Seyferth–Gilbert homologation

The Seyferth–Gilbert homologation is a the 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 oxaphosphatane 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
IUPAC name dimethyl (1-diazo-2-oxopropyl)phosphonate
Identifiers
CAS number	90965-06-3
Jmol-3D images	Image 1
SMILES O=P(OC)(OC)C(C(C)=O)=[N+]=[N-]
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

1. ^ D. Seyferth, R. S. Marmor and P. Hilbert (1971). "Reactions of dimethylphosphono-substituted diazoalkanes. (MeO)2P(O)CR transfer to olefins and 1,3-dipolar additions of (MeO)2P(O)C(N2)R". J. Org. Chem. 36 (10): 1379–1386. doi:10.1021/jo00809a014. 
2. ^ J. C. Gilbert and U. Weerasooriya (1982). "Diazoethenes: their attempted synthesis from aldehydes and aromatic ketones by way of the Horner-Emmons modification of the Wittig reaction. A facile synthesis of alkynes". J. Org. Chem. 47 (10): 1837–1845. doi:10.1021/jo00349a007. 
3. ^ D. G. Brown, E. J. Velthuisen, J. R. Commerford, R. G. Brisbois and T. H. Hoye (1996). "A Convenient Synthesis of Dimethyl (Diazomethyl)phosphonate (Seyferth/Gilbert Reagent)". J. Org. Chem. 61 (7): 2540–2541. doi:10.1021/jo951944n. 
4. ^ S. Müller, B. Liepold, G. Roth and H. J. Bestmann* (1996). "An Improved One-pot Procedure for the Synthesis of Alkynes from Aldehydes". Synlett 1996 (06): 521–522. doi:10.1055/s-1996-5474. 
5. ^ G. Roth, B. Liepold, S. Müller and H. J. Bestmann (2004). "Further Improvements of the Synthesis of Alkynes from Aldehydes". Synthesis 2004 (1): 59–62. doi:10.1055/s-2003-44346. 
6. ^ Lidija Bondarenko, Ina Dix, Heino Hinrichs, Henning Hopf* (2004). "Cyclophanes. Part LII:1 Ethynyl[2.2]paracyclophanes – New Building Blocks for Molecular Scaffolding". Synthesis 2004 (16): 2751–2759. doi:10.1055/s-2004-834872. 

See also

• Corey-Fuchs reaction
• Horner-Wadsworth-Emmons reaction
• Wittig reaction