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Yield in a Horner Wadsworth Emmons reaction

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Babcock_Hall:
I wonder if the deprotection conditions for FMOC cause self condensation of an aldehyde.  This would be useful to know in that I was contemplating working with the FMOC-version of the same compound later on.

pgk:
1). In general, aldol type reactions need heat under basic catalysis.
2). Contrary, the fluorene ring of FMOC can be deprotonated by bases forming the fluorenyl anion that can react with aldehydes (even at room temperature, in some cases), forming the corresponding 9-fluorenyl carbinol, which may further be dehydrated.
Condensation of Aldehydes with Fluorene and Nirofluorenes, J. Org. Chem., (1951), 16(11), 1690-1696
https://pubs.acs.org/doi/abs/10.1021/jo50005a005

Babcock_Hall:

--- Quote from: pgk on June 20, 2019, 01:17:28 PM ---FmocN- is described to be incompatible with aldehydes but BocN- is described as compatible.
Anyway, the Horner-Wadsworth-Emmons reaction might have a higher yield by slow addition of diluted reagents (say 1 drop/sec) at -5oC (NaCl/ice), left to react at -5oC for 1 h, followed by reaction at 0oC for another 1 h and then at room temperature for 2 hours (or something else like that).


--- End quote ---
Thank you very much.  The protocol that I will follow first calls for drop-wise addition of the base to the phosphonate at 0 °C then stirring for 90 minutes, then drop-wise addition of the aldehyde, followed by stirring at zero, then room temperature.  This sounds similar to your suggestion.

If this is unsatisfactory in the presence of FMOC, I would consider trying different conditions for the Horner reaction, possibly similar to Blanchette M et al., (1984) Tetrahedron Letters 25(21) 2183-2186.  These authors use DBU or DIPEA as the base and lithium ions to chelate the intermediate and

rolnor:
I dont think its neccesary to add the base so slow and the acid-base reaction is very rapid.

pgk:
No!
The base must slowly be added at low temperature otherwise, uncontrolled exothermic (acid-base) reaction will occur that will favor side reactions, like self-condensation of diethyl β-sulfonylphosphonate.

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