[Messi]

Hey guys,

When exactly can an amine be used as a proton source? If I just have lets say a secondary amine or a primary amine, how can that amine be used as a proton source? Will it actually be used as a proton source?

Thanks!

[Babcock_Hall]

Do you know the pK_a of a typical amine?

[Messi]

Pka of NH3 is 38!

But, I was reading online where a paper said you don't need to use an acid source if one uses two equivalence of amine because the amine will act as the proton source.

[Babcock_Hall]

Can you supply the context?

[Messi]

Yes, certainly. I have posted below snips of the paper where it explains the two equivalents of the amine. Let me know if you would like me to add more information:

[Babcock_Hall]

I have to wonder whether the proton source in question is an organic ammonium ion, as opposed to an amine.  Simple alkyl thiols have pK_a values near 8-10 and are not going to take a proton away from an amine.

[Messi]

Where would the organic ammonium ion come from...?

[Babcock_Hall]

I was just thinking out loud, with respect to the possibility of an ammonium ion, and I was probably going down a dead end.  But it occurs to me that if the equilibrium shown in equation 33 is very far from one, it would affect the pK_a of the sulfur.  I don't have any experience with thioketenes, however.

[Messi]

Yah, I'm as confused as you regarding the author's comment regarding the amine being a proton source

[Babcock_Hall]

I think that the amine as a proton source is plausible.  If we knew the pK_a of the thioketene in equation 33, it would be very helpful.  I checked a couple of tables of pK_a values, but I did not find anything.

[orgopete]

Yes, certainly. I have posted below snips of the paper where it explains the two equivalents of the amine. Let me know if you would like me to add more information:

When I look at this and assuming H⁺ is the acid, I don't get the inference that ammonia or an amine was used. If the text elsewhere indicateds an excess of amine was used, I don't think of ammonia as an acid except in limited cases, alkene reductions. It isn't unusual to use ammonium chloride or another buffered acid. If this is the case, then it probably is as valuable to know why an ammonium salt is being used. It provides a proton source and excess will not result in a strong acid. Carbonic acid can also be used similarly.

[Messi]

When I look at this and assuming H⁺ is the acid, I don't get the inference that ammonia or an amine was used. If the text elsewhere indicateds an excess of amine was used, I don't think of ammonia as an acid except in limited cases, alkene reductions. It isn't unusual to use ammonium chloride or another buffered acid. If this is the case, then it probably is as valuable to know why an ammonium salt is being used. It provides a proton source and excess will not result in a strong acid. Carbonic acid can also be used similarly.

Well from my understanding of the paper, the amine is used as the generation of the thioketene as clearly stated by the authors.

[Dan]

Messi, can you post the citation for that paper (minimum: journal name, year, page number). Those who have access to it can then read it in full and perhaps shed more light on the question.

NB: Do not post the full paper (breaches copyright law), only the citation.

[Babcock_Hall]

orgopete,  Drawing an analogy with a dissolving metal reduction of an alkyne might not be too far-fetched, perhaps.

[Messi]

Sorry, I was away on business.

The chemistry of thioketens.Tetrahedron, Volume 44, Issue 7, 1988, Pages 1827–1871.

http://www.sciencedirect.com/science/article/pii/S0040402001903291