Boffis
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Quaternary pyridine compounds
The route to quaternary pyridinium salts via alkyl halide is well known and there are several examples in posts on SM. But can other alkylating agent
such as methyl tosylate or dimethyl sulphate be used?
I have found several general chemistry books that state that they can but the claims are un-referenced and I can't find an actual example. I did find
one reference that I haven't checked out yet that claims that even methyl picrate (2,4,6-trinitroanisole) reacts with pyridine to give
1-methylpyridinium picrate!! So surely more reactive alkylating reagents such as dimethyl sulphate do but there appear to be few published examples in
spite of the fact that the presence of iodide ions in the reaction mixture would be problematic. Has anyone on the board even seen or used a procedure
that uses quaternarisation of simple pyridines with such reagents?
That then raises the question of reaction ratios. Would dimethyl sulphate react 1:1 or 1:2 i.e. would the product be methylpyridinium methylsulphate
or bis(methylpyridinium)-sulphate or are both possible?
My interest is in the possibility of producing iodide /halogen free cyanine dyes
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Quieraña
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I can't answer your question, but I recognized an otc methylating agent, very mild by comparison to methylene chloride: dimethyl carbonate. It's
available at hardware stores as dichloromethane's replacement.
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Boffis
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What is? Methylpyridinium methylsulphate?
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andyloris
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I think he meant that dimethyl carbonate was available at hardware stores as dichloromethane replacement.
I am not sure that dimethyl carbonate would alkylate pyridine since the latter is such a poor nucleophile...
Moreover, methylations using dimethyl carbonate seem only to proceed at temperatures above 120°C.
I found a paper on the N-methyl alkylation of pyridine using Alcohols and Carboxylate Esters.
[Edited on 2-1-2024 by andyloris]
Always trust your first idea, especially if you only have one.
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clearly_not_atara
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A few studies suggest that it works with mesylates:
https://www.sciencedirect.com/science/article/pii/S2210271X1...
The tosylate is very likely similar. Pyridine has very low steric hindrance, which is the only reason I can think of why tosylates would not react
like halides (even then for SN2 it shouldn't matter).
MeOTs can bee had by the rxn of TsOH with Me2CO3 according to some papers in the old methylation thread.
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Niklas
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Quote: Originally posted by Quieraña  | | but I recognized an otc methylating agent, very mild by comparison to methylene chloride: dimethyl carbonate. |
Using dimethylcarbonate as a methylating agent doesn’t really work unfortunately, to get its reactions going you pretty much always need an
autoclave making it quite inaccessible for amateur chemists..
For simpler substrates dimethyloxalate would have potential tho, since apparently, under addition of potassium carbonate, it can act as a methylating
agent to both alcohols and amines if kept under reflux in DMF [1]. In general I don’t think the method is too great, because of the high
temperatures required being able to decompose lots of substrates and the parallel decomposition of the solvent causing side reactions to occur, I
believe it may still be suitable to be used in home labs tho
[1] https://www.sciencedirect.com/science/article/abs/pii/S00404...
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kmno4
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Such methylation of pyridine by DMS is commonly known, for example here : http://www.orgsyn.org/demo.aspx?prep=CV2P0419
It is strange that you could not find this or similar procedure....
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