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Ohira–Bestmann reaction

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Ohira–Bestmann reaction

The Seyferth–Gilbert homologation is a chemical reaction of an aryl ketone 1 (or aldehyde) with dimethyl (diazomethyl)phosphonate 2 and potassium tert-butoxide to give substituted alkynes 3.[1][2] Dimethyl (diazomethyl)phosphonate 2 is often called the Seyferth–Gilbert reagent.[3]

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.

Bestmann modification

Ohira–Bestmann reagent
Identifiers
CAS number 90965-06-3
Jmol-3D images Image 1
Properties
Molecular formula C5H9N2O4P
Molar mass 192.11
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Dimethyl (diazomethyl)phosphonate can be generated in situ from dimethyl-1-diazo-2-oxopropylphosphonate (also called Bestmann's reagent) by reaction with methanol and potassium carbonate. Reaction of Bestmann's reagent with aldehydes gives terminal alkynes often in very high yield.[4][5]

The use of the milder potassium carbonate makes this procedure much more compatible with a wide variety of functional groups.

Another modification for less reactive aldehydes is made by replacement of potassium carbonate with caesium carbonate in MeOH and results in a drastic yield increase.[6]

References

See also

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