Ruthenium(II) Complexes with Chiral Tetradentate P₂N₂ Ligands Catalyze the Asymmetric Epoxidation of Olefins with H₂O₂

Abstract

The five-coordinate complexes of the type [RuCl(PNNP)]PF 6 (PNNP = tetradentate ligand with a P 2 N 2 donor set) are prepared by chloride abstraction from [RuCl 2 (PNNP)]. A mixture of Δ- cis -β- and Λ- cis -β-[RuCl 2 ( 1a -κ 4 P,N,N,P )] ( 2a; 1a = N,N ‘-bis[ o -(diphenylphosphino)benzylidene]-2,2‘-diimino-1,1‘-( S )-binaphthylene), prepared by reaction of 1a with [RuCl 2 (PPh 3 ) 3 ], reacts with Tl[PF 6 ], giving the five-coordinate [RuCl( 1a -κ 4 P,N,N,P )]PF 6 ( 3a ). The related trans -[RuCl 2 ( 1b -κ 4 P,N,N,P )] ( 2b; 1b = N,N ‘-bis[ o -(diphenylphosphino)benzylidene]-(1 S,2 S )-diiminocyclohexane) reacts with Tl[PF 6 ] to give [RuCl( 1b -κ 4 P,N,N,P )]PF 6 ( 3b ). With the amino ligand N,N ‘ -bis[ o- (diphenylphosphino)benzylidene]-(1 S,2 S )-diaminocyclohexane ( 1c ), the aqua complex [RuCl(OH 2 )( 1c -κ 4 P,N,N, P )]PF 6 ( 5c ) is obtained by reaction of Tl[PF 6 ] with [RuCl 2 (PPh 3 )( 1c -κ 3 P,N,N )] ( 4 ), which has been isolated and structurally characterized. The reactivity of the five-coordinate 2b with CO and oxygen donors such as water, Et 2 O, THF, and methanol is reported. Both 3 and 5 catalyze the asymmetric epoxidation of olefins with hydrogen peroxide as oxidant. Enantiomeric excesses up to 42% were obtained in the enantioselective epoxidation of styrene and of other unfunctionalized olefins. The reaction is highly stereospecific, as the epoxidation of ( Z )-2-methylstyrene gives a cis: trans ratio of 99:1.