New chiral ruthenium(II)–phosphinite complexes containing a ferrocenyl group in enantioselective transfer hydrogenations of aromatic ketones

A new and versatile class of unsymmetrical ferrocenyl-phosphinite ligands possessing a stereogenic center has been prepared from commercially available, inexpensive aminoacids such as, d-, l-phenylglycine and d-, l-phenylalanine, through a concise synthetic procedure. These ligands are not very sensitive to air and moisture, and display good enantioselectivities in the ruthenium-catalyzed asymmetric transfer hydrogenation of acetophenone derivatives, in which up to 91% ee was obtained. A comparison of the catalytic properties of amino alcohols and other analogues based on a ferrocenyl backbone is also discussed briefly. The structures of these ligands and their corresponding complexes have been elucidated by a combination of multinuclear NMR spectroscopy, IR spectroscopy, and elemental analysis.

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(2R)-2-[(Ferrocenylmethyl)amino]-2-phenylethan-1-olC19H21NOFe[α]D20=-42.8 (c 1.2, MeOH)Source of chirality: d-phenylglycinolAbsolute configuration: (R)

(2R)-2-[(Ferrocenylmethyl)amino]-3-phenylpropan-1-olC20H23NOFe[α]D20=+18.6 (c 1.2, MeOH)Source of chirality: d-phenylalaninolAbsolute configuration: (R)

(2R)-2-(Ferrocenylmethylamino)-2-phenylethyl diphenylphosphiniteC31H30NOPFe[α]D20=-55.6 (c 1.2, MeOH)Source of chirality: d-phenylglycinolAbsolute configuration: (R)

(2S)-2-(Ferrocenylmethylamino)-3-phenylpropyl diphenylphosphiniteC32H32NOPFe[α]D20=-25.4 (c 1.2, MeOH)Source of chirality: l-phenylalaninolAbsolute configuration: (S)

(2R)-2-(Ferrocenylmethylamino)-2-phenylethyl diphenylphosphinito(dichloro(η6-p-cymene)ruthenium(II))C41H44NOPFeRuCl2[α]D20=-43.2 (c 1.2, MeOH)Source of chirality: d-phenylglycinolAbsolute configuration: (R)

(2R)-2-(Ferrocenylmethylamino)-3-phenylpropyl diphenylphosphinito(dichloro(η6-p-cymene)ruthenium(II))C42H46NOPFeRuCl2[α]D20=+33.9 (c 1.2, MeOH)Source of chirality: d-phenylalaninolAbsolute configuration: (R)