Testing New Ruthenium Complexes bearing Chiral 1,2,4-Triazol-5-ylidene ­Ligands as Catalysts for Asymmetric Olefin Metathesis

Abstract

New ruthenium complexes bearing chiral 1,2,4-triazol-5-ylidene ligands were obtained and tested in model asymmetric metathesis reactions. Low enantioselectivity in asymmetric ring-closing metathesis (ARCM) and moderate enantioselectivity in asymmetric ring-opening/ring-closing metathesis (AROCM) was observed.

Key words

ruthenium - carbene complexes - NHC - olefin meta­thesis - asymmetric catalysis

• References and Notes


For reviews on asymmetric olefin metathesis, see:
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Ruthenium carbene complexes bearing achiral 1,2,4-triazol-5-ylidene have been obtained. See:
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• 13 Synthesis of 7b: (S)-5-sec-Butylmorpholin-3-one (13; 230 mg, 1.46 mmol; see ref. 12) was dissolved in anhyd CH₂Cl₂ (7 mL) and Me₃OBF₄ (259 mg, 1.75 mmol) was added. The mixture was stirred at r.t. for 24 h and then cooled to 0 °C. Sat. aq NaHCO₃ (10 mL) was added over 15 min and the mixture was further stirred at r.t. for 30 min. The mixture was extracted with CH₂Cl₂, and the combined organic layers were washed with H₂O and dried over Na₂SO₄. After filtration, the solvent was removed under reduced pressure to afford the crude (S)-3-sec-butyl-5-methoxy-3,6-dihydro-2H-1,4-oxazine (239 mg, 96%) as a white solid which was used in the next step without further purification. Mesitylhydrazine hydrochloride (270 mg, 1.46 mmol) was dissolved in anhyd MeOH (6 mL). The crude (S)-3-sec-butyl-5-methoxy-3,6-dihydro-2H-1,4-oxazine (239 mg, 0.48 mmol) was added in one portion followed by HCl (4 M in 1,4-dioxane; 36 μL, 0.20 mmol, 0.100 equiv) and the reaction mixture was stirred at r.t. for 24 h. The solvent was evaporated to give a yellow solid which was dried in high vacuum. The residue was suspended in chlorobenzene (1.5 mL) and triethyl orthoformate (2 mL, 11.7 mmol) was added. The mixture was stirred for 4 h at 110 °C and concentrated in vacuum. After purification by column chromatography (CH₂Cl₂–MeOH = 99:1 to 95:5) the desired product was obtained as an off-white solid (160 mg, 33%).
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• 16 Synthesis of 7d: (S)-5-sec-Butylmorpholin-3-one (13; 692 mg, 4.40 mmol) was dissolved in anhyd CH₂Cl₂ (10 mL) and Me₃OBF₄ (651 mg, 4.4 mmol) was added. The mixture was stirred at r.t. for 24 h. Pentafluorophenyl hydrazine (872 mg, 4.40 mmol) was added in one portion and the mixture was further stirred for 24 h at r.t. The solvent was evaporated and residue was dried in high vacuum. Chlorobenzene (5 mL) and triethyl orthoformate (1.8 mL, 11.0 mmol) were added and the mixture was heated to 130 °C for 24 h and then stored overnight in fridge. The precipitated crystals were filtered off and washed with toluene and dried in high vacuum to give the desired product as a white solid (1.40 g, 72%).
• 17 AROCM of 11 with In Situ Prepared Catalyst 8d: To a solution of bis(tricyclohexylphosphine)benzylidene ruthenium dichloride (8.2 mg, 0.01 mmol, 5 mol%) in anhyd THF (0.5 mL) silver complex 15 (6.2 mg, 0.007 mmol, 3.5 mol%) was added under argon and the reaction mixture was stirred at r.t. for 15 min. After that time the reaction mixture was transferred to a solution of 11 (32.8 mg, 0.2 mmol) and styrene (104.2 mg, 1.0 mmol) in anhyd THF (0.5 mL) and the formed mixture was stirred at 24 °C for 24 h under an argon atmosphere. The reaction was quenched by addition of ethyl vinyl ether and the reaction mixture was concentrated under vacuum. Conversion was determined by ¹H NMR while the ee value was determined by HPLC analysis using a chiral column (Daicel Chiralcel^® OJ).