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

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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?

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roymend
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    What if the reaction is not concerted? ;) – Klaus-Dieter Warzecha Nov 25 '16 at 20:09
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    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...) – roymend Nov 25 '16 at 20:17
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    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. – orthocresol Nov 25 '16 at 20:33
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    The other question to ask is: what is the symmetry of the orbitals on the phosphorus ylide? – Zhe Nov 25 '16 at 20:38
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    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
  • There's some evidence that it's not concerted, but far more evidence that it is. At least, that's what I gather from nearly every org chem textbook I've read. March in particular has references to primary literature. – orthocresol Nov 26 '16 at 03:54
  • @roymend, as for your earlier comment, all pericyclic reactions are indeed concerted. However not all cycloadditions are concerted, and therefore not all cycloadditions are pericyclic. In this case, the others are hinting that a 2-step cycloaddition can exist, and it would be neither pericyclic nor concerted. – orthocresol Nov 26 '16 at 03:56
  • For a somewhat recent review of the mechanism of the Wittig reaction proposed in the past, have a look at Chem. Soc. Rev., 2013, 16, 6670-6696 (DOI) – Klaus-Dieter Warzecha Nov 26 '16 at 06:46

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