kenofken
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Benzyl iodide prep and purification
I recently prepared some benzyl iodide for use as an alkylating group in some porphyrin-related work I'm doing. I made it from benzyl chloride with a
Finkelstein swap, and that was straightforward enough.
The problem I'm having is that my product is dark - seems to have free iodine in it. In one or two previous alkylation reactions, this seems to be
reducing some groups its not supposed to, probably in a way similar to ephedrine reduction. How can I purify this stuff? It's damn near impossible to
distill, though I might be able to try again with a stronger vacuum. I can't wash it with thiosulfate because it hydrolyzes quickly in water.
Activated carbon did no good. My next thought is to try a bit of separation on a column with silica gel, although in TLC, the Rf values of the iodine
and compound seem too similar.
If anyone has any ideas, I'd be mighty grateful. I may simply try the whole thing as a one-pot Finkelstein, to generate the benzyl iodide in-situ with
a bit of sodium iodide. I suspect I'll still have the subsequent I2 formation and maybe the formation of HI which will in turn trash my imines and
other groups I want untouched.
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Paddywhacker
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I have heard that some shiny copper turnings will keep the free iodine down, but iodine-containing organics will nearly always behave as you see.
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Dr.Bob
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I don't think benzyl iodides are very stable, that is why most come as the chloride or bromide. I have only made a few alkyl iodides, and the ones
that I tried that were allyl or benzyl types mostly were too unstable to store. But for non-benzylic, saturated, alkyl groups the iodides are
usually stable and react very well. I have also tried to generate a benzyl iodide from a benzyl alcohol and an iodinating reagent (from a vague lit.
prep), and that was not too pretty as well.
I would either just use the chloride as is, convert to the bromide if needed, or do the in-situ with KI, which might well work best. Did you try
alkylating with the original chloride already? I have had good luck with those being hot enough usually.
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kenofken
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We have the chloride and bromide versions in the supply room. I ought to try those. There was a concern that we would need a better leaving group
halogen, but I won't know if I don't try.
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kristofvagyok
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Quote: Originally posted by kenofken  | | We have the chloride and bromide versions in the supply room. I ought to try those. There was a concern that we would need a better leaving group
halogen, but I won't know if I don't try. |
Benzyl bromide usually works well for every purpose, if a special reaction is needed, then they usually prepare the benzyl-iodide freshly, if not...
Then they clean the reaction product.
Can I ask that what would be this reaction what needs benzyl iodide?
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kenofken
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The end game of what I'm doing is porphyrazines. The alkylations I'm working on will provide some side chains and subsitutions along the outside of
the macrocycle when all is said and done. I'm looking to alkylate amine groups which already bear an ester we'll need as a protecting group in
cyclization.
So why do I need an iodinated alkyl group as opposed to something else? I don't know for sure that I do, but I have reason to suspect I might. The
amines I want to alkylate are part of a system which includes nitrile groups and which seems to display some strong electron-withdrawing behavior. It
would explain why we could get a Fisher esterification to proceed at room temperature earlier in the workup. So if we have electron density being
pulled away from our reaction sites, those amines we're targeting are likely to be shabbier nucleophiles than usual, so we might need a more willing
leaving group.
We figured we'd give the reaction the best chance possible, and also figured that any fool with a jar of sodium iodide and some acetone could produce
an alkyl or benzyl iodide. We were right, but now there's this cleanup and/or stability issue. If I don't get lucky with silica gel Monday, I'll try
one of the other halides. Among its other faults, benzyl iodide is a hideous lachrymator, even in microliter amounts with a fume hood and the usual
precautions. The sooner I'm done with it the better!
Here's a look at the stuff, you can see it's fairly murky!
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kristofvagyok
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| Quote: Originally posted by kenofken | The end game of what I'm doing is porphyrazines. The alkylations I'm working on will provide some side chains and subsitutions along the outside of
the macrocycle when all is said and done. I'm looking to alkylate amine groups which already bear an ester we'll need as a protecting group in
cyclization.
....
Here's a look at the stuff, you can see it's fairly murky! |
Finally, a good research, hope you will have good results!
If you want to alkylate the inner nitrogens if the porphyrazine skeleton, then I think the problem will be that the benzyl group needs a lot space and
it simply won't fit in. Have this molecule been described somewhere? Or this is just your idea to protect it on this way?
Pyrrole-NH alkylation with benzyl works easily, usually goes in hot acetone or any other solvent (I last used toluene at 80Celsius), with some KHCO3
or K2CO2 as a base and it usually has a really good yield (I used bromo halides and had a 80% yield).
P.S. That benzyl iodide doesn't looks so bad
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kenofken
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The bulk of a benzyl ring hopefully is not an issue. I haven't even gotten that far to know. However, by our route, and the only way I know so far,
the substituents are added prior to forming up the macrocycle. I still have a bit of trouble visualizing the cyclization mechanism, but when all is
said and done, the subtitution groups end up on the beta carbons of the pyrroles, in other words, on the outside of the whole structure. The nitrogen
integral to porphyrazine are spoken for. The ones that link the rings together and take the place of meso-carbons in porphyrin, have their hands full,
so to speak. The innermost nitrogens pretty much only have the capacity to hold magnesium or a metal of similar radius, which is usually removed at
the end.
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