[synthnick]

In this reaction, the products are a,b or a only.I would be grateful if you could provide me with an explanation.



thank you

[fledarmus]

What do you know about the reaction mechanism for addition of diiodomethane to a double bond?

[synthnick]

syn addition

[fledarmus]

That's just a description of the product - what is the mechanism? What adds first to the double bond, and where?

[synthnick]

i know (few things) about directing groups that coordinate with zinc.
this is the mechanism?

[fledarmus]

Okay, so you have the methylene-zinc complex orienting to the center of the double bond. Now if you build a model of your bicyclic alkene, you will see that the pi-bond that the methylene is coordinating with extends above and below the plane of the double bond (the plane which contains the two carbons of the alkene, the two hydrogens, and the two bridgehead carbons). You have a fairly large electrophile in your methylene-zinc complex - would it be easier for the methylene to come in from the top or the bottom (make sure you have all the alkyl hydrogens on your model)?

If you can't find any real difference between the top and the bottom, you'll probably end up with a mixture of the two

[synthnick]

it is more crowded from the back side so the a is the major product.


thank you very much.