# Grubbs, Wittig & hydroboration reactions and Woodward-Hoffmann rules

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.

• en.wikipedia.org/wiki/Olefin_metathesis#Reaction_mechanism – Mithoron May 24 '15 at 12:57
• So it happens, regardless of the forbidden symmetry. – RBW May 24 '15 at 16:28
• 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. – J. LS May 24 '15 at 21:06
• Could you elaborate as an answer? – RBW May 25 '15 at 7:36
• I think you can try - don't be afraid :) – Mithoron Aug 9 '15 at 16:28