# Symmetry forbidden cycloaddition in the Wittig reaction?

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

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?

• What if the reaction is not concerted? ;) Nov 25 '16 at 20:09
• 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...) Nov 25 '16 at 20:17
• 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. Nov 25 '16 at 20:33
• The other question to ask is: what is the symmetry of the orbitals on the phosphorus ylide?
– Zhe
Nov 25 '16 at 20:38
• 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.
– Jan
Nov 25 '16 at 23:03