2. Ethyl (triphenylphosphoranylidene)acetate is available from Fluka AG and Tridom Chemical Inc. under the name
(ethoxycarbonylmethylene)triphenylphosphorane and from Aldrich Chemical Company, Inc. under the name (carbethoxymethylene)triphenylphosphorane. The
reagent may be prepared from triphenylphosphine and ethyl bromoacetate by the following procedure.3
A 1-L, two-necked, round-bottomed flask fitted with a dropping funnel and a mechanical stirrer is charged with 131.0 g (0.5 mol) of triphenylphosphine
(Fluka AG, purum) and 250 mL of benzene (Merck, pro analysi). The solution is stirred vigorously while 83.5 g (0.5 mol) of ethyl bromoacetate (Fluka
AG, practical-grade) is added dropwise at a rate that maintains the reaction mixture at, or slightly above, room temperature. After a total of 2 hr
the reaction is complete and the colorless phosphonium salt is filtered. The salt is washed with 300 mL of cold benzene and 200 mL of pentane and then
dissolved in 3 L of water at room temperature. Some further organic impurities are removed by extraction with ether, after which 2 drops of 2%
alcoholic phenolphthalein are added. The aqueous solution is stirred vigorously and cooled in an ice bath as 2 M aqueous sodium hydroxide is added
slowly until the pink endpoint is reached (pH 8–10). The crystalline phosphorane is collected by filtration, washed thoroughly with cold water, and
dried, first with a rotary evaporator under reduced pressure at 60°C and then overnight in a drying oven at 180 mm and 70°C. The white to
cream-colored crop of ethyl (triphenylphosphoranylidene)acetate, mp 124–126°C, weighs 150–156 g (86–90%) and may be used for the preparation of
α-allenic esters without further purification. |