Science of Synthesis

Schobert, R.; Hölzel, C.; Barnickel, B., Science of Synthesis, (2010) 47, 27.

The vast majority of published Wittig alkenations with unstabilized ylides lead to Z-1,2-disubstituted alkenes, owing to the importance of the latter in natural product chemistry. High Z/E selectivities, ranging from 94:6 to 99:1, are achieved[‌4‌,‌11‌–‌13‌] when the reactions are carried out under so-called “salt-free conditions”, i.e. in the absence of lithium salts that may coordinate to the ylide species and alter its reactivity.[‌8‌,‌10‌] However, examples are also known in the literature of Wittig alkenations that proceed with high Z selectivities even though alkyllithium bases are used to generate the ylide. While the mechanistic implications of this empirical finding are still under debate, it is uncontentious that high Z/E selectivities in Wittig reactions between unstabilized ylides and aldehydes are generally only achieved when a cis-oxaphosphetane intermediate, irreversibly formed via a puckered early transition state, undergoes rapid pseudorotation to a still cis-configured O-equatorial rotamer, which in turn collapses with syn elimination of an alkene.[‌5‌,‌252‌–‌255‌] Lithium salts are thought to mediate ring opening and cis–trans equilibration of the cis-oxaphosphetanes and hence lead to a greatly reduced Z selectivity.[‌8‌]

References

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