Y. Nishibayashi et al., NOVEL CHIRAL LIGANDS, DIFERROCENYL DICHALCOGENIDES AND THEIR DERIVATIVES, FOR RHODIUM-CATALYZED AND IRIDIUM-CATALYZED ASYMMETRIC HYDROSILYLATION, Organometallics, 15(1), 1996, pp. 370-379
As chiral ligands for transition metal complex-catalyzed asymmetric re
actions, a variety of novel chiral ferrocenyl chalcogen compounds, whi
ch possess planar chirality due to the 1,2-unsymmetrically disubstitut
ed ferrocene structure, have been prepared from chiral ferrocenes. The
re are seven diferrocenyl dichalcogenides (4-10), nine alkyl or aryl f
errocenyl chalcogenides (11-19), two bis(ferrocenylseleno)alkanes (20
and 21), two cogeno)-1-[2-(diphenylphosphino)ferrocenyl]ethanes (22 an
d 24), and two )-1-[1',2-bis(diphenylphosphino)ferrocenyl]ethanes (23
and 25). ene-2,3-dihydroxy-1,4-bis(phenylchalcogeno)butanes (26-28) ar
e also synthesized. The Rh(I) complex-catalyzed hydrosilylation of ket
ones with diphenylsilane in the presence of these chiral ligands inclu
ding the reported [1-(dimethylamino)ethyl]ferrocenyl]dichalcogenides (
1-3), followed by hydrolysis with dilute HCl, affords the correspondin
g chiral alcohols (R-configuration) in moderate to quantitative yield
with up to 88% enantiomeric excess (ee), Similar treatment of acetophe
none 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 yi
eld with up to 23% ee. The new complex prepared from a cationic rhodiu
m compound and the diferrocenyl diselenide (2) shows an activity for a
symmetric 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 s
ec-amines and amide with up to 53% and 69% ee, respectively. A catalyt
ic cycle involving the formation of tetracoordinated rhodium(I)-dichal
cogenide complex (two Se and two N atoms to one Rh) followed by oxidat
ive addition of the Si-H bond to Rh(I) and carbonyl addition to the pr
oduced rhodium(III) hydride complex is proposed for hydrosilylation of
ketones.