Studies of immobilized homogeneous metal catalysts on silica supports

Abstract

<p>Rhodium complexes of the chiral, chelating diphosphine (2S,4S)-4-(diphenylphosphino)-2-(diphenylphosphinomethyl)pyrrolidine tethered on silica were characterized by 31P NMR and IR. The tethered complex catalyzes the enantioselective hydrogenation of methyl-alpha-acetamidocinnamate (MAC). Spectral and catalytic investigations indicate that the tethered complex reacts by the same mechanism as the untethered complex in solution.;The rhodium complexes, [Rh(COD)H]4, [Rh(COD)2] +BF4-, [Rh(COD)Cl]2, and RhCl3·3H2O, adsorbed on SiO2 are optimally activated for toluene hydrogenation by pretreatment with H2 at 200°C. The same complexes on Pd-SiO2 are equally active without pretreatments. The active species in all cases is rhodium metal. The catalysts were characterized by XPS, TEM, DRIFTS, and mercury poisoning experiments.;Rhodium on silica catalyzes the hydrogenation of fluorobenzene to produce predominately fluorocyclohexane in heptane and 1,2-dichloroethane (DCE) solvents. In heptane/methanol and heptane/water solvents, hydrodefluorination to benzene and subsequent hydrogenation to cyclohexane occurs exclusively. Benzene inhibits the hydrodefluorination of fluorobenzene. In DCE or heptane solvents, fluorocyclohexane reacts with hydrogen fluoride to form cyclohexene. Reaction conditions can be chosen to selectively yield fluorocyclohexane, cyclohexene, benzene, or cyclohexane.;The oxorhenium(V) dithiolate catalyst [-S(CH2)3S-]Re(O)(Me)(PPh 3) was modified by linking it to a tether that could be attached to a silica support. Spectroscopic investigation and catalytic oxidation reactivity showed the heterogenized catalyst's structure and reactivity to be similar to its homogeneous analog. However, the immobilized catalyst offered additional advantages of recycleability, extended stability, and increased resistance to deactivation.</p>