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SolutionsWittig Reaction

1. Double Wittig

Predict the result of the following reactions:

Cyclohexane-1,4-dione with 2 equivalents of benzyltriphenylphosphonium bromide and 2 equivalents of potassium tert-butoxide in THF
The two stereoisomers of 1,4-dibenzylidenecyclohexane are formed: with the two phenyl groups on opposite sides of the ring and on the same side

tert-Butoxide deprotonates the phosphonium salt to give the ylide, which reacts with both carbonyls. Since each double bond can form with the phenyl pointing one way or the other, a mixture of isomers is obtained.

2. Choose the reagents

Propose two combinations of ylide and carbonyl compound to prepare 2-methylpent-2-ene.

  1. Acetone + Ph3P=CHCH2CH3 (ylide of propyltriphenylphosphonium bromide).
  2. Propanal + Ph3P=C(CH3)2 (ylide of isopropyltriphenylphosphonium bromide).

In practice the combination whose alkyl halide is primary is preferred, because the phosphonium salt is prepared by SN2 of triphenylphosphine on it.

3. Advantage over elimination

What advantage does the Wittig reaction have over dehydration of an alcohol for preparing methylenecyclohexane?

The Wittig reaction (cyclohexanone + Ph3P=CH2) places the double bond exactly where the carbonyl was. Dehydration of 1-methylcyclohexanol would give mainly 1-methylcyclohexene, which is more substituted and more stable (Saytzev rule).

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