Transition metal catalysis in fluorous media: Practical application of a new immobilization principle to rhodium-catalyzed hydroborations of alkenes and alkynes
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Addition of a yellow-orange toluene solution of [Rh(C1)(COD)]2 to a colorless CF3C6F11 solution of P(CH2CH2R(f6))3 (R(f6) = (CF2)5CF3)3) gives a colorless toluene solution of COD and an orange CF3C6F11 solution of ClRh[P(CH2CH2R(f6))3]3 (1). Evaporation of CF3C6F11 gives analytically pure 1 (94%), which is insoluble in most organic solvents and stable to 300 C. Alkenes, catecholborane, and CF3C6F11 solutions of 1 (950:950:1 mol ratio for norbomene) are stirred for 1-24 h at 40 C (heterogeneous conditions). The resulting alkylboranes are extracted with benzene (2x; turnover number (TON) 854 (90%) for norbornene), toluene, or THF, and the catalyst solution is reused (TON 2409 for three cycles). Subsequent reactions with H202/NaOH give alcohols, which are isolated in 92-77% yields (11 examples). Longer reaction times afford TON values higher than 10000 (<0.1 mol % 1). Analogous reactions of alkynes yield alkenylboranes (89-88%). Pinacolborane additions are also catalyzed. A higher homologue of 1, ClRh[P(CH2CH2R(f8))3]3 (2), and the nonfluorinated analogue ClRh[P((CH2)7CH3)3]3 are similarly prepared. Solubilities and reactivities are compared. Atomic absorption analyses shows rhodium losses of 0.4% (1) and 0.2% (2) per cycle, corresponding to 4.52.2 ppm rhodium/mol of addition product. These data demonstrate the viability and practicality of an exciting new approach to catalyst immobilization.