An Easy Stereoselective Copper(I)-Catalyzed Synthesis of (E)-3-Trifluoromethylbut-2-en-3-ynoate via Palladium-Free Stille Coupling Reaction

Abstract

A general and efficient Cu(I)-catalyzed cross-coupling reaction of alkynyl bromides and β-tributylstannyl-α,β-unsaturated ester bearing a trifluoromethyl group in β-position was developed under very mild conditions. This method provides easy access to a variety of 2,3-enynoate bearing a trifluoromethyl group from good to excellent yields with excellent stereoselectivity. This procedure does not require the use of any expensive supplementary additives and is palladium-free.

Key words

β-tributylstannyl-α,β-unsaturated ester - palladium-free - copper-catalyst - trifluoromethylbut-2-en-3-ynoate

References and Notes

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21

Typical Experimental Procedure for the Preparation of 2a-l: Synthesis of ( E )-Ethyl 3-(Trifluoromethyl)undec-2-en-4-ynoate (2a) Compound 1β (200 mg, 0.437 mmol), 1-bromooct-1-yne (83 mg,0.437 mmol), and dry DMF (5 mL) were introduced into a dry Schlenk flask under argon. The mixture was degassed under agitation (10 min), then CuI (8 mg, 0.04 mmol) was introduced under argon flux. The mixture was brought to r.t. and left for 5 h under stirring. The reaction mixture was diluted with Et₂O, washed with aq KF solution (1 M, 10 mL) and the ether layer was separated, dried over MgSO₄, concentrated, and separated on a silica gel column (pentane-Et₂O = 95:5) to provide 100 mg (83%) of enyne 2a as a colorless liquid. ^¹H NMR (300 MHz, CDCl₃): δ = 0.91 (t, J = 7.2 Hz, 3 H), 1.22-1.66 (m, 11 H), 2.48 (d, J = 7.1 Hz, 2 H), 4.26 (q, J = 7.2 Hz, 2 H), 6.54 (s, 1 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 13.6, 14.1, 20.0, 22.4, 27.9, 28.4, 31.2, 61.1, 72.3, 106.8, 120.9 (q, J _C-F = 273.4 Hz), 126.4 (q, J _C-F = 34.6 Hz), 127.1 (q, J _C-F = 4.4 Hz), 163.5. ^¹9F NMR (282 MHz, CDCl₃): δ = -67.9. IR (ATR): ν = 2955, 2926, 2856, 2219 1734, 1634 cm^-¹.