Asymmetric transfer hydrogenation of ferrocenyl ketones: a new simple route to chiral ferrocenyl alcohols

The asymmetric transfer hydrogenation (ATH) of ferrocenyl ketones, such as FcC(O)CH2Y [Fc = ferrocenyl, Y = H (1a), CH3 (1b), Cl (1c) or N3 (1d)] has been carried out using the Noyori/Ikariya catalysts [(−)-(1R,2S)-ephedrine] or N-tosyl-(1R,2R)-diphenylethylenediamine [(R,R)-TsDPEN] as chiral ligands combined with [RuCl2(η6-benzene)]2 and 2-PrOH or HCO2H–Et3N as the hydrogen sources, respectively. The best results were achieved with the [(R,R)-TsDPEN–RuIIHCO2H–Et3N] catalytic system, which produced the ferrocenylalcohols (R)-2a, (R)-2c, and (R)-2d in good yields and excellent enantiomeric excesses (>98% ee).

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(R)-1-FerrocenylethanolC12H14FeOEe >98% ee[α]D21 = −29.4 (c 1.43, benzene)Source of chirality: asymmetric transfer hydrogenationAbsolute configuration: (R)

(R)-2-Chloro-1-ferrocenylethanolC12H13FeClOEe >98% ee[α]D21 = −18 (c 0.93, benzene)Source of chirality: asymmetric transfer hydrogenationAbsolute configuration: (R)

(R)-2-Azido-1-ferrocenylethanolC12H13FeN3OEe >98% ee[α]D21 = −80 (c 0.79, benzene)Source of chirality: asymmetric transfer hydrogenationAbsolute configuration: (R)