Preparation of new, optically active 1,2-ferrocenyldiamine ligands and their application to ruthenium-catalyzed asymmetric transfer hydrogenation of ketones

The treatment of (R,R)-1,2-bis(1-acetoxy-1-phenylmethyl)ferrocene (1) with azidotrimethylsilane in CH2Cl2 in the presence of a catalytic amount of a Lewis acid such as Cu(OTf)(2) or Sc(OTf)(3), at -40 degrees C for 24 h, gives a mixture of two diastereomeric ferrocenyl diazides 4 and 5 in an 80:20 ratio. The major isomer (R,R)-4 is formed with retention of configuration at both benzylic chiral centers whereas the minor isomer meso- (R, S) -5 is formed with inversion of configuration at one of the two chiral centers. After a shorter reaction time (12 h), the (R,R)-ferrocenylazido acetate 6 was isolated as a single diastereomer, which gave 4 and 5 under the same conditions with almost the same ratio as with the longer reaction time. These results show that the first step of the reaction is the substitution, with retention of configuration, of the exo acetate of 1 by means of an iron-assisted ionization, i.e., neighboring-group participation, then the endo acetate is replaced by the azide ion with retention or inversion of configuration to give 4 or 5, respectively. The optically active complex 4 could be converted into the corresponding ferrocenyldiamme 2 by reduction with LiAIH(4), and its ditosylamide 8a and di-acetamide 8b were isolated as stable compounds. The ferrocenyldiamine and both diamides were used as ligands in the ruthenium-catalyzed asymmetric transfer hydrogenation of aryl ketones, and the complex with 2 gave 1-arylethanols in good yields with up to 75% ee. (c) Wiley-VCH Verlag GmbH & Co.