Radical Addition to (2,3-Epoxy-4-pentenyloxy)trialkylsilanes Yielding α,β-Unsaturated Aldehydes via Carbon-carbon Bond Cleavage

Abstract

Treatment of (2,3-epoxy-4-pentenyloxy)trialkylsilane with radical precursors such as triphenylgermane and α-halo carbonyl compounds in the presence of Et₃B yields α,β-unsaturated aldehydes. The reaction involves β-scission of a secondary alkoxy radical that releases a siloxymethyl radical.

Key words

radical reactions - α,β-unsaturated aldehydes - carbon-carbon bond cleavage - vinyloxirane - germanium compounds

References

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4

Experimental Procedure: Triphenylgermane (90 mg, 0.30 mmol) and t-butyldimethyl(2,3-epoxy-4-pentenyloxy)silane (1a, 0.10 g, 0.45 mmol) were placed in a 50 mL reaction flask. The mixture was set under argon with a toy ballon, and benzene (3 mL) was introduced. A hexane solution of triethylborane (1.0 M, 0.20 mL, 0.20 mmol) was added to the reaction flask with stirring. The whole mixture was heated at reflux for 3.5 h. TLC analysis indicated the completion of the reaction. (In the case of the other radical precursors, when the reaction did not finish, an additional triethylborane was added and stirring continued at reflux.) Evaporation of the solvent and following silica gel column purication with hexane/AcOEt = 10/1 as an eluent provided 4-triphenylgermyl-2-butenal (75 mg, 0.20 mmol, 65%). ( E )-4-Triphenylgermyl-2-butenal(4): IR(neat): 2924, 2855, 1682, 1462, 1377, 1092, 741, 700 cm^-1; ¹H NMR (CDCl₃): δ = 2.79 (d, J = 9.0 Hz, 2 H), 6.00 (dd, J = 15.3, 8.1 Hz, 1 H), 6.93 (dt, J = 15.0, 9.0 Hz, 1 H), 7.36-7.47 (m, 15 H), 9.30 (d, J = 8.4 Hz, 1 H); ¹³C NMR (CDCl₃): δ = 22.60, 128.58, 128.65, 129.65, 132.44, 134.80, 157.20, 193.43. Calcd for C₂₂H₂₀GeO: 374.0730. Found: 374.0735.

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Acid-induced olefination starting from β-alkoxy germane was reported. See ref. [5c] and ref. [5d] .