Novel Chiral Ligands, Diferrocenyl Dichalcogenides and Their Derivatives, for Rhodium- and Iridium-Catalyzed Asymmetric Hydrosilylation

As chiral ligands for transition metal complex-catalyzed asymmetric reactions, a variety of novel chiral ferrocenyl chalcogen compounds, which possess planar chirality due to the 1,2-unsymmetrically disubstituted ferrocene structure, have been prepared from chiral ferrocenes. There are seven diferrocenyl dichalcogenides ( 4 − 10 ), nine alkyl or aryl ferrocenyl chalcogenides ( 11 − 19 ), two bis(ferrocenylseleno)alkanes ( 20 and 21 ), two 1-(phenylchalcogeno)-1-[2-(diphenylphosphino)ferrocenyl]ethanes ( 22 and 24 ), and two 1-(phenylchalcogeno)-1-[1‘,2-bis(diphenylphosphino)ferrocenyl]ethanes ( 23 and 25 ). 2,3- O, O ‘-Isopropylidene-2,3-dihydroxy-1,4-bis(phenylchalcogeno)butanes ( 26 − 28 ) are also synthesized. The Rh(I) complex-catalyzed hydrosilylation of ketones with diphenylsilane in the presence of these chiral ligands including the reported [ R, S; R, S ]-bis[2-[1-(dimethylamino)ethyl]ferrocenyl] dichalcogenides ( 1 − 3 ), followed by hydrolysis with dilute HCl, affords the corresponding chiral alcohols ( R -configuration) in moderate to quantitative yield with up to 88% enantiomeric excess (ee). Similar treatment of acetophenone in the presence of diferrocenyl dichalcogenides ( 1, 2, 3, and 10 ) and a catalytic amount of Ir(I) complex gives chiral 1-phenylethanol of the opposite configuration ( S ) compared with the Rh case in high yield with up to 23% ee. The new complex prepared from a cationic rhodium compound and the diferrocenyl diselenide ( 2 ) shows an activity for asymmetric hydrosilylation of acetophenone to afford 1-phenylethanol in 60% chemical yield with 60% ee. Asymmetric hydrosilylation of imines and asymmetric hydrogenation of an enamide also proceed smoothly using the Rh(I)−diselenide ( 2 ) catalytic system to give the corresponding sec -amines and amide with up to 53% and 69% ee, respectively. A catalytic cycle involving the formation of tetracoordinated rhodium(I)−dichalcogenide complex (two Se and two N atoms to one Rh) followed by oxidative addition of the Si−H bond to Rh(I) and carbonyl addition to the produced rhodium(III) hydride complex is proposed for hydrosilylation of ketones.