Wittig Reaction

The Wittig reaction is a unique reaction that can be carried out in various ways as our prompt exemplifies. We will be primary focusing on the second of the three schemes. In The usual formation of an ylide occurs via Sn2 attack of a PR3 group on an alkyl halide, which is then followed by a strong base deprotonating of one of the hydrogens on the methyl which the PR3 attacked. Before we dive into the details of the reaction occurring in scheme 2 it will be important to understand the mechanism which I will provide below:

The reason that no base is needed in scheme 2 is due to the two ester groups present in Maleate. Upon the initial attack of PBu3 we see that electrons are pushed onto what was originally one of the double bonded oxygen’s. In the next step, the electrons on the oxygen can be pushed down via resonance which causes the carbon-carbon double bond that was formed to intramolecularly grab the hydrogen that was on the same carbon as the PBu3. The electrons of this hydrogen then aid in forming the enolate. The ability of the carbon-carbon double bond to deprotonate the hydrogen is why this scheme does not require a base like the usual ylide reaction.
The reason it is able to react with the Maleate alkene is due to the presence of the ester groups. these groups are allowing the maleate to resonate and pull the electrons from the alkene towards the esters creating partial positive charges on the carbons of the carbon-carbon double bonds formed next to the carbon-oxygen bonds. Pbu3 can attack these partial positive charges and thereby form a stable