21. Aldehydes and Ketones: Nucleophilic Addition

Determine the carbonyl and ylide that formed the following product.

Trimethylphosphine is a stronger nucleophile than triphenylphosphine, but it is rarely used to make ylides. Why is trimethylphosphine unsuitable for making most phosphorus ylides?

Propose mechanisms for the following reactions.

(c)

The Wittig reaction is useful for placing double bonds in less stable positions. For example, the following transformation is easily accomplished using a Wittig reaction.

(a) Show how you would use a Wittig reaction to do this.

(b) Show how you might do this without using a Wittig reaction, and then explain why the Wittig reaction is a much better synthesis.

Like other strong nucleophiles, triphenylphosphine attacks and opens epoxides. The initial product (a betaine) quickly cyclizes to an oxaphosphetane that collapses to an alkene and triphenylphosphine oxide.

(a) Show each step in the reaction of trans-2,3-epoxybutane with triphenylphosphine to give but-2-ene. What is the stereochemistry of the double bond in the product?

Like other strong nucleophiles, triphenylphosphine attacks and opens epoxides. The initial product (a betaine) quickly cyclizes to an oxaphosphetane that collapses to an alkene and triphenylphosphine oxide.

(b) Show how this sequence might be used to convert cis-cyclooctene to trans-cyclooctene.

Show how you would accomplish the following syntheses efficiently and in good yield. You may use any necessary reagents.

(e)

Show how you would accomplish the following synthetic conversions efficiently and in good yield. You may use any necessary additional reagents and solvents.

(h)

What two sets of reagents (each consisting of a carbonyl compound and phosphonium ylide) can be used for the synthesis of the following alkene?

1.

Suggest the appropriate carbonyl and Wittig reagent to make the following alkenes.

a. (E)-7-methylnon-4-en-3-one