Highly efficient iridium catalysts based on C2-symmetric ferrocenyl phosphinite ligands for asymmetric transfer hydrogenations of aromatic ketones

A series of chiral modular C2-symmetric ferrocenyl phosphinite ligands have been synthesized in good yields by using 1,1′-ferrocenedicarboxyaldehyde and various amino alcohols as starting materials, and applied in the iridium(III)-catalyzed asymmetric transfer hydrogenations of aromatic ketones to give the corresponding secondary alcohols with good enantioselectivities and reactivities using 2-propanol as the hydrogen source (up to 98% ee and 99% conversion). The substituents on the backbone of the ligands were found to have a significant effect on both the activity and enantiomeric excess. The structures of these complexes have been clarified by a combination of multinuclear NMR spectroscopy, IR spectroscopy, and elemental analysis.

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(S)-Bis[[N-2-diphenylphosphinite-1-benzyl)ethyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethyl-cyclopentadienyl iridium(III))]C74H84N2O2P2FeIr2Cl4[α]D20 = +53.9 (c 1.2 MeOH)Source of chirality: l-phenyl alanineAbsolute configuration: (S,S)

(S)-Bis[[N-2-diphenylphosphinite-1-phenyl)ethyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethyl cyclopentadienyl iridium(III))]C72H80N2O2P2FeIr2Cl4[α]D20 = +89.2 (c 1.2 MeOH)Source of chirality: l-phenyl glycineAbsolute configuration: (S,S)

(S)-Bis[[N-2-diphenylphosphinite-1-iso-butyl)ethyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethylcyclopentadienyl iridium(III))]C68H88N2O2P2FeIr2Cl4[α]D20 = +72.5 (c 1.2 MeOH)Source of chirality: l-leucineAbsolute configuration: (S,S)

(S)-Bis[[N-2-diphenylphosphinite-1-sec-butyl)ethyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethylcyclopentadienyl iridium(III))]C68H88N2O2P2FeIr2Cl4[α]D20 = +83.8 (c 1.2 MeOH)Source of chirality: l-isoleucineAbsolute configuration: (S,S)

(S)-Bis[[N-2-diphenylphosphinitepropyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethyl cyclopentadienyl iridium(III))]C62H76N2O2P2FeIr2Cl4[α]D20 = +77.6 (c 1.2 MeOH)Source of chirality: (S)-1-amino-2-propanolAbsolute configuration: (S,S)

((R)-Bis[[N-2-diphenylphosphinite-2-phenyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethyl cyclopentadienyl iridium(III))]C72H80N2O2P2FeIr2Cl4[α]D20 = −85.5 (c 1.2 MeOH)Source of chirality: (R)-2-amino-1-phenylethanolAbsolute configuration: (R,R)

((R)-Bis[[N-2-diphenylphosphinite-1-ethyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethyl cyclopentadienyl iridium(III))]C64H80N2O2P2FeIr2Cl4[α]D20 = −63.5 (c 1.2 MeOH)Source of chirality: (R)-2-amino-1-butanolAbsolute configuration: (R,R)

(1S,2R)-Bis[[N-2-diphenylphosphinite-1,2-diphenyl)ethyl]-1,1′-ferrocenylmethyldiamine(dichloro η5-pentamethylcyclopentadienyl iridium(III))]C84H88N2O2P2FeIr2Cl4[α]D20 = +36.6 (c 1.2 MeOH)Source of chirality: (1S,2R)-2-amino-1,2-diphenyl ethanolAbsolute configuration: (S,R,S,R)