Woodward-Hoffman Rules confusion

Having a lot of trouble with the question below.

I understand the basics of the rules, I can do it when it's a cycloaddition involving only pi bonds as drawing the diagram to work out the number of suprafacial/antarafacial components is much more simple. Electrocyclics are usually ok when it's just a simple chain.

However for this I've got myself pretty confused. I know its an electrocyclic reaction because the change in the number of sigma bonds is 1. I can tell from a simple diagram it has to be conrotary, which is what you'd expect for a 6 electron photochemical process. However I can't work out how to calculate the number of suprafacial and antarafacial components when there are sigma bonds involved - which is what I presume they want, unless I don't understand things properly (very possible) and that's only relevant for cycloadditions?

(edited 11 years ago)

Reply 1

EierVonSatan

Yeah, it's a reverse electrocyclisation but I always do it from the ring forming direction. If it's 6e (4n +2) and photochemcial then it should be conrotation/antrafacial, no? :colondollar:

You promote an electron into the LUMO so your two end orbitals are of opposite sign on each side (this would be much easier to explain with pen and paper :p:

) and rotate together to form the sigma bond. The reverse process is then just that.

Reply 2

illusionz

Original post by EierVonSatan

Yeah, it's a reverse electrocyclisation but I always do it from the ring forming direction. If it's 6e (4n +2) and photochemcial then it should be conrotation/antrafacial, no? :colondollar:

You promote an electron into the LUMO so your two end orbitals are of opposite sign on each side (this would be much easier to explain with pen and paper :p:

) and rotate together to form the sigma bond. The reverse process is then just that.

Ah yeah it is conrotary. I'd made a silly error when working it out, then tried to check online and wikipedia says that you get opposite rotations for opening/closing, but that seems wrong when the reaction must reverse by the same way it happens. Checked Organic Chemistry by Clayden, Greeves, Warren and Wothers and it doesn't mention anything about opposite types for opening/closing, so I guess that's one of the cases where wikipedia is just wrong.

I know exactly what you mean in terms of being a lot easier to explain with pen and paper! I'm actually starting to think I was getting the methodology for cycloadditions and electrocyclics mixed up. Am I correct in saying you don't actually need to work out the number of (4q+2)_s and (4r)_a components for an electrocyclic reaction? only for cycloadditions?

The thing which makes me think that I should be doing just that for this question is that the stereochemistry is very obvious as there are rings involved, so working out if it's con/dis is rather easy. It's a 15 minute question so I'm feeling like I should be doing more than simply noting the number of electrons and what direction the ring opens in! It just seems like there's not very much to talk about.

Pericyclics are definetely the most conceptually challenging bit of organic chemistry I've come across so far...

(edited 11 years ago)

Reply 3

EierVonSatan

Original post by illusionz

Yeah, you worried me there for a second :p:

I drew a pretty diagram and everything :lol:

I'm pretty dubious about what it says there :s-smilie:

I know exactly what you mean in terms of being a lot easier to explain with pen and paper! I'm actually starting to think I was getting the methodology for cycloadditions and electrocyclics mixed up. Am I correct in saying you don't actually need to work out the number of (4q+2)_s and (4r)_a components for an electrocyclic reaction? only for cycloadditions?

I never learnt to consider components when doing this, I was taught to do it just by considering the frontier orbitals, so I can't help you with that :colondollar:

The thing which makes me think that I should be doing just that for this question is that the stereochemistry is very obvious as there are rings involved, so working out if it's con/dis is rather easy. It's a 15 minute question so I'm feeling like I should be doing more than simply noting the number of electrons and what direction the ring opens in! It just seems like there's not very much to talk about.

I think the time taken is thinking time :p:

Drawing out the diagrams, rubbing them out and starting over. Then drawing the opposite and saying how it isn't the one shown - you'd get a nasty E double bond if it was disrotation here for example.

Pericyclics are definetely the most conceptually challenging bit of organic chemistry I've come across so far...

Fear not, I've barely encountered them since doing this module in my 3rd year. It gets better :awesome:

Reply 4

illusionz

Original post by EierVonSatan

Yeah, you worried me there for a second :p:

I drew a pretty diagram and everything :lol:

I'm pretty dubious about what it says there :s-smilie:

I remember one of our lecturers made a rather big point that wikipedia has some very dubious mechanisms for some reactions. Think he mentioned the schmidt rearrangement is wrong.

I never learnt to consider components when doing this, I was taught to do it just by considering the frontier orbitals, so I can't help you with that :colondollar:

Yeah I think that's all we were taught to do as well. I was trying to work out whether the interactions between the sigma and pi orbitals was a or s and getting into a horrible mess.

I think the time taken is thinking time :p:

Drawing out the diagrams, rubbing them out and starting over. Then drawing the opposite and saying how it isn't the one shown - you'd get a nasty E double bond if it was disrotation here for example.

I hope you're right! Guess they just want me to say that the trans double bond in a 6 membered ring is impossible.

Fear not, I've barely encountered them since doing this module in my 3rd year. It gets better :awesome:

Most of it doesn't seem too hard. It's just we did all of pericyclics in 2 hours of lectures so it was all a little brief lol. The worse ones from an exam point of view in my opinion are NMR and retrosynthetic analysis. They either come to me in 20 seconds or I can spend ages getting nowhere!

(edited 11 years ago)

Reply 5

EierVonSatan

Original post by illusionz

I remember one of our lecturers made a rather big point that wikipedia has some very dubious mechanisms for some reactions. Think he mentioned the schmidt rearrangement is wrong.

Online resources like wikipedia are really good on the whole, but as the material gets increasingly advanced it just can't stand up to the quality of texts or journals. I've spotted a few errors here and there, it's bound to happen. Hell, you still get it in journals after the editing process :p:

Well it makes my head hurts, and I utter a sigh of disappointment when asked about it...we spent a few lectures on it, but it doesn't really sink in until you've done plenty of problems on them.