retrosynthesis with the Wittig reaction

Performing retrosynthesis on an alkene with the Wittig reaction gives two possible routes for assembly the alkene.  Each route involves the reaction of a simple carbonyl compound with a phosphonium ylide.  The key difference, if any, between the two routes is normally in the preparation of the phosphonium ylide precursor, a phosphonium salt.  Phosphonium salts that are formed from the reaction of a primary halide are much easier to prepare than salts from secondary halides.  Triphenylphosphine is often not a good enough nucleophile to react well with secondary halides in an SN2 reaction.  In deciding between two Wittig pathways, one should favor the pathway that begins with a primary halide for making the phosphonium salt.

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