Comparing Chiral Ferrocenyl and Ruthenocenyl Ligands: How Subtle Structural Changes Influence Their Performance in Asymmetric Catalysis

Abstract

A route to enantiopure heteroleptic ruthenocenyl derivatives has been found; the diastereoselective addition of MeLi to (R)-CyCH(Me)N(Me)CHC5H4 (de = 74%), followed by a transmetalation reaction with either [Cp*Ru(μ3-Cl)]4 or [(p-cymene)RuCl2]2/KPF6, afforded the heteroleptic complexes (S,R)-[Cp*Ru(η5-C5H4CH(Me)N(Me)CH(Me)Cy)], 6, or (S,R)-[(p-cymene)Ru(η5-C5H4CH(Me)N(Me)CH(Me)Cy)]+PF6-, 5, in 93% and 72% yields, respectively. Whereas 5 displayed a somewhat inert behavior, 6 reacted with NHMe2, in acetic acid, to afford its dimethylamino congener (S)-7 in 93% yield. The latter was converted in two steps into the bis(phosphine) derivatives (S)-(R)-Cp*RuC5H3CH(Me)PCy2PPh2-2, (S)-(R)-9, and (S)-(R)-Cp*RuC5H3CH(Me)PC8H14PPh2-2, (S)-(R)-10, and into the P,N derivative (S)-(R)-Cp*RuC5H3CH(Me){N2C3HMe2-3,5)PPh2}-1,2, (S)-(R)-11. These products were obtained in >99% ee after recrystallization. The ruthenocenyl derivatives were probed for their use as chiral ligands for the palladium-catalyzed enantioselective allylic alkylation and the rhodium catalyzed hydroboration reactions. By employing the ruthenocenylpyrazole (S)-(R)-11, styrene was converted to (S)-1-phenylethanol with 87% ee, whereas its isostructural ferrocenyl congener (S)-(R)-16 afforded 94% ee. The following compounds were characterized by X-ray diffraction: (S,R)-5, (S)-(R)-9, (S)-(R)-10, [((S*)-(R*)-14)Pd(η3-C3H5)]+[OTf]- ((S*)-(R*)-12), and [((S*)-(R*)-9)Pd(η3-C3H5)]+[OTf]- ((S*)-(R*)-13).