Fluorous Versions of Wilkinson's Catalyst. Activity in Fluorous Hydrogenation of 1-Alkenes and Recycling by Fluorous Biphasic Separation

Two novel fluorous tris(arylphosphine)rhodium(I) chloride complexes RhCl[P{C₆H₄-p-SiMe₂(CH₂)₂C_nF_2n+1}₃]₃ (3a:  n = 6, 3b:  n = 8) being fluorous analogues of Wilkinson's catalyst show high activity in hydrogenation of 1-alkenes under single phase fluorous conditions and can be effectively recycled using the new concept of fluorous biphasic separation. For comparison the p-(trimethylsilyl)-substituted derivative RhCl[P(C₆H₄-p-SiMe₃)₃]₃ (3c) has also been synthesized and the X-ray structure of dimeric [Rh(μ-Cl){P(C₆H₄-p-SiMe₃)₃}₂]₂ (4c) was determined. A comparison of the catalytic activity of the fluorous catalysts and the nonfluorous derivatives under identical conditions revealed a decreasing activity in the order 3c > 3a > RhCl(PPh₃)₃ > 3b. The recycling efficiency of the new catalysts (> 98% for 3b) is much better than expected on the basis of the fluorous phase affinity of the free phosphine ligands themselves and is in fact the result of the much higher fluorous phase affinity of the phosphine-containing rhodium species that are present during and after catalysis. Hence it is demonstrated that assembling multiple fluorous ligands in a specific configuration at one metal center can drastically improve the fluorous phase affinity of the resulting complexes.