[Rutherford]

How does the reaction leading from G to H in the presence of KNH₂/NH₃ proceed?

I think it is H elimination form the C atom that has a methyl group bonded to it, migration of π bond and then the carbanion takes a hydrogen from ammonia. Correct?

[discodermolide]

Rather than carbanion chemistry I think you should consider the formation of radicals causing the bond migration.
KNH₂/NH₃ as far as I remember is a source of electrons.
Think Birch reduction.

[Rutherford]

But one C atom has to switch from sp₃ to sp₂ while in Birch is the opposite. Can't think of the radical mechanism.

[discodermolide]

Try breaking the C-H bond by moving single electrons around same with the C=C.

[Rutherford]

Thank you. But this doesn't seem as Birch reduction, nothing is reduced here. Is this a special case or another named reaction?

[discodermolide]

Surely G is the intermediate in a Birch reduction??

[Rutherford]

There was an aromatic ring, then Li/NH₃ were added and G was produced. Afterwards, KNH₂/NH₃ were added. The first reaction was Birch, the second can't be?

[discodermolide]

I said it looks like an intermediate in a Birch reduction, which it is.
The second reaction is an isomerisation to give a more stable conjugated system.

[Rutherford]

Sorry, didn't understand you. Thank you for the help.

[orgopete]

Rather than carbanion chemistry I think you should consider the formation of radicals causing the bond migration.
KNH₂/NH₃ as far as I remember is a source of electrons.
Think Birch reduction.

This is not a Birch or Birch-like reaction. It is simply a base catalyzed tautomerization.

[Rutherford]

Tautomerization doesn't occur by a free-radical mechanism, but rather the one I proposed first. Now I am confused  .

[spirochete]

Yes you were totally correct in the beginning. Orgopete confirmed you were correct.  I think somebody else was confused when they proposed a birch, and/or radical type mechanism.

The fact that a molecule is a product of some reaction does not tell you how it will act as a substrate for a different set of reaction conditions.

Calling an acid base reaction an "H elimination" in your original post is an imprecise use of language, though. That could mean H⁺, H minus or H radical. Usually simply saying "H" is used to mean hydrogen radical. It's better to say "proton removal" or "H⁺ abstraction or something like that.

[Rutherford]

Okay, thank you. Yeah, I didn't think too much about language here .