Melgar
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How to choose a base catalyst for nitroaldol/Henry reaction?
I've read quite a bit on the nitroaldol reaction, though for some reason, there's rarely any explanation on choice of base to catalyze it. I've read
that weak bases are generally preferred, and yet, the base that ends up being used is often a very strong base, like potassium hydroxide. Considering
that potassium hydroxide seems to catalyze the decomposition of nitromethane into a brown, unusable liquid, this does not seem like a very good idea,
and yet there are numerous examples of it being used in literature. Why is this? Is it just faster, and so better for demonstration purposes? It it
because it's a desiccant?
I've also read that amines make good catalysts, and that it matters whether you choose a primary, secondary, or tertiary amine. IIRC, choice of
catalyst is based on whether you want a ketone, a nitroalkene, or a nitroalcohol. There also seems to be a lot of papers out there about how to
choose a base such that the reaction is stereoselective for certain substrates, but if you hydrogenate the double bond and reduce the nitro group
afterwards, stereoselectivity at this step isn't very useful, unless you want the secondary alcohol or are able to do a stereoselective alkene
hydrogenation, correct?
So, if you want the nitroalcohol, you would perform the reaction in a mixed solvent that included water, and use a bicarbonate or tertiary amine as a
base, correct? As for using secondary or primary amines as a base, under what conditions would those be desireable? Are there any conditions under
which ammonia should be used as the base?
Also, presumably some substrates are more prone to forming the nitroalcohol, and some the nitroalkene, and you can use whichever conditions are most
convenient if the substrates are prone to forming the one you want. Is there a good rule of thumb for determining which product would be favored?
I rarely seem to be able to find good explanations along these lines anywhere. Is this because that information is typically covered in the regular
aldol reaction? Because this is a reaction I know comparatively little about, so that could very well be the case.
Incidentally, I just reacted vanillin and nitromethane in methanol, using caffeine as the base catalyst. It turned from pale yellow, to orange
overnight. How would I determine when the reaction is complete? I'd guess when it stops smelling of vanillin, but I'm trying to get away from using
"nose chromatography" so much.
[Edited on 7/20/16 by Melgar]
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stoichiometric_steve
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It's rather difficult to determine what exactly your question is here, TBH.
Different substrates (aldehydes and nitroalkanes) favour different amine catalysts. It would be best for you to conduct a google search on the
substrates you're interested in reacting.
Ever heard of TLC?
[Edited on 20-7-2016 by stoichiometric_steve]
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Melgar
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| Quote: | | It's rather difficult to determine what exactly your question is here, TBH. |
The ones that end in "correct?" are yes or no questions, for one. I'm trying to make sure that the bits and pieces I've picked up are fitting
together correctly. And yes, I've been doing a lot of research. For some reason, it's been very difficult to find a writeup for any logic concerning
base selection, unless it's for stereoselectivity. For instance, why does anyone ever use KOH as the base for a nitromethane reaction, when it
degrades nitromethane into brown goop? Do all bases do this?
| Quote: | | Different substrates (aldehydes and nitroalkanes) favour different amine catalysts. It would be best for you to conduct a google search on the
substrates you're interested in reacting. |
But that's just it. I don't want a recipe. I want to understand how the reaction works, what changing reaction conditions are likely to do, come up
with hypotheses based on that understanding, and test those hypotheses via experimentation. That is, I want to do science, not cookery.
That girl band from the 90s?
Seriously though, yes, I've heard of thin layer chromatography, but I'm not really sure what's the best way to get started with it. Even on eBay, TLC
plates are a couple bucks each when you buy like 100, and I could burn through money very fast, considering how often I'd want to test things. I'd
imagine it's possible to make your own TLC plates, but don't you need some sort of reactive indicator to make the lines visible? You apply the
indicator after waiting a set amount of time, correct? How do you apply the indicator? What if your reaction products are volatile, and there's
nothing you can do about it? Just use a really volatile carrier so that the carrier evaporates first, and then make sure to apply your indicator as
soon as your carrier evaporates? With water glass and plaster of Paris, DIY chromatography plates should be pretty cheap and easy, as long as you're
consistent with them. Chromatography is a whole branch of chemistry that I've really wanted to get into, but haven't, mostly because there are
usually enough visual or olfactory cues as to whether a reaction has taken place, but I'm getting to the point where that will no longer suffice.
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stoichiometric_steve
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What's stopping you from actually researching the matter? That's what science is all about. The primary literature has a vast array of previous
research on this topic.
To me, your questions appear just borderline to spoonfeeding.
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Melgar
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Quote: Originally posted by stoichiometric_steve  | What's stopping you from actually researching the matter? That's what science is all about. The primary literature has a vast array of previous
research on this topic.
To me, your questions appear just borderline to spoonfeeding. |
Would it kill you to post a link or something (to the nitroaldol stuff, not the TLC stuff, I admit I haven't researched TLC extensively), rather than
being insultingly unhelpful? Did you see the two times where I mentioned researching this, and only being able to find information that wasn't
relevant to my questions? It'd be better if you didn't respond to this thread at all, rather than just smugly telling me to "research it", as though
I'm so stupid that I didn't even bother to type some search terms into Google. Considering this is the "Beginnings" forum, isn't this the place where
basic questions are supposed to be asked and answered?
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AvBaeyer
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Melgar,
Take a look at this reference. Perahps it will give you a start on what you seek.
AvB
Attachment: The Henry reaction_recent examples Tet Report.pdf (584kB)
This file has been downloaded 428 times
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Melgar
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Thank you! This is the kind of thing I was looking for: something more in-depth than the Wikipedia page, but not some writeup of an ultra-specialized
organometallic catalyst containing two platinum-group metals in a porphyrin complex or whatever unobtanium-catalyzed reaction happened to be popular
that year. It'll take some time to fully absorb the contents, but I'll definitely be less of a noob the next time I have questions. 
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