[Hunt]

I have trouble finding where the nucleophile attacks this dienophile ( for diels alder rxn )

In the first case, the carboanion is stablized by resonance.




In the second case, the carboanion is stabilized by an electron withdrawing group NO₂.



Which is more stable ?

[Dan]

I would expect, based on the pKa of nitromethane (~10) vs the pKa of ethyl acetate (~25), that the nucleophile would add alpha to the carbonyl, as the negative charge will be more effectively stabilised alpha to the nitro group than the carbonyl group. I don't know any examples off the top of my head, but if I was put on the spot that would be my answer.

[spirochete]

I agree with Dan's analysis based on PkA if we're talking about general nucleophilic addition to the pi bond.  I think both carbanion intermediates would be stabilized by resonance though, in fact the nitro one looks more stabilized by resonance.

 But I'm curious why you are referring to it as a diels alder reaction?  A DA reaction is concerted and wouldn't have a carbanion intermediate.

[Hunt]

Thanks guys

You're right spirochete . I'm studying diels alder reactions and in precise their regioselectivity. Our professor said we need to consider the dienophile to act as an electrophile being attacked by a nucleophile ( dienes ) . This way we can figure out how the reaction proceeds for unsymmetrical dienes. I connect the positive carbon on the dienophile to the negative carbon on the diene. It's meaningless but helps me figure out the regioselectivity.

If you have a better method dont mind telling me.

[spirochete]

Your way of thinking works fine for predicting products, especially if you have access to pKas.

I'd think of it in terms of which carbon was more electrophilic, ie has more cationic character.  I would draw the two (or more) resonance structures which place a positive charge on each carbon.  Then I'd compare the electron withdrawing "strength" of the ester group and the nitro group.  In this case it's obvious to me that the nitro group is more E-Withdrawing because it has the electronegative nitrogen directly bonded to the carbon (meaning more inductive power probably) and also there are at least 2 structures that contribute to its ability to pull electron density away from that carbon.

As you do more problems of course drawing the resonance structures won't be necessarry, it's just good to do at first.