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

A route to enantiopure heteroleptic ruthenocenyl derivatives has been found; the diastereoselective addition of MeLi to ( R )-CyCH(Me)N(Me)CH C 5 H 4 (de = 74%), followed by a transmetalation reaction with either [Cp*Ru(μ 3 -Cl)] 4 or [( p -cymene)RuCl 2 ] 2 /KPF 6, afforded the heteroleptic complexes ( S, R )-[Cp*Ru(η 5 -C 5 H 4 CH(Me)N(Me)CH(Me)Cy)], 6, or ( S, R )-[( p -cymene)Ru(η 5 -C 5 H 4 CH(Me)N(Me)CH(Me)Cy)] + PF 6 -, 5, in 93% and 72% yields, respectively. Whereas 5 displayed a somewhat inert behavior, 6 reacted with NHMe 2, 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*RuC 5 H 3 CH(Me)PCy 2 PPh 2 -2, ( S )-( R )- 9, and ( S )-( R )-Cp*RuC 5 H 3 CH(Me)PC 8 H 14 PPh 2 -2, ( S )-( R )- 10, and into the P,N derivative ( S )-( R )-Cp*RuC 5 H 3 CH(Me){N 2 C 3 HMe 2 -3,5)PPh 2 }-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(η 3 -C 3 H 5 )] + [OTf] - (( S* )-( R *)- 12 ), and [(( S *)-( R *)- 9 )Pd(η 3 -C 3 H 5 )] + [OTf] - (( S* )-( R *)- 13 ).