2,3‐Dibromo‐1‐(phenylsulfonyl)‐1‐propene

[128496-94-6] C9H8Br2O2S (MW 340.03) InChI = 1S/C9H8Br2O2S/c10-6-8(11)7-14(12,13)9-4-2-1-3-5-9/h1-5,7H,6H2/b8-7+ InChIKey = NMJCDFKJASKHFY-BQYQJAHWSA-N (furan synthesis; cyclopentenone synthesis; conjugate addition; multifunctional sulfone reagent) Physical Data: white crystalline solid; mp 62–63 °C. Solubility: sol CH2Cl2, THF, acetonitrile, toluene. Preparative Methods: two general methods of preparation are available (eq 1), both proceeding through the intermediacy of the well-known phenylsulfonylallene.1-3 In one approach (Path A), propargyl alcohol is treated with phenylsulfenyl chloride to give a thermally unstable thioperoxide intermediate, which spontaneously undergoes a thermal [2,3] rearrangement to the more stable phenylsufinyl allene; oxidation with hydrogen peroxide in acetic acid at 95 °C provides phenylsulfonyl allene in 68 % yield, from which 2,3-dibromo-1-phenylsulfonylpropene (DBP) is prepared by bromination in acetic acid in 87 % yield.4, 5 Another more operationally straightforward method (Path B) is available through the reaction of sodium phenolthiolate with propargyl chloride, oxidation with hydrogen peroxide, alumina-catalyzed isomerization, and bromination according to the following method. To a stirred solution of thiophenol (97 %, 60.0 mL, 0.568 mol, 1.00 equiv) in methanol (600 mL, ACS grade) under nitrogen is added dropwise over 5 min at 0 °C sodium hydroxide (10 N, 57.0 mL, 0.570 mol, 1.00 equiv), followed by propargyl chloride (98 %, 43.2 mL, 0.584 mol, 1.03 equiv). The mixture is allowed to warm to room temperature and stir for 3 h. The reaction mixture is acidified with hydrochloric acid (37.2 %, 1.2 mL, 17 mmol, 0.03 equiv) and treated with titanium(III) chloride (20 %, 7.2 mL, 11 mmol, 0.02 equiv). Hydrogen peroxide (33 %, 232 mL, 2.27 mol, 4.00 equiv) is added over 20 min, allowing the temperature to rise to 65 °C, cooling with an ice bath as necessary. After addition is complete, the mixture is heated to reflux for 1 h. While still hot, the mixture is diluted with water (600 mL) and allowed to cool to room temperature and stand overnight, during which time white crystals of phenyl propargyl sulfone precipitate. The slurry is cooled to 0 °C to further facilitate crystallization, isolated on a Buchner funnel (Whatman No. 2 paper), washed with water, and dried, to yield 93.8 g (92 % of theory) of phenyl propargyl sulfone as a white crystalline solid. A solution of phenyl propargyl sulfone (20.0 g, 111 mmol, 1.00 equiv) in methylene chloride (300 mL) is treated with neutral alumina, (Brockman activity I, 20.0 g) and allowed to stir for 4 h at room temperature, and then treated with bromine (6.0 mL, 18.6 g, 116 mmol, 1.05 equiv) while irradiating with a 23 W compact fluorescent light. Stirring is continued for 20 min at room temperature, after which the alumina is removed by filtration. The filtrate is washed with a dilute sodium thiosulfate solution to remove excess bromine, dried over sodium sulfate, and concentrated to give a light-yellow oil containing ca. 35 g (93 %) of DBP as a 4:1 mixture of E/Z isomers, which is typically used without further purification. However, pure E-DBP is easily obtained by flash chromatography or crystallization of this crude mixture.6 (1) Purification: chromatography on silica gel eluting with 25 % ethyl acetate in hexane; recrystallization from ether. Handling, Storage, and Precautions: neither air- nor moisture-sensitive; in a tightly closed bottle under nitrogen, shelf-stable for several years. Toxicity has not been determined.