Photochemical Electrocyclic Reactions

Photochemical Electrocyclic reactions are pericyclic reactions in which 1 pi bond is destroyed after a light-catalyzed cyclic mechanism.

Concept: MO Theory of Photochemical Electrocyclics

Video Transcript

Hey everyone. In this video we're going to discuss a type of pericyclic reaction called a photochemical electrocyclic reaction. So, photochemical electrocyclic reactions are simply going to be intramolecular pericyclic reactions in which 1 pi bond is destroyed after a light-activated cyclic mechanism, I know that's a mouthful but you guys should already be really comfortable with all those key terms, it's intermolecular because all electrocyclic reactions are intermolecular it destroys 1 pi bond because all electrocyclic reactions destroy 1 pi bond and it's light activated because we're using photo chemical energy. So, here's an example, we have a molecule that is reacting with itself in the presence of light to form a new ring and we are changing 1 pi bond in the process, you start off with 3 and we end up with 2, and this mechanism would be the same exact mechanism for the thermal conversion electrocyclic reactions because nothing has changed, all it's going to happen is that you're going to form a new Sigma bond and then go through the rest of your concerted mechanism and basically in the meantime you exchange one sigma bond for 1 pi bond, cool? Awesome. So, by the way, just want to throw this out there, every single conjugated polyene is capable of doing this. So, it's not unique to specific types any polyene can do this but the stereochemistry does depend on frontier molecular orbital theory. So, we're going to be doing, we're going to not focus too much on the general mechanism because that's the easy part, we know, we're going to form a ring, we're going to focus more on the idea of HOMO and LUMO frontier orbitals so that we can figure out what the stereochemistry of the product will be, okay? Now, something that is unique to a photochemical electrocyclic reaction is that light is going to be involved in exciting ground state electrons and kicking them up one energy level. So, it's going to take those electrons in their ground state and it's going to move them to a higher energy state. So, usually that means we're going to go from a bonding psi to an antibonding psi and that means that your HOMO and your LUMO orbitals are going to change and since the stereochemistry of an electrocyclic reaction is dependent on understanding the HOMO orbital, that means that we need to take light into account, that's going to change the identity of the HOMO molecular orbital, okay? So, let's go ahead and look here at just basically a diene, which is a very simple example and before we even start, why don't we fill in what the molecular orbitals would look like for a diene, just remember I'm just going to go through this very quickly because this is not the point of this video, but very quickly, the first one doesn't change, the last one always changes and my node keep increasing. So, this would be one node and this would be two nodes and this would be three nodes, cool? Awesome. So, we know about a typical diene is that four pi electrons will fill orbital psi 1 and psi 2 making my HOMO psi 2 usually, and my LUMO psi 3, right? But after I react with light what's going to happen is that one of these electrons is going to get kicked up to a higher energy state, meaning that now my molecular orbital diagram will look like this, meaning that my HOMO and, or LUMO orbitals have changed. Now this is my new HOMO, psi 3, and this is my new LUMO, psi 4, cool? Now, guys actually LUMO is going to be completely irrelevant for this specific reaction because the electrocyclic reaction is intermolecular and only involves the HOMO of the molecule but it's just interesting to see how light has now changed the identity of my HOMO orbital. So now, when I go ahead and consider the stereochemistry of this molecule, I'm going to have to draw my orbital differently because light was included. So, in the in the next example, in the next video, I'm going to go through an example showing how to draw the stereochemistry from scratch of an electrocyclic reaction that's using light.

Example: Predicting Electrocyclic Products