Wittig reaction
Abstract
The Wittig reaction converts a ketone (or aldehyde) into an alkene by reaction with a phosphorus ylide, with concomitant formation of triphenylphosphine oxide (Ph3P=O) as by-product. It is one of the most powerful and widely used methods for the stereoselective synthesis of alkenes with complete control over the position of the double bond.
Keywords
Aldehyde, Ketone, Phosphorus ylide, C-C bond formation, Alkene
Category
Aldehyde/Ketone
Mechanism
The ylide is prepared by nucleophilic substitution of triphenylphosphine on an alkyl halide, yielding a phosphonium salt. The phosphorus ylide is then obtained by deprotonation of the carbon atom in the α-position relative to the phosphorus.
The Wittig reaction consists of the reaction between this ylide and a ketone (or aldehyde). The nucleophilic ylide carbon attacks the electrophilic carbonyl carbon, while the nucleophilic carbonyl oxygen attacks the electrophilic phosphorus of the ylide ([2+2]-cycloaddition), forming a four-membered ring intermediate. This intermediate then undergoes syn-elimination ([2+2]-cycloreversion), with simultaneous breaking of the C–O and P–C bonds, releasing the alkene and triphenylphosphine oxide (Ph3P=O).
It is important to note that the stereochemical outcome depends on the nature of the ylide. Ylides stabilized by conjugation with an electron-withdrawing group (EWG) predominantly yield (E)-alkenes, whereas non-stabilized ylides bearing only alkyl substituents mainly produce (Z)-alkenes.
