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Wikipedia says that with unstabilised ylides they react very quickly in step 1-2 and step 4-5 is the rate limiting step. But with stabilised ylides step 1-2 is the rate limiting step and this is why stabilised ylides give E-alkenes. But there is no explanation as to why this makes E-alkenes and where in the mechanism it occurs.

I would have thought stabilised and unstabilised ylides would give E-alkenes to reduce steric hinderance but this is not the case. Any help explaining this would be very appreciated. :)

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    $\begingroup$ The Wittig reaction is generally considered to proceed via a concerted [2+2] cycloaddition, to form the oxaphosphetane 5 directly, without the intermediacy of the betaine 3. Philipp's answer (based on thermodynamic oxaphosphetane formation) is a mechanism that was accepted and taught many years ago (Clayden 1st ed was published in 2002). Nowadays, the generally accepted explanation is different; please see the duplicate questions. $\endgroup$ – orthocresol Aug 24 '17 at 9:17
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I have given some justification on why unstabilized ylids give Z-alkenes in this answer of mine.

A possible explanation why stabilized ylids show E-selectivity is given in the organic chemistry textbook by Clayden, Warren, Wothers and Greeves on p. 817:

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