[azmanam]

Sorry for the delay... again.  Monday's are busy for me.  Anyway, here's this weeks problem.

QUESTION: Provide a complete, arrow-pushing mechanism for the following one-carbon ring expansion of an oxetane.

[Heory]

 

[azmanam]

very close.  the authors propose a different order of electron transfer than what you have shown.  as a comparison, if the lewis acid is not added, the regioselectivity is reversed.  That is, the cyclohexenone inserts between the other carbon-oxygen bond of the oxetane.

[Heory]

 

[azmanam]

let's combine those two.  the initial radicals are not proposed to be in equilibrium.  And actually, looking back at your first answer, it's closer than I initially thought.  I missed your equilibrium arrow between the oxetane and the zwitterion.  That is not proposed to be in equilibrium, but it's closer to the answer than your seccond answer.  The regiodifferentiating step is proposed to be the oxetane fragmentation step, also not proposed to be in equilibrium.  The very first, probably very fast, step is association of oxetane and lewis acid, as you have in your first answer.

[Heory]

 

[azmanam]

The second is the answer, good work.  I suppose the equilibrium arrows you've drawn in the top row are really more accurately resonance arrows.  The oxetaine-LA and the carbocation/alkoxide are really just resonance structures, right?  Those resonance structures are probably more important than the corresponding resonance structures in the case without lewis acid present.  The reversal of regioselectivity is what I'm basing that statement on.  Clearly the tertiary radical is more stable.  Given the choice, you'd imagine the regioselectivity would not change with reaction conditions, as the formation of the tertiary radical should be more stable.  But in the absence of Lewis acid, the authors postulate the more branched alkoxide is more stable than the more substituted radical.  The references probably explain it better than I do

synthesis: http://dx.doi.org/10.1021/jo00020a011
methodology: http://dx.doi.org/10.1021/ja00206a063

[Heory]

Thank you very much for your work!

[Heory]

In the fist sheme, in the presence of AlMe3 I supposed the regioselectivity resulted not only from the more stable tertiary carbon radical, but also from the more stable tertiary  carbon cation. They can be resonance structures. And they can also be different molecules. With the help of AlMe3 and the stability of the tertiary  carbon cation, I suppose the C-O be broken, which is followed by the reductive procedure.