Chapter 17, Problem 17.58P

Interpretation Introduction

Interpretation:

It is to be explained why the reaction of phosphonium ylide with ketone or aldehyde produces an alkene in comparison to the reaction of a sulfonium ylide that produces an epoxide.

Concept introduction:

In a Wittig reaction, the carbonyl ($\text{C=O}$) bond of aldehyde or ketone is converted into a ($\text{C = C}$) bond. In step one, the negatively charged C atom from the phosphonium ylide attacks the aldehyde or ketone and produces a betaine, that is, the species in which negative charge is present on O atom and a positive charge on P atom. In step two, a bond is formed between a positively charge P atom and negatively charged O atom. This results in an oxaphosphetane that contains a four-membered ring. This is followed by elimination of triphenylphosphineoxide producing the alkene.

In a sulfonium ylide reaction, the carbonyl ($\text{C=O}$) bond of aldehyde or ketone is converted into an epoxide group. In the first step, the negatively charged C atom from the sulfonium ylide attacks the aldehyde or ketone and forms a new species containing the negative charge present on O atom and a positive charge on S atom. The negatively charged O atom is a good nucleophile, and ${{\text{(CH}}_{\text{3}}\text{)}}_{\text{2}}\text{S}$ is a good leaving group. Finally, an intramolecular ${\text{S}}_{\text{N}}\text{2}$ reaction is carried out, and ${{\text{(CH}}_{\text{3}}\text{)}}_{\text{2}}\text{S}$ departs from it and produces an epoxide functional group.