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

The five-coordinate complexes of the type [RuCl(PNNP)]PF₆ (PNNP = tetradentate ligand with a P₂N₂ donor set) are prepared by chloride abstraction from [RuCl₂(PNNP)]. A mixture of Δ-cis-β- and Λ-cis-β-[RuCl₂(1a-κ⁴P,N,N,P)] (2a; 1a = N,N‘-bis[o-(diphenylphosphino)benzylidene]-2,2‘-diimino-1,1‘-(S)-binaphthylene), prepared by reaction of 1a with [RuCl₂(PPh₃)₃], reacts with Tl[PF₆], giving the five-coordinate [RuCl(1a-κ⁴P,N,N,P)]PF₆ (3a). The related trans-[RuCl₂(1b-κ⁴P,N,N,P)] (2b; 1b = N,N‘-bis[o-(diphenylphosphino)benzylidene]-(1S,2S)-diiminocyclohexane) reacts with Tl[PF₆] to give [RuCl(1b-κ⁴P,N,N,P)]PF₆ (3b). With the amino ligand N,N‘-bis[o-(diphenylphosphino)benzylidene]-(1S,2S)-diaminocyclohexane (1c), the aqua complex [RuCl(OH₂)(1c-κ⁴P,N,N,P)]PF₆ (5c) is obtained by reaction of Tl[PF₆] with [RuCl₂(PPh₃)(1c-κ³P,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, Et₂O, 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.