Fe(BF₄)₂-Catalyzed Inter- and Intramolecular Carbonyl-Ene Reaction of Trifluoropyruvate

Abstract

Inter- and intramolecular carbonyl-ene reactions have been developed using 5 mol% Fe(BF₄)₂ as catalyst, affording homoallylic alcohols in 36–87% isolated yields. This catalyst, prepared from FeCl₂ and AgBF₄, is the first Fe^II Lewis acid reported for the carbonyl-ene reaction using ethyl trifluoropyruvate. The method was successfully applied to the reaction of various 1,1-disubstituted alkenes with ethyl trifluoropyruvate and to the cyclization of citronellal.

Key words

ene reaction - Lewis acids - iron - trifluoropyruvate - cyclization

• References and Notes


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Isolated yields are given for 5 and 6, ¹H NMR spectra are in agreement with those reported by:
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• 29 General Procedure for the Carbonyl-Ene Reaction of Alkenes with Ethyl 3,3,3-Trifluoropyruvate FeCl₂ (1.7 mg, 0.0125 mmol) and AgBF₄ (5 mg, 0.025 mmol) were added into a flame-dried test tube. The test tube was placed under high vacuum and purged with argon three times. Afterwards, distilled THF (1 mL) was added, and the resulting solution was stirred for 0.3 h, then filtered under argon atmosphere through a pre-dried pipet plugged with cotton and Celite^® to remove the precipitated AgCl. To the resulting solution was added 4 Å MS (50 mg), and the solution was then evaporated under reduced pressure. The test tube was then placed under high vacuum (1 Torr) for 0.3 h. Under an argon atmosphere freshly distilled CH₂Cl₂ (0.5 mL) was then added, and the solution stirred for 0.1 h. Then, α-methyl styrene (30 mg, 0.25 mmol) and ethyl 3,3,3-trifluoropyruvate (64 mg, 0.375 mmol) were added via syringe into the solution. The solution was then stirred for 48 h under an argon atmosphere. Then, the crude reaction mixture was directly purified by silica flash chromatography (eluent: hexane/ethyl acetate) to give pure carbonyl-ene product 3a (63 mg, 87%). Ethyl 2-Hydroxy-4-phenyl-2-(trifluoromethyl)pent-4-enoate (3a) ^{30 1}H NMR (400 MHz, CDCl₃): δ = 7.35–7.26 (m, 5 H), 5.39 (d, J = 1.3 Hz, 1 H), 5.28 (d, J = 1.3 Hz, 1 H), 4.03 (dq, J = 10.6, 7.2 Hz, 1 H), 3.76 (d, J = 0.9 Hz, 1 H), 3.64 (dq, J = 10.6, 7.2 Hz, 1 H), 3.28 (d, J = 14.0 Hz, 1 H), 3.04 (d, J = 14.0, 1.0 Hz, 1 H), 1.11 (t, J = 7.2 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 168.9, 141.0, 128.1, 127.9, 126.8, 123.4 (q, J = 286.2 Hz), 119.4, 77.09 (q, J = 28.9 Hz), 63.5, 37.0, 13.5. IR (NaCl): 3491, 2985, 1741, 1629, 1446, 1370, 1312, 1227, 1184, 1136, 1050, 911, 778, 701 cm^–1.
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