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

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journal contribution
posted on 2000-08-25, 00:00 authored by Robert M. Stoop, Stephan Bachmann, Massimiliano Valentini, Antonio Mezzetti
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(1a4P,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(1a4P,N,N,P)]PF6 (3a). The related trans-[RuCl2(1b4P,N,N,P)] (2b; 1b = N,N‘-bis[o-(diphenylphosphino)benzylidene]-(1S,2S)-diiminocyclohexane) reacts with Tl[PF6] to give [RuCl(1b4P,N,N,P)]PF6 (3b). With the amino ligand N,N-bis[o-(diphenylphosphino)benzylidene]-(1S,2S)-diaminocyclohexane (1c), the aqua complex [RuCl(OH2)(1c4P,N,N,P)]PF6 (5c) is obtained by reaction of Tl[PF6] with [RuCl2(PPh3)(1c3P,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.