Grubbs, Wittig & hydroboration reactions and Woodward-Hoffmann rules

Asked May 24 '15 at 12:51

Active Aug 17 '17 at 11:30

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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.

edited Aug 17 '17 at 11:30

orthocresol

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asked May 24 '15 at 12:51

EJC

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https://en.wikipedia.org/wiki/Olefin_metathesis#Reaction_mechanism – Mithoron May 24 '15 at 12:57
So it happens, regardless of the forbidden symmetry. – EJC May 24 '15 at 16:28
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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? – EJC May 25 '15 at 07:36
I found an answer in Ahn.N.T "Frontier Orbitals" for hydroboration: "The transition state for a Woodward–Hoffmann- allowed reaction is aromatic in character, whereas a forbidden one is antiaromatic. However, the rules of aromaticity only apply when each atom of the annulene uses one, and only one, AO to bind to its neighbors." – EJC Aug 09 '15 at 16:22
Could someone expand this into an answer? – EJC Aug 09 '15 at 16:23
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I think you can try - don't be afraid :) – Mithoron Aug 09 '15 at 16:28
I'm afraid I can't. Would be good if someone could expand the answer to account for the Wittig and Grubbs reaction. Wittig may go through a betaine intermediate, but what about Grubbs? – EJC Aug 09 '15 at 16:44
Same question but specifically on the Wittig – orthocresol Aug 17 '17 at 11:32

Grubbs, Wittig & hydroboration reactions and Woodward-Hoffmann rules

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