Alkyl Ylides Requiring n-Butyl Lithium

The quaternary salt is now treated with ethoxide ion in the presence of cinnamalde-hyde so that the ylide reacts in situ as it is produced. A solution of sodium ethoxide is prepared by slowly adding 0.75 g of sodium metal to 100 ml of absolute ethanol in a dry Erlenmeyer flask (hood). In a second flask, the phosphonium chloride is dissolved in 150 ml of absolute ethanol, cinnamaldehyde (2.9 g) is added and the flask is swirled while the ethoxide solution is added a transient orange-red color indicates the formation of the ylide. 

The reaction mixture is allowed to stand at room temperature for 30 minutes, during which time crystals of the product form. The product is collected by filtration, washed with a little cold ethanol, and recrystallized from ethanol or methylcyclohexane. The faintly yellow product has mp 150-151°. 

Alkyltriphenylphosphonium salts normally require extremely strong bases for the generation of the ylide. In this experiment, the alkyl group is methyl and the base required to generate the ylide is -butyl lithium. 

Triphenylmethylphosphonium bromide A pressure bottle is charged with a solution of 55 g (0.21 mole) of triphenylphosphine in 45 ml of dry benzene and cooled in an ice-salt bath. A commercially available ampoule of methyl bromide is cooled below 0° (ice-salt bath), opened, and 28 g (0.29 mole, approx. 16.2 ml) is added to the bottle in one portion. The pressure bottle is tightly stoppered, brought to room temperature, and allowed to stand for 2 days. After this time, the bottle is opened and the product is collected by suction filtration, the transfer being effected with hot benzene as needed. The yield of triphenylphosphonium bromide is about 74 g (99%), mp 232-233°. This material should be thoroughly dried (vacuum oven at 100°) before use in preparing the ylide. 

Methylenecyclohexane A 500-ml, three-necked, round-bottom flask is fitted with a mechanical stirrer, a reflux condenser leading to a mercury filled U-tube, a pressureequalizing dropping funnel, and a gas inlet tube. The system is flushed with nitrogen 

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