Derivatives via the Wittig Reaction

The reaction of cyclopropyl bromide with triphenylphosphine gives the expected phosphonium salt in less than 1 % yield. An alternate route to the salt by the thermal decomposition of 2-oxo-3-tetrahydrofuranyltriphenylphosphonium bromide gives a 

2 Oxo-3-tetrahydrofuranylt riphenylphosphomum Bromide (7) In 100 ml of dry tetrahydrofuran are dissolved 26.2 g (0.1 mole) of triphenylphosphine and 16.5 g (0.1 mole) of a-bromo-y-butyrolactone (5), and the solution is refluxed for 12 hours or longer. After cooling in an ice bath, the phosphonium salt is collected by filtration and dried. It may be purified by dissolving in hot methanol (1 ml/g of salt) and precipitating by the addition of cold ethyl acetate (2.5 ml/g of salt), mp 196-197°. 

Cyclopropyltriphenylphosphonium Bromide (5). The lactone salt is pyrolyzed by placing it in a round-bottom flask fitted with an adaptor attached to a vacuum source (aspirator is sufficient). The flask is heated (oil bath) to 180-190° for 48 hours. The residue is a virtual quantitative yield of the tan product, which may be crystallized from ethyl acetate giving cream crystals, mp 189-190°. An alternate setup is convenient if a drying pistol (Abderhalden) is available. The compound is placed in the pistol, which is then evacuated. Decalin (bp approx. 187°) is refluxed over the pistol to provide the heating source. The work-up is the same. 

The reaction is carried out in a manner similar to that described above (Chapter 11, Section II). In a 250-ml flask fitted with stirrer, condenser, and dropping funnel is placed a solution of 19.25 g (0.0505 mole) of the phosphonium salt in 180 ml of THF. The nitrogen atmosphere is established and 0.05 mole of phenyllithium added (as a solution in benzene, available from Foote Mineral Co.). The mixture is stirred for 45 minutes at room temperature and then refluxed for 15 minutes. To the red-brown solution is added dropwise over 20 minutes 4.91 g (0.05 mole) of distilled cyclohexanone stirring is continued for 24 hours. The mixture is then concentrated by distillation at 

Following the procedure given above, cyclopropylidenecyclopentane is prepared in 85% yield from 34.4 g (0.09 mole) of the phosphonium salt, 3.83 g (0.097 mole) of sodium amide (used instead of phenyllithium), and 8.4 g (0.1 mole) of cyclopentanone in ether as solvent (350 ml). The product has bp 69-70770 mm. 

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