b-Hydroxyethyltriphenylarsonium Bromide

[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.

1. Shi, L. L.; Wang, W. B.; Huang, Y. Z. TL 1988, 29, 5295.
2. Manske, R.; Gosselck, J. T 1975, 31, 2121.
3. (a) Still, W. C.; Novack, V. J. JACS 1981, 103, 1283. (b) Ousset, J. B.; Mioskowski, C.; Solladie, G. TL 1983, 24, 4419.
4. Wang, W. B.; Shi, L. L.; Li, Z. Q.; Huang, Y. Z. TL 1991, 32, 3999.
5. Melot, J. M.; Texier-Boullet, F.; Foucand, A. S 1987, 364.

Yao-Zeng Huang, Li-Lan Shi & Zhang-Lin Zhou

Shanghai Institute of Organic Chemistry, China

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