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  • I want to compare and contrast the behavior

  • of 5-membered and 6-membered rings

  • in this mode of reactivity

  • in which the π system serves as the nucleophile.

  • The electrophilic aromatic substitution reactions

  • like we were con- considering in the previous webcast,

  • but now with the 5-membered heteroaromatics.

  • We'll take as our 5-membered ring our prototypical reaction,

  • the reaction of thiophene, that system there,

  • with acetyl nitrate.

  • Acetyl nitrate is just a good source of an electrophilic

  • nitronium cation, but it's going to react

  • in a slightly different mode of reactivity.

  • Two things to note; first of all,

  • the reaction takes place at a very low temperature,

  • -10ºC.

  • Obviously, this is much accelerated

  • compared to the previous case of the 6-membered ring.

  • And the second thing to note is that the mode, the, of

  • substitution takes place in this position,

  • the so called 2-position.

  • In the 2-position, that hydrogen atom

  • is going to be displaced so this is carbon-2 of thiophene,

  • and we call that carbon-3 of thiophene,

  • it's the carbon-2, its hydrogen,

  • that undergoes substitution with the nitryl group.

  • There's a couple of ways we could understand why

  • the substitution takes place at C2 rather than C3.

  • One thing that we might do is to do a SHMO calculation

  • and look at the highest occupied molecular orbital

  • and we'd find that the grayest- greatest coefficient

  • of the highest occupied molecular orbital

  • is going to be on C2.

  • That's one thing.

  • Another way we might anticipate this reactivity

  • is to compare the two different types of intermediates

  • that form by attack at C2 and by attack at C3.

  • Let me outline that for you and what I would encourage you to do

  • while you're listening to this

  • is to make a reaction coordinate diagram

  • and compare the reaction pathway for C2 attack

  • versus C3 attack.

  • C2 attack involves three resonance contributors,

  • whereas C3 attack only involves two.

  • This is a π to σ* type interaction.

  • We're going to break the σ bond

  • between nitrogen and oxygen in acetyl nitrate

  • and we'll end up making a new carbon nitrogen bond,

  • and this intermediate is a carbocation intermediate

  • that is delocalized onto those positions in the ring

  • as well as on the sulfur atom.

  • And so a N to A type interation,

  • resonance interation gives us three resonance contributors

  • one of which, the best one, is the last one

  • because it has an octet of electrons on every atom.

  • Attack at C3 ult- provides us with a less stable intermediate.

  • That less stable intermediate

  • only has 2 resonance contributors

  • one of which is again, having a positive charge on sulfur,

  • an octet of electrons on every atom,

  • but you can see that we have less d- delocalization

  • of that positive charge.

  • The intermediate that results from attack at C2

  • is more stable, the reaction pathway proceeds

  • through a transition state to the C2 attack

  • that’s lower in energy than the C3 attack.

  • That pathway, the C2 pathway, is faster than C3.

  • Why the enhanced reactivity?

  • We just explained the regioselectivity,

  • but why the enhanced reactivity?

  • And that has to do with the highest occupied

  • molecular orbital energy level being elevated.

  • The higher the energy of the HOMO,

  • the more nucleophilic that pair of electrons.

  • And so that’s one- one reason.

  • So there’s really two reasons why benzene,

  • of these four, is the least reactive

  • towards electrophilic aromatic substitution.

  • So the first thing that I’ve already mentioned

  • is this idea down here

  • where the π electrons are raised up in energy,

  • theyre more nucleophilic than the benzene ring

  • pair of electrons.

  • And if you did a SHMO calculation

  • you would in fact see those electrons in the HOMO

  • of the thiophene ring

  • or the other 5-membered heteroaromatics

  • are higher in energy than is benzene.

  • But the other thing to note is that the transition state,

  • and the intermediate that results is more stable

  • and the main reason for that is that

  • intermediate can be drawn

  • with a complete octet on every single atom.

  • Here’s one resonance contributor and that’s the carbocation,

  • but we have- don’t forget

  • we have this nonbonding pair of electrons

  • that can do an end to A or an end to π*-type donation

  • and provide us with an intermediate-

  • a resonance contributor of that intermediate

  • that has an octet of electrons on every atom.

  • You cannot find that in the case of benzene,

  • all’s you have is open shelve carbocations.

  • So the intermediate is more stable

  • and because the transition state

  • is going to resemble the intermediate

  • the pathway is going to be lower.

  • So, we have two things going for us;

  • the energy is raised- the energy of that

  • higher occupied molecular orbital is raised,

  • and the transition state is lowered

  • and that’s what makes these heteroaromatics

  • more reactive than benzene.

  • Alright, let’s take a look at the imidazole

  • very quickly and say that, if we have

  • an N2 nitrogen in the ring,

  • were going to undergo deactivation

  • for the reasons that we encountered with pyridine.

  • Were going to first do acid-base chemistry

  • and that’s going to facilitate lowering that HOMO,

  • making that π system much less reactive.

  • Those π electrons now being part of a positively charged ring,

  • are no longer very reactive and so the end-

  • the presence of an N2 nitrogen in a molecule like imidazole,

  • make it very unfavorable toward nucleophilic-

  • toward electrophilic aromatic substitution

  • in which the π-system acts as a nucleophile. 47 making that π system much less reactive.

  • Those π electrons now being part of a positively charged ring,

  • are no longer very reactive and so the end-

  • the presence of an N2 nitrogen in a molecule like imidazole,

  • make it very unfavorable toward nucleophilic-

  • toward electrophilic aromat

I want to compare and contrast the behavior

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B2 中高級 美國腔

五元異構體作為C-親核體的作用 (Five-membered Heteroaromatics as C-Nucleophiles)

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    Scott 發佈於 2021 年 01 月 14 日
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