Question

What carbonyl compound and what phosphorus ylide might you use to prepare each of the following compounds?

(a)

(b)

(c)

(d)

(e)

(f)

Short answer

(a)

Formation of ethylidene cyclohexane

(b)

Formation of vinyl cyclohexane

(c)

Formation of 2-methyl-hex-2-ene

(d)

Formation of desired product

(e)

Formation of desired product

(f)

Formation of 3-methylene cyclohexene

Step-by-step solution

Carbonyl compounds

Carbonyl compounds on treatment with a phosphorus ylide gives an alkene in which the oxygen atom of the carbonyl group reactant is replaced by the of the ylide. Preparation of the phosphorus ylide itself involves the reaction of a primary alkyl halide with the triphenylphosphine in manner. So, the ylide part is obtained from the reaction between primary alkyl halides and triphenylphosphine. This means di-substituted carbon on the alkene double bond is from ketone and monosubstituted carbon is from the ylide in the witting reaction.

Reaction with Witting reagents

Formation of the following compounds

(a)

When cyclohexanone reacts with a ethylidene phenyl phosphane, ethylidene cyclohexane is obtained which is represented as follows:

Formation of ethylidene cyclohexane

(b)

Cyclohexane carbaldehyde when treated with the phosphorous ylide, vinyl cyclohexane is obtained.

Formation of vinyl cyclohexane

(c)

Propanone reacts with the phosphorus ylide, 2-methyl-hex-2-ene is formed

Formation of 2-methyl-hex-2-ene

(d)

Propanone reacts with the phosphorous ylide, (2-methyl-propenyl)-benzene is formed.

Formation of desired product

(e)

Benzaldehyde reacts with phosphorous ylide to form the desired product.

Formation of desired product

(f)

When cyclohexanone reacts with the phosphorous ylide 3-methylene cyclohexene is obtained.

Formation of 3-methylene cyclohexene