June 25, 2024, 03:35:07 PM
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Topic: Regioselectivity in Se-Reactions in the context of polycyclic compounds  (Read 1077 times)

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Offline Venoxis

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Hello dear forum members,

I'm new here and I have registered to ask a question which for some reason is difficult for me to solve on my own as I struggle to wrap my head around it. The problem pertains to electrophilic aromatic substitution reactions. I know how it works, both in theory as well as in practice when it comes to monocyclic arenes like benzene itself and how activating/deactivating ortho/para/meta directors decide over at which C-X the next group will be bonded to (please note that "X" here doesn't stand for halogens but is a placeholder for the position number of the carbon in question).
However, things got complicated for me once I moved on to polycyclic compounds because I have no idea how to "jump" back and forth between neighbouring rings. To make matters worse my textbook only explains Se-Reactions in the context of monocyclic arenes like benzene and phenol.

Let's take for example biphenyl. Assuming the Se-Reaction starts at ring A, how does a chemist ensure that his next reaction takes place at ring B? No matter whether I use activating/deactivating ortho-para-meta directors, the reactions will always take place on ring A, right? What does it take to affect the regioselectivity of the molecule in such a way to "jump" to ring B?

Another example would be the phenanthrene molecule (I have uploaded an image with the numbering system of the (4,5-epoxy)morphinan family). This one is different as ring B is essentially a cycloalkene which responds to E1/E2/E1cb-Reactions as well as Addition-Reactions, so I would assume that simply doing one of these reactions automatically removes/adds functional groups to ring B instead of the neighbouring arenes. But what if I am currently at C-3 of ring A after having substituted the H-atom with an OH-group via Se and want to skip ring B and go directly to ring C (which is also an arene that also responds to Se-Reactions) and add another OH-group at C-6? The OH-group at C-3 would be a strongly activating ortho-para director, so the next reaction will take place at either C-2 or C-4 (ring A). Para isn't possible because C-12 doesn't have any valence electrons left (and even if it had any, there is too much steric hindrance, so a reaction at ortho is much more likely).
I can't imagine that a chemist has to go through the entire ring structure and do a whole range of unnecessary and time-consuming reactions until he reaches his desired C-6 position to manipulate. There must be some kind of standard ruleset that chemists apply in order to address specific rings while leaving others alone. Something that gives us more control over the regioselectivity of the reaction we want to conduct.

Maybe the best way to answer my question would be to tell me how YOU would synthesize the compound 3,6-dihydroxyphenanthrene if you were to start with phenanthrene.

I hope I haven't expressed myself in too complicated a manner and if I did I apologize as english isn't my mother tongue. I have really researched book after book, searched websites and sifted through youtube videos but I haven't found anything that discusses how to affect the regioselectivity of a polycyclic molecule in order to conduct reactions at different rings instead of a single ring. Maybe I'm thinking in overly complicated ways and can't see the forest for the trees anymore. Whatever it is, I need help because this is driving me insane. Thank you.
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Offline Venoxis

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Is this question really that difficult to answer or is it unclear what exactly is being asked here? I see multiple threads have been answered since my post but unfortunately not mine :-(
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Offline Borek

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I am not an organiker, so take my answer with a grain of salt.

Regioselectivity in aromatic compounds is in a large part related to deviations in the electron density - which is to some extent related to the presence of resonance structures. In general, the more resonance structures possible, the lower their individual effect (ie the deviations from the equal distribution become smaller). The larger the molecule, the more atoms involved, the more the resonance structures possible - and the less prominent effects.

So if the selectivity of the reaction on a single ring is such that you get a 90:10 mixture of products, you can speak about the reaction being selective. When you get to several rings and you get mixture of 5:6:7:5:5:4:... it is hard to speak about "selectivity" as such.

This is not to say there are no specific examples when the reaction can be selective, it is just that the general trend seems to be working against.
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Offline Venoxis

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Thank you for your reply Borek. I think it might be a combination of electron distribution (nucleophilic/electrophilic potential of individual C positions), the amount of steric hindrance and the type of reaction applied that determines where the next reaction takes place. Let's see what an organic chemist will say about this.

Z ukłonami, (hope that was correct)
Venoxis
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