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Several organic mechanisms I know involve [2+2] cycloadditions and retro-[2+2] cycloadditions, namely:

  • The Wittig reaction
  • Hydroboration of an alkene
  • Olefin metathesis (using Grubbs' catalyst)

How are these in accordance with the Woodward–Hoffmann rules? I think that there are two $(4q+2)_s$ components and that's not an odd number, which is the criterion for a reaction to be thermally allowed.

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  • $\begingroup$ en.wikipedia.org/wiki/Olefin_metathesis#Reaction_mechanism $\endgroup$
    – Mithoron
    May 24, 2015 at 12:57
  • $\begingroup$ So it happens, regardless of the forbidden symmetry. $\endgroup$
    – EJC
    May 24, 2015 at 16:28
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    $\begingroup$ The Woodward-Hoffmann rules for pericylic reactions assumes essentially that you can describe the orbitals in terms of Huckel approximations, which is definitely not the case for a Ru catalyst. $\endgroup$
    – J. LS
    May 24, 2015 at 21:06
  • $\begingroup$ Could you elaborate as an answer? $\endgroup$
    – EJC
    May 25, 2015 at 7:36
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    $\begingroup$ I think you can try - don't be afraid :) $\endgroup$
    – Mithoron
    Aug 9, 2015 at 16:28

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