Stereoselective synthesis of chiral atropisomerically stable ferrocenyldiols containing a biphenyl unit

Two different ferrocenyldiketones 1 and 2 containing a biphenyl unit were prepared by Ni(0) promoted homocoupling of suitable ferrocenyl-aryl halides. The atropisomeric stability of the corresponding diols and other related derivatives was investigated. Chiral 2,2′-bis-(ferrocenylhydroxymethyl)-1,1′-biphenyl, (−)-5c, and 1(1,1′)-ferrocena-2,5-dihydroxy-3,4(1,2)dibenzenacyclopentaphane, (+)-6a and (−)-6b, were prepared as single diastereoisomers with defined central and axial chiralities by CBS-catalyzed asymmetric reduction of 1 and 2, respectively. During the reaction, that proceeded with high stereoselectivity affording the above mentioned diols in satisfactory enantiomeric excess, we noticed the occurrence of an unusual reductive deoxygenation process. Absolute configurations of the chiral diols were assigned by means of X-ray crystallographic and circular dichroism analyses.

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(aR,R,R)-2,2′-Bis-(hydroxymethylferrocenyl)-1,1′-biphenylC34H30Fe2O2Ee = 95%[α]D = −22.5 (c 1.10, CHCl3)Source of chirality: asymmetric reduction of the corresponding diketoneAbsolute configuration: aR,R,R

(aR,R,R)-3-Oxa-2,4-diferrocenyl-1,5(1,2)-dibenzenacyclopentaphaneC34H28Fe2OEe = 95%[α]D = +44.9 (c 0.15, CHCl3)Source of chirality: asymmetric reduction of the corresponding diketoneAbsolute configuration: aR,R,R

(aS,R,S)-1(1,1′)-Ferrocena-2,5-dihydroxy-3,4(1,2)dibenzenacyclopentaphaneC24H20FeO2Ee = 92%[α]D = +88.9 (c 0.68, CH3CN)Source of chirality: asymmetric reduction of the corresponding diketoneAbsolute configuration: aS,R,S

(aR,R,R)-1(1,1′)-Ferrocena-2,5-dihydroxy-3,4(1,2)dibenzenacyclopentaphaneC24H20FeO2Ee = 95%[α]D = −56.8 (c 0.44, EtOH)Source of chirality: asymmetric reduction of the corresponding diketoneAbsolute configuration: aR,R,R

(aS,S)-1(1,1′)-Ferrocena-2,5-dihydroxy-3,4(1,2)dibenzenacyclopentaphaneC24H20FeOEe = 52%[α]D = +57.3 (c 0.19, CH3CN)Source of chirality: asymmetric reduction of the corresponding diketoneAbsolute configuration: aS,S