Mechanism of Grignard addition to carbonyls So from the general reaction we can see that the carbonyl is reduced while the Grignard is oxidized to an alkene. 

So for this hypothetical reaction (does this really occur?) we are asked to provide a plausible mechanism. This is what I got, but the prof. wants something better - why transfer negative charge from oxygen to an $\text{sp}^3$ carbon? 



Click on the image to enlarge, if you wish.

Here are his hints:


  Remember, prior to Grignard reaction the $\ce{Mg}$ center swaps out an ether
  solvent molecule for a reactant oxygen.  This interaction will lead to
  the reaction but in this case, due to steric interference a hydride is
  delivered instead as the Grignard is oxidized to an alkene.  In this
  way the Grignard reagent is almost reacting in the way $\ce{NaBH4}$ would in
  the presence of this ketone. Try to get all of this action working
  together in a single, concerted step.


So ... I get the first part just fine. The $\ce{Mg}$ center has an octet; it's solvated by two ether molecules in solution usually. Okay and in this case the carbonyl oxygen helps solvate the $\ce{Mg}$ center. And then hydride transfer occurs ... but how exactly do we get the hydride to leave without creating a carboanion? And is it really that important that everything be concerted? 

This is my second attempt ... if this is all concerted in real life then I must have hit the orbital alignment lottery! All the stars must have been aligned ... ! 

The only thing I see about my second attempt is that in the end the $\ce{Mg}$ center doesn't have a complete octet - perhaps I should have shown another ether molecule popping in to help $\ce{Mg}$ out with its octet fantasies.
This reaction can be viewed as an example of a pericyclic (concerted) Ene reaction.  The ene reaction is actually fairly common in organic chemistry.   The following diagram details the mechanism as a concerted reaction with a cyclic 6 membered transition state involving 6 electrons (an "aromatic" transition state).



A note on the Ene reaction...

The Ene reaction is really just a variation on the Diels-Alder theme.  The Diels-Alder reaction is a concerted [4 pi + 2 pi] reaction, whereas the Ene reaction is a [(2 pi + 2 sigma) + 2 pi] concerted reaction.