Recycling and Delivery of Homogeneous Fluorous Rhodium Catalysts Using Poly(tetrafluoroethylene): "Catalyst-on-a-Tape"
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abstract
2017 American Chemical Society. The red-orange fluorous rhodium(I) complexes ClRh(P((CH2)mRfn)3)3 (m/n = 2/6 (1a), 2/8, 3/6, 3/10; Rfn = (CF2)n-1CF3) are essentially insoluble in organic solvents at 20 C but have measurable solubilities in dibutyl ether at 55-65 C. Under these conditions, they are effective catalyst precursors for the hydrosilylation of cyclohexanone by PhMe2SiH. Upon cooling, the catalyst rest states precipitate, giving colorless solutions of C6H11OSiPhMe2. When this sequence is conducted in the presence of poly(tetrafluoroethylene) (PTFE; Teflon) tape, the catalysts precipitate onto the tape but desorb when used in subsequent cycles. The catalyst precursors can also be precoated onto the tape, allowing quantities to be delivered by length instead of mass. Rate measurements (1a) show an induction period in the first cycle, excellent retention of activity in the second and third cycles, and significant activity loss in the fourth. Rhodium leaching is 0.57% and 5.3% for the first two cycles (atomic absorption spectroscopy inductively coupled plasma analysis); (CF2)5CF3 leaching is 11.4% over the first three cycles (19F NMR). Reactions with added mercury show that metallic rhodium is not responsible for catalysis. Identical protocols are applied to 2-octanone, acetophenone, and benzophenone, albeit with some activity loss in the third cycle. Other forms of PTFE can be similarly employed (e.g., Gore-Tex membrane). However, fluorous/organic liquid/liquid biphase conditions can give better retention of catalyst activity. Nonetheless, the diverse morphologies of PTFE that are commercially available suggest avenues for further optimization.
