Question

Most wittig reactions are not stereoselective and yield a mixture of cis and trans products. can you think of a reason why this particular witting shows such a high degree of stereospecificity. which product will result?

Answer 1

The Wittig reaction is one of the most common techniques used for the stereoselective preparation of alkenes. , the reaction allows for the formation of an alkene product and a triphenylphosphine oxide side product from the reaction of an aldehyde or ketone and a “Wittig reagent” (a triphenylphosphonium ylide).

The key step of the mechanism of the ylide reaction is the nucleophilic addition of the ylide to the electrophilic carbonyl group, forming a 4-membered ring that dissociates into the product molecules. The stereoselectivity of the reaction is predicated on the stability of triphenylphosphonium ylide, which determines which of two ring intermediates form: the sterically-unfavored cis intermediate that forms via a fast yet reversible process, or the slow, irreversible trans intermediate.

We can thus classify three distinct levels of stabilization of the ylide by its residue: neutral substituents that do not stabilize negative charge (like alkyl groups or hydrogens) produce an unstabilized ylide that preferentially forms cis product, whereas electron-withdrawing residues (like nitro or ester groups) stabilize the charge and preferentially form trans product, while “semi-stabilized” substituents (generally aromatic rings) produce a mixture of the two.