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-<i>cis</i>-β- and Λ-<i>cis</i>-β-[RuCl₂(<b>1a</b>-κ⁴<i>P,N,N,P</i>)] (<b>2a</b>; <b>1a</b> = <i>N,N</i>‘-bis[<i>o</i>-(diphenylphosphino)benzylidene]-2,2‘-diimino-1,1‘-(<i>S</i>)-binaphthylene), prepared by reaction of <b>1a</b> with [RuCl₂(PPh₃)₃], reacts with Tl[PF₆], giving the five-coordinate [RuCl(<b>1a</b>-κ⁴<i>P,N,N,P</i>)]PF₆ (<b>3a</b>). The related <i>trans</i>-[RuCl₂(<b>1b</b>-κ⁴<i>P,N,N,P</i>)] (<b>2b</b>; <b>1b</b> = <i>N,N</i>‘-bis[<i>o</i>-(diphenylphosphino)benzylidene]-(1<i>S</i>,2<i>S</i>)-diiminocyclohexane) reacts with Tl[PF₆] to give [RuCl(<b>1b</b>-κ⁴<i>P,N,N,P</i>)]PF₆ (<b>3b</b>). With the amino ligand <i>N,N</i><i>‘</i>-bis[<i>o-</i>(diphenylphosphino)benzylidene]-(1<i>S</i>,2<i>S</i>)-diaminocyclohexane (<b>1c</b>), the aqua complex [RuCl(OH₂)(<b>1c</b>-κ⁴<i>P,N,N</i>,<i>P</i>)]PF₆ (<b>5c</b>) is obtained by reaction of Tl[PF₆] with [RuCl₂(PPh₃)(<b>1c</b>-κ³<i>P,N,N</i>)] (<b>4</b>), which has been isolated and structurally characterized. The reactivity of the five-coordinate <b>2b</b> with CO and oxygen donors such as water, Et₂O, THF, and methanol is reported. Both <b>3</b> and <b>5</b> 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 (<i>Z</i>)-2-methylstyrene gives a <i>cis</i>:<i>trans</i> ratio of 99:1.