When the substituents are very different in size ( t-butyl vs. Reaction with unsymmetrical ketones resulted in mixtures of E/ Z isomers (entry 14). Ketones were also successfully homologated (entries 12 to 15). Two-carbon homologation of aliphatic (Table 1, entries 1 to 3), aromatic (entries 4 and 8), heteroaromatic (entries 5 to 7), a, β-unsaturated (entries 9 and 10), propargylic aldehydes (entry 11), and dialdehydes (entries 16 and 17) was achieved with high isolated yields. The broad substrate scope of the reaction is presented in Table 1. Tris(ethoxyvinyl)borane has the following spectroscopic properties: 1H NMR pdf(500 MHz, THF-d 8. The tris(ethoxyvinyl)borane was about 90% pure after concentration based on NMR analysis. The 20% excess charge of ethoxyacetylene was reflected in its constant level in samples taken between 10 min and 1 h after addition of borane dimethyl sulfide. The reaction was complete within 10 min after addition of borane dimethyl sulfide. The conversion of ethoxyacetylene to tris(ethoxyvinyl)borane (Scheme 1) was followed by NMR by diluting a 0.05 mL reaction aliquot in 0.6 mL THF-d 8.ĭiagnostic signals for 1H NMR were δ 4.05 (q, 2H) for ethoxyacetylene and 3.84 (q, 6H) for tris(ethoxyvinyl)borane 13C NMR diagnostic signals were δ 74.9 and 91.5 for ethoxyacetylene and 65.3 and 163.6 for tris(ethoxyvinyl)borane. The crude product is purified by flash column chromatography (Note 10) and concentrated by rotary evaporation (40 ☌ bath temperature, 60 mmHg) to afford 7.7-8.2 g (71-75 % yield) as a pale yellow oil (Notes (Note 11) and (Note 12)). The volatile materials are removed by rotary evaporation (40 ☌ bath temperature, 60 mmHg) to afford 12 g of a brown oil. The combined organic layer is washed with saturated aqueous sodium bicarbonate (200 mL) and brine (50 mL), then dried by filtration through 50 g sodium sulfate. The aqueous layer is separated and back-extracted with MTBE (100 mL). The quenched reaction mixture is transferred to a 1-L separatory funnel. The reaction mixture is stirred for 20 min until the white precipitate of zinc oxide dissolves (Note 9). The ice bath is removed and aqueous 2M HCl (200 mL) is added over 5 min via the addition funnel. Gas (ethane) is evolved and a white precipitate of zinc oxide is formed. Vigorous stirring of the reaction mixture is commenced and water (100 mL) is carefully added over 5 min, keeping the temperature below 20 ☌. The nitrogen gas adapter is replaced with a 300-mL addition funnel. The reaction is then cooled to 5 ☌ in an ice-bath and t-butyl methyl ether (100 mL) is added. The mixture is allowed to warm to 20 ☌ over 1 h and stirred for 3 h (Note 8). 75 ☌ in a dry ice/acetone bath and 1.1M diethylzinc in toluene (96 mL, 105 mmol, 1.5 equiv) (Note 6) is added by cannula over 10 min, keeping the temperature below -65 ☌.Īfter the solution is stirred for 30 min between -70 and -75 ☌, hydrocinnamaldehyde (9.1 g, 68 mmol, 1.0 equiv) (Note 7) is added dropwise via a 20 mL syringe over 2 min. The volatile materials are removed at reduced pressure (60 mmHg) by applying gentle heating to the reaction flask (Note 4).Īfter removal of volatile materials, the reaction flask is back-filled with nitrogen and the brown oil is dissolved by addition of toluene (65 mL) (Note 5). The nitrogen adapter is replaced with a vacuum-distillation head connected to a 500-mL round bottom flask immersed in a dry-ice bath. The reaction mixture is stirred for 1 h at 2 ☌ (Note 3), then the ice-bath is replaced with a heating mantle. With stirring, 2M borane dimethyl sulfide complex in THF (17.5 mL, 35 mmol, 0.5 equiv) is added dropwise via a 30 mL syringe over 30 min, keeping the temperature below 7 ☌. The flask is charged with a 50 wt % solution of ethoxyacetylene in hexanes (22 mL, 16.8 g, 0.12 mol, 1.8 equiv) (Note 2) and THF (70 mL), then cooled to 2 ☌ in an ice-bath. An oven-dried, 1-L, 3-necked round-bottomed flask containing a 3-cm oval PTFE-coated magnetic stir bar is equipped with a nitrogen inlet adapter and two rubber septa, one of which is pierced with a thermocouple probe (Note 1).
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