Science of Synthesis

Schobert, R.; Hölzel, C.; Barnickel, B., Science of Synthesis, (2010) 47, 28.

The Wittig synthesis of Z-1,2-disubstituted alkenes normally employs solutions of phosphorus ylides, prepared in situ from the corresponding phosphonium salts in aprotic solvents. The stable aldehyde is usually added in one portion to the ylide solution at room temperature or below, and the mixture is then stirred at room temperature until completion of the reaction. However, procedures are also known in which the starting phosphonium salts, and thus the resulting phosphorus ylides, are immobilized by attachment to an inert polymer resin in order to facilitate workup and removal of byproducts. The application of phase-transfer conditions has also been found advantageous in certain cases. Aside from its stereoselectivity and its flexibility in terms of procedural variants, synthetic variability, and generation, as well as application of precursors, the Wittig alkenation is also quite chemoselective. It tolerates a broad variety of functional and protective groups, such as the often-used silyl ethers and acetals. As the reactivity of carbonyl compounds toward ylides decreases noticeably from aldehydes via ketones to esters and amides, selective alkenations of aldehydes are possible in the presence of other carbonyl groups.

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