Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: phenylacetate on November 03, 2017, 02:11:59 PM
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Can someone explain the underlying mechanism for this reaction (see attachment)? The reaction was performed in N-methylpyrrolidone at 80°C. I have found it in a paper and replicated it successfully, but I cannot understand the mechanism behind it. It looks to me like an intramolecular base-catalyzed hydroamination reaction, but what happens to the trimethylsilyl group is a mystery to me. I have been through the references in the paper I got this reaction from, but they do not mention this reaction in detail. I appreciate any useful info!
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1). Does this reaction occur in presence of a catalyst, such as an organometallic catalyst or something else?
2). What is the yield of the final product?
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1). Does this reaction occur in presence of a catalyst, such as an organometallic catalyst or something else?
2). What is the yield of the final product?
1. No, no organometallic catalyst, only the NaH and the NMP solvent.
2. The yield was 79%.
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I have my own question to spur discussion, can NaH deprotonate an aniline?
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Yes, NaH (pka = 35) can deprotonate 2-amiinopyridine (pka = 6.9) and aromatic amines (pka ≈ 4-5) because it is a stronger base. But this doesn’t lead to anywhere.
What about NaH attack to the C-Si bond that forms trimethylhydrosilane and the corresponding alkyne anion?
Besides, the so formed trimethylhydrosilane cannot reduce alkenes and alkynes, neither can cleave the aromatic bromine without the appropriate organometallic catalyst.
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Correction: Besides, the so formed trimethylhydrosilane cannot reduce alkenes and alkynes, neither can cleave the aromatic bromine without the appropriate organometallic or Lewis acid catalyst.
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Yes, NaH (pka = 35) can deprotonate 2-amiinopyridine (pka = 6.9) and aromatic amines (pka ≈ 4-5) because it is a stronger base.
that is is the pKa of the conjugate acid of aniline, the anilinium ion (Ph-NH3+), the pKa of of aniline which is the conjugate acid of ph-NH- is about 30 which is 5 units less than NaH so yes NaH can still deprotonate aniline
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Yes, you are right OrganicDan96 and I was wrong.
Thank you for the remark.
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As previously mentioned, trimethylhydrosilane cannot reduce alkenes and alkynes, neither can cleave the aromatic bromine without the appropriate organometallic or Lewis acid catalyst.
Furthermore, the pka of phenylacetylene is 28.8, which is very close to 30.
Please, see the proposed mechanism in the attached file below and let’s comment it.
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Please, see the proposed mechanism in the attached file below and let’s comment it.
i'm not convinced by the first step as hydride is a terrible nucleophile.
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Just a minute!
Alkali and alkali metal hydrides are bad nucleophiles due to their high basicity but this doesn’t mean that they are not nucleophiles at all.
To detail, alkali and alkali metal hydrides cannot reduce carbonyls due to the formation of the enol salt, neither can reduce esters due to the metalation of the α-carbon atom, nor substitute alkyl halides due to the formation of the corresponding alkenes and so on.
But in this case, there are no acidic or labile hydrogen atoms, except the aminopyridine ones that have about the same acidity with the silyl hydrogens (pka ≈ 30).
Determination of the Solution Acidity of Tris(trimethylsilyl)silane, Organometallics, (2002), 21, 3157-3161
http://www.admin.technion.ac.il/ApeloigYitzhak/ListOfScientific/154.pdf
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I agree with pgk after some scratchpaper work. There is no other way to get rid of that TMS group aside from as trimethylsilane, and the only way for that to happen is hydride attack followed by acetylene anion formation.
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Please also note that in this case, the 2-aminopyridine moiety further stabilizes the so formed amine anion by conjugation with the corresponding 2-imino-1,2-dihydropyridinine anion (see the attachment).
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Or even, by a more extended conjugation with the corresponding (2-imino-2,3-dihydropyridyl)allene anion, which also participates in the ring closure mechanism (see the attachment).