The Julia olefination is stereoselective
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص686-687
2025-07-09
550
Here are the results of a few simple Julia olefi nations. Notice that deprotonations can be with BuLi or EtMgBr and that the acylation step works with acetic anhydride or with benzoyl chloride. As you can see, they are all highly stereoselective for the E isomer, and the Julia olefination is one of the most important ways of making E double bonds connectively.
The reason for the E selectivity lies in the mechanism of the elimination. The details are not fully clear, but the first step, under the basic conditions of the reduction, appears to be the elimination of the acetate or benzoate ester to give a vinyl sulfone.
The stereochemistry of the vinyl sulfone does not matter because it is immediately reduced by an electron from sodium to give a vinyl radical. Much as you saw above, in the Birch reduction of alkynes, the vinyl radical collects a second electron and becomes a vinyl anion, which chooses to adopt the more stable E configuration before being protonated to give the predomi-nantly E alkene.
We know that there must be an anion intermediate because the elimination is not stereo specific—in other words, whichever diastereoisomer of the starting material you use (all of the examples in this section have been mixtures of diastereoisomers) you always get the E alkene product.
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