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47.1.1.1.4 E-1,2-Disubstituted Alkenes

DOI: 10.1055/sos-SD-047-00004

Schobert, R.; Hölzel, C.; Barnickel, B.Science of Synthesis, (20104749.

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[‌4‌,‌11‌‌13‌] (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 HornerWadsworthEmmons (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 WittigHorner reaction,[‌369‌] and the use of ylides bearing groups other than triphenylphosphine.[‌370‌‌373‌] 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.

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