Phospholane-oxazoline ligands for Ir-catalyzed asymmetric hydrogenation.

Although a lot of progress has been made in Rhor Rucatalyzed asymmetric hydrogenation, Ir-catalyzed asymmetric hydrogenation is relatively unexplored.[1] Pfaltz and coworkers first reported several Ir-phosphinooxazoline complexes as catalysts for asymmetric hydrogenation. With leading efforts by Pfaltz and co-workers,[2] and Burgess and Experimental Section Representative experimental procedure: VO(OiPr)3 (5 mL, 20 mmol) and hydroxamic acid (26.6 mg, 60 mmol) were dissolved in toluene (1 mL), stirred for 1 hour, and cooled to 0 8C. Cumene hydroperoxide (275 mL, 1.5 mmol) and 3-(1-naphthyl)-3-buten-1-ol (2j) (198 mg, 1.0 mmol) were added at 0 8C. The reaction mixture was stirred for 10 h, then trimethylphosphite (177 mL, 1.5 mmol) was added at that temperature. The mixture was allowed to reach room temperature, then it was extracted with ethyl acetate, dried over sodium sulfate, and evaporated. The crude product was purified by column chromatography on a silica gel (eluent ethyl acetate/hexane, 1:1) to give 3,4epoxy-3-(1-naphthyl)-1-butanol in 42% yield with 91% ee. 1H NMR (300 MHz, CDCl3, 25 8C, TMS): d= 8.13 (d, J= 8.0 Hz, 1H; Ar-H), 7.90 (d, J= 8.0 Hz, 1H; Ar-H), 7.83 (d, J= 8.0 Hz, 1H; Ar-H), 7.50 (m, 4H; Ar-H), 3.71 (m, 2H; CH2OH), 3.35 (d, J= 6.0 Hz, 1H; OCH2), 3.00 (d, J= 6.0 Hz, 1H; OCH2), 2.45 (m, 1H; CCH2CH2), 2.29 (m, 1H; CCH2CH2), 1.69 ppm (br, 1H; OH). HPLC analysis (column: OD-H, Daisel): retention times 44.9 (main peak) and 68.1 min (minor peak) using hexane/2-propanol (40:1) as the eluent at a flow rate of 1.0 mLmin 1. For the epoxy alcohols 3a–e, a saturated aqueous solution of sodium sulfite instead of trimethylphosphite was used for quenching the reaction.