Seyferth–Gilbert homologation
The Seyferth–Gilbert homologation is a chemical reaction of an aryl ketone 1 with dimethyl phosphonate 2 and potassium tert-butoxide to give substituted alkynes 3. Dimethyl phosphonate 2 is often called the Seyferth–Gilbert reagent.
Image:Seyferth-Gilbert Homologation [Scheme.png|center|300px|The Seyferth–Gilbert homologation]
This reaction is called a homologation because the product has exactly one additional carbon more than the starting material.
Reaction mechanism
Deprotonation of the Seyferth–Gilbert reagent A gives an anion B, which reacts with the ketone to form the oxaphosphetane D. Elimination of dimethylphosphate E gives the vinyl diazo-intermediate Fa and Fb. The generation of nitrogen gas gives a vinyl carbene G, which via a 1,2-migration forms the desired alkyne H.[Image:Seyferth-Gilbert Homologation Mechanism.png|center|600px|The mechanism of the Seyferth–Gilbert homologation]
Bestmann modification
The dimethyl phosphonate carbanion can be generated in situ from dimethyl-1-diazo-2-oxopropylphosphonate by reaction with methanol and potassium carbonate as the base by cleavage of the acetyl group as methyl acetate. Reaction of Bestmann's reagent with aldehydes gives terminal alkynes often in very high yield and fewer steps than the Corey–Fuchs reaction.[Image:Bestmann's Reagent Scheme.png|center|300px|Bestmann's reagent]
The use of the milder potassium carbonate makes this procedure much more compatible with a wide variety of functional groups.
Traditional syntheses of the Ohira−Bestmann reagent use the rather explosive tosyl azide, but better diazo transfer agents are available.