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

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 R(P)PR(2) were synthesised. They are covalently linked to a monomethyl polyethylene glycol ether with a mass of 2000 Dalton (R(P)=MeOPEG(2000)) R(P)PR(2): -PR(2)= -CH(2)C(6)H(4)CH(2)P(1-Ad)(2), -C(6)H(4)-P(1-Ad)(2), -C(6)H(4)-PPh(2). To couple aryl iodides and acetylenes, the catalyst [(MeCN)(2)PdCl(2)]/2 R(P)-C(6)H(4)-PPh(2) was used in CH(3)CN/Et(3)N/n-heptane (5/2/5). The combined yields of coupling product over five reaction cycles are between 80-95 percent. There is no apparent leaching of the catalyst into n-heptane, as evidenced by (1)H NMR spectroscopy. The new catalyst [(MeCN)(2)PdCl(2)]/2 (1-Ad)(2)PBn can be used for room-temperature coupling of various aryl bromides and acetylenes in THF with HNiPr(2) as a base. A closely related catalyst Na(2)[PdCl(4)]/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 60 degrees C with 0.5 mol percent Na(2)[PdCl(4)], 1 mol percent R(P)PR(2) and 0.33 mol percent CuI. The combined yield of coupling products over five cycles is always greater than 90 percent, 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 (1)H NMR and TXRF; this is indicative of >99.995 percent catalyst retention in the DMSO solvent.

A Convenient High Activity Catalyst for the Sonogashira Coupling of Aryl Bromides

“…This is basically one lesson we learned from our studies of cross-coupling reactions under biphasic reaction conditions. [14] Results and Discussion…”

9‐Fluorenylphosphines for the Pd‐Catalyzed Sonogashira, Suzuki, and Buchwald–Hartwig Coupling Reactions in Organic Solvents and Water

“…Trialkylphosphines with bulky substituents are highly useful ligands for palladium-complex catalysts in various types of cross-coupling reactions of the Suzuki, [1][2][3][4][5][6][7][8][9][10][11] Sonogashira, [12][13][14][15][16][17][18][19][20][21] Heck, [22][23][24][25][26][27] Buchwald-Hartwig amination [28][29][30][31][32][33][34] and ether formation, [35] Negishi, [36] Stille, [37][38][39] Hiyama, [40] Kumada, [41] a-arylation, [42,43] and carbonylation type. [44] The main reasons for the favorable catalytic properties of trialkylphosphine-palladium complexes are the electron-richness and the steric bulk of trialkylphosphine ligands, which favor the formation of low-coordinate and highly active Pd complexes, possibly of the L 1 Pd type, [45][46]…”

Improved Palladium‐Catalyzed Sonogashira Coupling Reactions of Aryl Chlorides

“…[13] During the last few years important catalyst developments have been described such as the minimization of the catalyst amount, [14] the activation of less-reactive starting materials (aryl chlorides, alkyl halides), [15] the selective transformation of more functionalized substrates, [16] and the application of cost effective and/or sustainable methods. [17] With respect to the latter point, aryl-alkyne coupling methods catalyzed by more cost-effective metals such as iron [18] or copper [19] have become an interesting alternative to Pd-catalyzed procedures regarding the efficient coupling of aryl iodides. However, so far there is no general procedure for the efficient coupling of deactivated aryl bromides and inexpensive aryl chlorides with these metals.…”