Seyferth–Gilbert homologation

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Seyferth–Gilbert homologation
Named after Dietmar Seyferth
John C. Gilbert
Reaction type Homologation reaction
Identifiers
Organic Chemistry Portal seyferth-gilbert-homologation
RSC ontology ID RXNO:0000387

The Seyferth–Gilbert homologation is a

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]

The Seyferth–Gilbert homologation
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.

The mechanism of the Seyferth–Gilbert homologation
The mechanism of the Seyferth–Gilbert homologation

Bestmann modification

Ohira–Bestmann reagent
Names
IUPAC name
dimethyl (1-diazo-2-oxopropyl)phosphonate
Identifiers
3D model (
JSmol
)
ChemSpider
UNII
  • InChI=1S/C5H9N2O4P/c1-4(8)5(7-6)12(9,10-2)11-3/h1-3H3
    Key: SQHSJJGGWYIFCD-UHFFFAOYSA-N
  • InChI=1/C5H9N2O4P/c1-4(8)5(7-6)12(9,10-2)11-3/h1-3H3
    Key: SQHSJJGGWYIFCD-UHFFFAOYAK
  • O=P(OC)(OC)C(C(C)=O)=[N+]=[N-]
Properties
C5H9N2O4P
Molar mass 192.11
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

The dimethyl (diazomethyl)phosphonate carbanion can be generated in situ from dimethyl-1-diazo-2-oxopropylphosphonate (also called the Ohira-Bestmann reagent) 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.[4][5]

Bestmann's reagent
Bestmann's reagent

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

Improved in situ generation of the Ohira-Bestmann reagent

Safe and scalable synthesis of alkynes from aldehydes

Recently a safer and more scalable approach has been developed for the synthesis of alkynes from aldehydes. This protocol takes advantage of a stable sulfonyl azide, rather than tosyl azide, for the in situ generation of the Ohira−Bestmann reagent.[6]

Other modifications

Another modification for less reactive aldehydes is made by replacement of potassium carbonate with caesium carbonate in MeOH and results in a drastic[quantify] yield increase.[7]

See also

References

  1. doi:10.1021/jo00809a014
    .
  2. doi:10.1021/jo00349a007
    .
  3. doi:10.1021/jo951944n
    .
  4. S2CID 196767504
    .
  5. S2CID 98558022
    .
  6. ^ Jepsen, T.H, Kristensen, J.L. J. Org. Chem. 2014, "In Situ Generation of the Ohira–Bestmann Reagent from Stable Sulfonyl Azide: Scalable Synthesis of Alkynes from Aldehydes". http://pubs.acs.org/doi/abs/10.1021/jo501803f
  7. doi:10.1055/s-2004-834872
    .
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