Synthesis of Polysubstituted Enynes through Iron-Catalyzed Carbomagnesiation of Conjugated Diynes

Abstract

Symmetrical and unsymmetrical conjugated diynes are chemo-, regio-, and stereoselectively carbomagnesiated with a ­Grignard reagent at room temperature in the presence of a catalytic amount of FeCl₂ without the need for an external ligand. The resulting magnesium intermediate can be further functionalized to give polysubstituted 1,3-enyne derivatives.

Key words

iron - catalysis - enynes - organometallic reagents - addition reactions

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• 14 Typical Procedure (Scheme 1): In an oven-dried Schlenk tube, FeCl₂ (5.0 mg, 0.040 mmol) was added to a solution of 1,4-bis(trimethylsilyl)-1,3-butadiyne (1; 78 mg, 0.40 mmol) in Et₂O (1.6 mL). After 15 min, a solution of phenyl-magnesium bromide in Et₂O (0.35 mL, 1.46 mol/L, 0.48 mmol) was added to the reaction mixture at r.t., and the resulting mixture was stirred at r.t. for an additional 3 h, then H₂O (3.0 mL) was added. The organic phase was extracted with EtOAc (3 × 5 mL), then the mixture was passed over a pad of Florisil. The volatiles were removed in vacuo to obtain an oily brown residue. The crude mixture was purified by column chromatography (hexane 100%) to afford the desired compound (74 mg, 68%) as a colorless oil. GC analysis and ¹H NMR analysis of the crude product indicated that only one isomer was obtained. (Z)-1-Phenyl-1,4-bis(trimethylsilyl)but-1-en-3-yne (2): ¹H NMR (500 MHz, CDCl₃): δ = 7.29–7.21 (m, 3 H), 7.05–7.03 (m, 2 H), 6.14 (s, 1 H), 0.25–0.22 (m, 18 H); ¹³C NMR (125 MHz, CDCl₃): δ = 159.8, 145.1, 128.0, 126.7, 126.4, 122.0, 104.9, 100.5, –0.3, –0.4. GC MS (EI): m/z (%) = 272 (27), 257 (52), 199 (17), 155 (16), 73 (100).