.
- 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
Dimethyl (diazomethyl)phosphonate can be generated in situ from dimethyl-1-diazo-2-oxopropylphosphonate (also called the Ohira-Bestmann 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]Ohira–Bestmann reagent 
Names IUPAC name
dimethyl (1-diazo-2-oxopropyl)phosphonateIdentifiers 90965-06-3 ChemSpider 9281325 Jmol interactive 3D Image PubChem 11106189 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).Infobox references
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
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]Safe and scalable synthesis of alkynes from aldehydes
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.[7]
References
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
Can Tho, Vietnam
Can Tho
City in Vietnam
Can
Tho is a city in southern Vietnam’s Mekong Delta region. Set on the
southern bank of the Hau River, it’s known for its network of canals and
nearby floating markets. The modern, cable-stayed Can Tho Bridge spans
the river. The busy Ninh Kiều waterfront is a hub for boat trips on the
waterways surrounding the city.
Can Tho,
Can Tho, my country, is waiting for
us. Mom and her friends want to see where I
live and work. The speedboat being back in service,
we get to Can Tho in 4 hours.
The next day, boat trip on the arroyos
of course, with the traditional stop in the fruit
garden around a good table.
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Going around in the rice fields, after
lunch...
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And in Can Tho's market after going
back to town.
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| The street children shelter
Interests the Daltons a lot, but
that's not surprising : it's a wonderful place. Yen (right on the picture)
takes daily care of about 92 kids, of which about 50 stay there permanently.
Among them, about 20 orphans.
A roof (and a bed !), meals,
education (basic and professional), counseling : Yen and her staff are there !
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We (the Daltons) set up a budget, out of
which I regularly take the money Yen asks for (mostly for medications).
The Daltons at school
For the great pleasure of the teachers (two with
us on the picture), the principal, but mostly of the kids !
Those
were a lot shyer than usual though, having 4 French-speaking guests in the classrooms
being quite rare !
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And voilà...
The Daltons split up again, Averell staying in Can Tho (all holidays come to an end...). After the others left in a minibus to Saigon, I went back to work...
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