This multistep synthesis enables the preparation of (E)-alkenes.
The addition of a phenylsulfonyl carbanion to an aldehyde or ketone leads to an intermediate alcohol, which is esterified in situ. The reductive elimination with sodium amalgam to furnish the alkene takes place in a second step.
The Julia-Kociensky Olefination is an alternative procedure, which leads to the olefin in one step.
Mechanism of the Julia Olefination
The acetoxysulfone synthesis produces diastereomers:
A first possible mechanism proceeds through a planar radical that can rotate freely about the C-C bond. Both diastereomers would thus pass through the same radical intermediate, which can be used to explain the (E)-selectivity.
Even though the carbanion is not configurationally or conformationally stable, it will prefer an arrangement with the R-groups further apart that will later lead to the (E)-alkene:
Keck demonstrated in 1995 that when the sodium amalgam reaction is run in MeOD as solvent, deuterium is incorporated into the product, in contrast to the absence of incorporation seen in the SmI2 reduction. Thus, the mechanism proposed above is completely consistent with the SmI2 reduction (M = SmI2).
The classical Julia Olefination with sodium amalgam might possibly proceed via an initial elimination to an alkenyl sulfone, which would then undergo homolytic cleavage involving single electron transfer.
Since the cis- and trans-vinyl radicals can equilibrate at this stage and the trans-radical is the more stable of the two, both diastereomeric acetoxy sulfones would still lead selectively to the same product.
A disadvantage of the Julia Olefination is its low tolerance for reducible functional groups. The (E)-selectivity is generally good to very good for alkenes with a low degree of substitution, while the selectivity improves as a function of increased branching in the substitutents.