Science of Synthesis

Schobert, R.; Hölzel, C.; Barnickel, B., Science of Synthesis, (2010) 47, 49.

While the Wittig reaction with salt-free unstabilized ylides yields Z-alkenes in good to excellent yields and with Z/E ratios ranging from 94:6 to 99:1[‌4‌,‌11‌–‌13‌] (see also Section ‌47.1.1.1.3‌), E-selective alkenations are not normally possible. Even with lithium salts added, reasonable E selectivity is not observed owing to an equilibrium of cis- and trans-oxaphosphetanes.[‌8‌,‌10‌,‌11‌] E-Selective alkenations can be achieved only with stabilized phosphoranes,[‌4‌,‌6‌] or with phosphonate anions as used in the so-called Horner–Wadsworth–Emmons (HWE) reaction.[‌353‌,‌367‌,‌368‌] The synthesis of E-alkenes is also possible, though, by variants of the Wittig procedure such as the Schlosser modification,[‌345‌] the E-selective Wittig–Horner reaction,[‌369‌] and the use of ylides bearing groups other than triphenylphosphine.[‌370‌–‌373‌] There are also some conventional ylides which provide selective access to E-alkenes including the Salmond γ-oxido ylide[‌344‌] (see Section ‌47.1.1.1.3.3.2‌) and the Maryanoff carboxylate ylide[‌346‌,‌347‌] that will not be discussed here.

References

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• 8.Meeeee-Meee, (8888) M 8e, 888.
• 8.Meeeee-Meee, (8888) M 8e, 888.