Thermomorphic fluorous imine and thioether palladacycles as precursors for highly active Heck and Suzuki catalysts ; evidence for palladium nanoparticle pathways
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p-Iodobenzaldehyde is elaborated to the fluorous alcohol p-Rf8(CH2)3C6H4CH(OH) (CH2)2Rf8 (three steps/80%; Rf8 = n-C8F17), which is converted to imine p-Rf8(CH2)3C6H4C(= N(CH2)3Rf8)(CH2)2R f8 (6, two steps/93%) and thioether p-Rf8(CH2)3C6H4CH(S (CH2)3Rf8)(CH2)2R f8 (12, 64%). Reactions with Pd(OAc)2 (AcOH, 95°C) give palladacycles with [RC6H3CR′=N(R)Pd(μ-OAc)]2 (7, 87%) and [RC6H3CHR′S(R)Pd(μ-OAc)]2 (13, 84%) cores. The former reacts with LiCl and LiI to give the corresponding bridging halide complexes (8, 9); LiCl/PPh3 affords monomeric RC6H3CR′=N(R)Pd(Cl)(PPh3) (10). Palladacycles 7-9 and 13 are poorly soluble or insoluble in many solvents at 20-24°C, but much more soluble at higher temperatures. The CF3C6F11/toluene partition coeffcients of 6, 7, 12, and 13 are > 91: < 9 (24°C). Both 7 and 13 are excellent catalyst precursors for Heck reactions of aryl halides. Turnover numbers exceed 106 with phenyl iodide under homogeneous conditions in DMF at 140°C. The palladacycles precipitate as bridging halides upon cooling, and can in theory be recovered by liquid/solid phase separations. However, since the quantities are small, the solvent C8F17Br is added for recycling. Induction periods in both the first and second cycles, and progressively lower activities, are noted. Transmission electron microscopy indicates the formation of soluble palladium nanoparticles. Together with other data, it is proposed that the nanoparticles are the active catalysts, for which the recyclable palladacycles constitute a steady state source, until exhausted. Complex 7 similarly catalyzes the Suzuki reaction (K3PO4, toluene, 130°C).
author list (cited authors)
Rocaboy, C., & Gladysz, J. A.