[aspiringphysician]

Regarding the Wittig reaction mechanism, my lab manual states that "the oxaphosphetane intermediate can open, allowing equilibration to the more stable (E)-isomer." I'm just wondering: does this ring-opening and equilibration occur only when the ylide is resonance-stabilized, or does it occur with all ylides?

[discodermolide]

Regarding the Wittig reaction mechanism, my lab manual states that "the oxaphosphetane intermediate can open, allowing equilibration to the more stable (E)-isomer." I'm just wondering: does this ring-opening and equilibration occur only when the ylide is resonance-stabilized, or does it occur with all ylides?

I think there is some discussion about this mechanism, at least concerning the formation of the oxaphosphatane intermediate, some think it is a 2+2 cycloaddition, others a stepwise process.
As far as the opening goes as far as I remember the ring opening and equilibration to the (E) isomer occurs with stabilized ylides. There are exceptions, for example the Still-Gennari olefination which gives almost exclusively >90:10 (depending upon the substrates) the (Z) isomer.

[aspiringphysician]

Ah, I see...thank you very much!