Wittig reaction…

basically illustrates the transformation of an aldehyde or ketone to an alkene. You start with building the triphenylphosphonium ylide (Wittig reagent). This can simply be done through the reaction of an alkyl halide with triphenylphosphine and a strong base like nBuLi, done in dry THF of ether. Once you have formed the phosphonium ylide, you quickly throw in the aldehyde or ketone and boom (no boom goes on in the reaction), you get the alkene.

Key point to take from this reaction: you form cis alkenes, as you have no stabilizing component built into the formed ylide. Oh, and another thing: esters cannot be used in the Wittig reaction, probably because they are less reactive than aldehydes or ketones. For the case of converting esters to alkenes, it’s best to perform the Tebbe-Petasis olefination. If you want to form trans alkenes, you could either do a Horner-Wadsworth-Emmons reaction or Julia olefination, or even the Julia-Kociensky olefination, which is a modification of the Julia olefination to avoid the use of a sodium-mercury amalgam.

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