My textbook (Klein) describes the mechanism for the Wittig reaction as a [2+2] cycloaddition, followed by a collapse of the ring.

enter image description here

As far as I'm aware, [2+2] cycloadditions are symmetry forbidden, unless the reaction is light-initiated, which the text does not mention. Is the book wrong? If not, what is going on here?

  • 5
    $\begingroup$ What if the reaction is not concerted? ;) $\endgroup$ Nov 25 '16 at 20:09
  • 1
    $\begingroup$ Then the reaction is not a cycloaddition and a completely different mechanism is needed? (I learned that pericyclic reactions are always concerted, but I may be wrong...) $\endgroup$
    – roymend
    Nov 25 '16 at 20:17
  • 3
    $\begingroup$ Very interesting question! I'd like to point out that a thermal $[2_\mathrm{s} + 2_\mathrm{s}]$ cycloaddition is Woodward-Hoffmann forbidden, but a $[2_\mathrm{s} + 2_\mathrm{a}]$ cycloaddition is (e.g. the reaction of a ketene with an alkene proceeds in this manner). See: Why is the Wittig reaction syn stereoselective? Also, Grossman talks about this a bit in his book The Art of Writing Reasonable Reaction Mechanisms. I've attached the relevant pages here. $\endgroup$
    – orthocresol
    Nov 25 '16 at 20:33
  • 2
    $\begingroup$ The other question to ask is: what is the symmetry of the orbitals on the phosphorus ylide? $\endgroup$
    – Zhe
    Nov 25 '16 at 20:38
  • 2
    $\begingroup$ I think, the simplest explanation here is that the reaction is not concerted. But then remember that the Wittig ylide is not exactly identical to your average alkene either. $\endgroup$
    – Jan
    Nov 25 '16 at 23:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.