The Wittig reaction

The Wittig reaction or Wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (often called a Wittig reagent) to give an alkene and triphenylphosphine oxide.

The Wittig reaction was discovered in 1954 by Georg Wittig, for which he was awarded the Nobel Prize in Chemistry in 1979. It is widely used in organic synthesis for the preparation of alkenes.

The reaction works for a wide variety of R groups, and with both aldehydes and ketones, and with simple alkyl or aryl groups it generally gives mainly the Z- alkene product, though if the R groups are similar it may give E/Z mixtures.  The driving force is the formation of a the highly stable triphenylphosphine oxide (Ph₃P=O).

The classical mechanism

This involves an initial nucleophilic addition step giving betaine 3 (=4), followed by a coordination step to form a four-membered ring 5 (called an oxephosphetane), which decomposes to the product 7 via a ring rearrangement reaction.