Asymmetric Reactions Catalyzed by Fluorous Chiral Catalysts

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We prepared chiral BINOLs (F_<13>BINOL and F_<17>BINOL), BNAP and ephedrine derivatives which bear long fluorous tags, and also a FiaBINOL derivative the hydroxyl groups of which are (S)-1-phenyl-1-hydroxylethylated (FDHPEB). The BINOLs were easily recovered by using a fluorous reverse phase silica gel (FRP) in a BINOL-Ti complex catalyzed asymmetric addtion reaction of Et2Zn to aldehydes and were reused without further purification to give similar enantioselectivities to the original non-fluorous reaction. The chiral fluorous catalyst in the fluorous phase was reused at least 5 times to give similar enantioselectivities to the original one in a toluene/FC-72 biphasic system. FDHPEB was also reused with the use of the FRP silica gel in SmI2 mediated reductive cleavage of 1-methoxy-1-phenylcyclohexanone followed by an asymmetric protonation by FDHPEB. After separation of the organic product by FRP silica gel, the crude product was analyzed by a chiral column HPLC with CD detector. Only two peaks due to the enantiomers of the product were observed and> the data revealed that the enantioselectivity was much higher (95 % ee) man that'of the sample which was obtained by purification with a preparative TLC (88 % ee). The results demonstrated that racemization occured to some extent during the purification. Thus the fluorous chiral ligands and catalysts are recyclable with the use of the FRP silica gel or in a fluorous/organic biphase system and the easy separation of the organic product allowed us to perform a quick analysis using HPLC with CD detector.