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 ∆-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]-(1S,2S)-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]-(1S,2S)-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.

Expanding the scope of asymmetric electrophilic atom-transfer reactions: Titanium- and ruthenium-catalyzed hydroxylation of β-ketoesters

“…As already observed for catalyst 1, the enantioselectivity of the reaction ref lects that of a related transformation. In fact, the ee observed with 3f (36%) is similar to that obtained with the related complex [RuCl(PNNP)]PF 6 in the epoxidation of 1,2-dihydronaphthalene (41%) (19). This analogy is suggestive of the involvement of the enol form of 3k in the oxygen transfer step.…”

“…Thus, in a complementary approach, we also report the hydroxylation of highly enolized 1,3-dicarbonyl compounds with hydrogen peroxide in the presence of the chiral ruthenium catalyst 2 ( Fig. 1 previously applied in the asymmetric epoxidation and cisselective cyclopropanation of olefins (19)(20)(21) Titanium-Catalyzed Hydroxylation. The ␤-ketoester 3 (0.2 mmol), complex 1 (8 mg, 0.01 mmol), and 2-(phenylsulphonyl)-3-(4-nitrophenyl)oxaziridine (67 mg, 0.22 mmol) were successively added to freshly distilled dichloromethane (2 ml) in a Schlenk tube, and the reaction was stirred at room temperature for 2-24 h. The course of the reaction was monitored either by TLC or by 1 H NMR spectroscopy [by integration of the signals of the oxaziridine proton at ␦ 5.62 vs. that of the N-(benzenesulfonyl)-(4-nitrophenyl)imine proton at ␦ 9.15 (approximate reaction times for full consumption of the starting material were as follows: 3b, 2 h; 3c, 4 h; 3f, 5 h; 3g, 1 h; 3h, 2 h; 3i, 6 h; and 3j, 30 min)].…”

Transition Metal Peroxides. Synthesis and Role in Oxidation Reactions*Dedicated, on the occasion of his 80th birthday, to our ‘vitae ac studiorum magister in secunda et in adversa fortuna’ Emeritus Professor Giorgio Modena, Padova University, who showed us the way toward Chemistry.

“…Similar chiral inductions were reported for Ru derivatives, although the nature of the active intermediate was still in question 237 .…”

Ruthenium‐Catalyzed Asymmetric Epoxidation of Olefins Using H2O2, Part II: Catalytic Activities and Mechanism

“…In general, after 12 h of addition of H 2 O 2 good yield for the epoxidation of styrene and an excellent yield for that of trans-stilbene are obtained. It is worth noting that aryl groups on the C4-position generally gave a better reactivity towards styrene than alkyl groups (Table 1, entries 1-16 and entries [18][19][20][21][22]. When a highly sterically demanding group is introduced, either on the C4-or C5-position, the reactivity dropped tremendously (Table 1, entries 17 and 20).…”