Orbital Symmetry Controlled Reactions

[future_vet]

Homework Statement

http://avogadro.chem.iastate.edu/CHEM332/332Fall06Exam5Key.pdf
Please go to page 5.

Homework Equations

No equations, just concepts.

The Attempt at a Solution

I tried the first problem (A), and got the solution wrong. I thought I'd check the concepts before attempting any other problem and checking the solution.
I thought that it was photochemically allowed, not thermally because from what I understood, an odd number of double bonds in the initial reactant and a trans product indicated a conrotatory ring closure using light. Obviously, I was wrong.

Could you please explain how I should solve this and similar problems?

Thank you.

 

[chemisttree]

read http://www.usm.maine.edu/~Newton/Chy251_253/Lectures/Pericyclic/PericyclicFS.html"

 

[future_vet]

I am sorry, I still don't have a clear understanding of what I should do. Could you please tell me how you would reason for just the 1st problem? Thank you!

 

[chemisttree]

Do you understand that it is a 4+2 cycloaddition?

 

[future_vet]

Yes, the way I do it (correct me if I am wrong) is to count around the new sigma bond. The "1" would be the carbon attached to CO2Et, and "2" the other side of the double bond where the new sigma bond will be (hence the "2"), while the "4" comes from counting from the other side ("1" being the carbon to the right of the molecule). It is a cycloaddition because we have 2 double bonds forming a single bond. We have 2 less double bonds, and 2 new single bonds.

What I don't understand is why it is thermally allowed and suprafacial on alkene and diene.

 

[chemisttree]

Yes, the way I do it (correct me if I am wrong) is to count around the new sigma bond. The "1" would be the carbon attached to CO2Et, and "2" the other side of the double bond where the new sigma bond will be (hence the "2"), while the "4" comes from counting from the other side ("1" being the carbon to the right of the molecule). It is a cycloaddition because we have 2 double bonds forming a single bond. We have 2 less double bonds, and 2 new single bonds.

What I don't understand is why it is thermally allowed and suprafacial on alkene and diene.

The numbers "2" and "4" come from the number of electrons in the interacting system. You should not count carbons... It means that a 2-electron system (isolated alkene) is interacting with a 4-electron system (conjugated diene), thus a 4+2 cycloaddition.

Do you know what the terms 'suprafacial' and 'antarafacial' mean?

 

[future_vet]

I understand now, I was thinking of sigmatropic rearrangements. It is like a Diels-Alder reaction then?

Suprafacial is when we have bonding of lobes of the same sign, while antarafacial has bonding of lobes of the same side as well as bonding of lobes of opposite signs.
Do we have to draw the orbitals in order to know which applies to the problem?

 

[chemisttree]

I understand now, I was thinking of sigmatropic rearrangements. It is like a Diels-Alder reaction then?

Suprafacial is when we have bonding of lobes of the same sign, while antarafacial has bonding of lobes of the same side as well as bonding of lobes of opposite signs.
Do we have to draw the orbitals in order to know which applies to the problem?

This is a Diels-Alder reaction. Rethink your definition of suprafacial. It doesn't refer to the sign of lobes. Also rethink what 'antarafacial' means. It would be helpful if you would revisit the link I provided and carefully read the whole thing.

 

[future_vet]

I did, but combined with the lecture, I got really confused and the prof. is not helpful at all.

Here's a pattern I derived from all the related notes/websites/exams I read:

In an electrocyclic addition:
Even double bonds with trans product = thermal + conrotatory
cis product = photochemical + disrotatory
Odd double bonds with trans product = photochemical + conrotatory
cis product = thermal + disrotatory

Cycloaddition reaction:
Even double bonds with trans product = photochem. + suprafacial
cis product = thermal + antarafacial
Odd double bonds with cis product = photochemical + antarafacial
trans product = thermal + supra.

Sigmatropic:
Even double bonds with trans product - heat/conrotatory
cis product - thermal/disrotatory
Odd double bonds with trans product - photochem. + conrotatory
cis product - thermal - disrotatory

Does this sound correct? I will not learn this by heart even if it is correct, I will most likely find diene and triene example, but I just want to know if I am on the right path.

Thanks!

 

[chemisttree]

I'm confused as to why you are grouping these reactions as you have them. A cycloaddition is a type of electrocyclic reaction. Sigmatropic reactions are isomerizations. The same principles apply to all of these reactions if you understand the principles. It is best to understand the principles first and the types of reactions (a distant) second.

 

[future_vet]

This is how they were broken down in the lecture... I will go back and try to get the principle down first.