl iodides and acetylenes, the catalyst [(MeCN)(2)PdCl2]/2 R-p-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 H-1 NMR spectroscopy. The new catalyst [(MeCN)(2)PdCl2]/2 (1-Ad)(2)PBn can be used for room-temperature coupling of various aryl bromides and acetylenes in THF with HNiPr2 as a base. A closely related catalyst Na-2[PdCl4]/2 R-p-CH(2)C(6)H(4)CH(2)p(1-Ad)(2) linked to the polymer was used to couple aryl bromides and acetylenes in DMSO or DMSO/n-heptane at 60degreesC with 0.5 mol % Na-2[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 H-1 NMR and TXRF; this is indicative of >99.995% catalyst retention in the DMSO solvent.