[57813-28-2] · C20H20AsBrO · b-Hydroxyethyltriphenylarsonium Bromide · (MW 431.22)
(epoxidation reagent for aromatic aldehydes with solid KOH under phase-transfer conditions)
Physical Data: mp 199 °C.
Preparative Methods: prepared from triphenylarsine and bromoethanol without solvent at 110 °C for 4 h in 69% yield;1 mp and 1H NMR spectrum coincide with those reported by another method2 in which the reaction was carried out in an autoclave.
Handling, Storage, and Precautions: the toxicity of this reagent is unknown at present. Use in a fume hood.
2,3-Epoxy alcohols can be directly synthesized from aromatic aldehydes at ambient temperature in good yields by treatment with b-hydroxyethyltriphenylarsonium bromide using solid Potassium Hydroxide as a base (eq 1).
Weak bases such as triethylamine, piperidine, and K2CO3 do not promote this reaction; strong bases such as KN(SiMe3)2 and BuLi, usually employed to generate nonstabilized arsonium ylides,3 are not suitable for this reaction. Using solid-liquid phase-transfer process (KOH (s)/THF (trace H2O) system), the 2,3-epoxy alcohol products are obtained in good yields. The (E):(Z) ratio in each case is around 9:1 with the (E)-isomer predominating.
The general procedure is as follows. Arsonium salt (1) (1.2 mmol), aldehyde (1 mmol), and KOH (s) (1.2 mmol) are mixed in 4 mL of THF (trace H2O) under N2 and stirred at 25 °C. After the reaction is complete, the reaction mixture is filtered. The products (2) (eq 1) are obtained after flash chromatography.
The related compound, (E)-4-hydroxy-2-butenyltriphenylarsonium halide (3), is a precursor of a good ylide epoxidation reagent for aromatic aldehydes. This reagent is prepared (75% yield) by treatment of Ph3As with (E)-4-chloro-2-buten-1-ol and a catalytic amount of Potassium Iodide in MeCN. (E)-4-Chloro-2-buten-1-ol, in turn, is prepared via Lithium Aluminum Hydride reduction of butyne-1,4-diol with subsequent treatment with Thionyl Chloride (eq 2).4
Aromatic aldehydes react with arsonium salt (3) in MeCN (trace H2O) at ambient temperature in the presence of KF.Al2O35 for 12-35 h to give exclusively (E)-5-aryl-4,5-trans-epoxy-2-penten-1-ols in moderate to good yields (53-78%) (eq 3). When cinnamaldehyde is used as substrate, (E)-5-(2-styrenyl)-4,5-trans-epoxy-2-penten-1-ol is obtained in 70% yield.4 The formation of highly stereoselective trans-epoxide may be explained by the mechanism shown in eq 4.
The following is a typical procedure. A mixture of arsonium salt (3) (310mg), PhCHO (0.5 mmol), and KF.Al2O3 (120 mg) in MeCN (2 mL, trace H2O) is stirred at ambient temperature under N2 for 28 h (monitored by TLC). The reaction mixture is filtered and washed with acetone. The filtrate is evaporated under reduced pressure and the product is purified by flash chromatography.
Yao-Zeng Huang, Li-Lan Shi & Zhang-Lin Zhou
Shanghai Institute of Organic Chemistry, China