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Author: Subject: Terminal double bonds
StrangelyBromine
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[*] posted on 17-10-2016 at 06:31
Terminal double bonds


Is there a reagent that has a preference for terminal C=C bonds over non terminal? Something akin to the way mercuric acetate would bond, but showing selectivity for the bond being terminal.

I'm having trouble separating isomers of a compound with a double bond that likes to move its way up the carbon chain if the reaction gets too hot. I'm hoping to isolate the compound when it is in the terminal bond state as recrystallisation via solvents is proving troublesome.

Any information would be greatly appreciated.
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Melgar
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[*] posted on 17-10-2016 at 11:43


I believe the word you're looking for is "terminal alkene/olefin", correct?

As for your question, I know a lot of reactions that are selective for terminal alkenes, but then you no longer have an alkene, you have something else. I can't think of any that are easily reversed, but maybe someone else here has more experience with olefin chemistry than I do.
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ziqquratu
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[*] posted on 17-10-2016 at 14:53


I've never used it - or even seen it used - but I understand that chromatography using AgNO3-impregnated silica is an effective way to separate isomeric olefins.

A quick search turned up this review that may be of interest:

Chromatography with silver nitrate
Craig M Williams, Lewis N Mander
Tetrahedron, 2001, 57(3), 425–447

Attachment: Chromatography on silver nitrate.pdf (296kB)
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CuReUS
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[*] posted on 17-10-2016 at 17:44


Quote: Originally posted by StrangelyBromine  
I'm having trouble separating isomers of a compound with a double bond

You will need to do a proper fractional vacuum distillation to separate safrole from isosafrole.
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Maroboduus
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[*] posted on 17-10-2016 at 21:13


Quote: Originally posted by CuReUS  
Quote: Originally posted by StrangelyBromine  
I'm having trouble separating isomers of a compound with a double bond

You will need to do a proper fractional vacuum distillation to separate safrole from isosafrole.


I think in safrole the double bonds like to move down to the propenyl position when you heat it.
If I'm right, then he is talking about something else.

But a little more info characterizing what you're doing might help us to help you.
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ziqquratu
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[*] posted on 17-10-2016 at 21:37


"Recrystallisation via solvents" sort of rules out (iso)safrole, given both are liquids. I recall that safrole, at least, can be purified by low-temperature fractional crystallisation, but solvents would surely be counterproductive. Also, if I recall correctly, heat alone isn't sufficient to isomerise safrole to isosafrole. And the majority of routes to MDMA prefer the higher-boiling and thermally-stable isosafrole anyway (the same generally holding true for other allylbenzene-type precursors).

Any particular reason not to give a new member the benefit of the doubt before reflexively labelling them a drug cook? Especially when the facts as presented are plausibly inconsistent with the compounds you're assuming are involved? There are many reasons that a person might not post extensive detail - including employment restrictions (such as would limit my posting), intellectual property concerns, or just a lack of familiarity with the practices of the forum. It doesn't have to be nefarious.
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DJF90
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[*] posted on 18-10-2016 at 01:35


As Ziqquratu has mentioned, argentation chromatography is generally the go-to for separating regiomeric/geometric isomers of alkenes. If you want to isomerise to the terminal alkene then you may have a couple of options:

The first is by hydroboration and thermal rearrangement of the obtained borane (it will migrate to the end of the chain). React with iodine to get the primary alkyl iodide, and eliminate with appropriate base.

The other option I could think of would be to halogenate/dehalogenate the alkene to get the internal alkyne, and then do an alkyne zipper reaction to get the terminal alkyne (https://en.wikipedia.org/wiki/Alkyne_zipper_reaction). Selective reduction to the alkene is then required, but should be possible.

Obviously both approaches require compatibility of the rest of the molecule to the reaction conditions.
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StrangelyBromine
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[*] posted on 18-10-2016 at 08:46


Thank you all for the informative responses. I apologise for not giving a satisfactory level of characterisation bit if it would help convince you I'm not a drug cook then sure: My starting material was 2-ethyl pyridine, I am now dealing with the propenyl tail that I've attached. I understand the chemistry I'm asking is relatable to isosafrole and safrol but unfortunately for the imaginative I'm doing chemistry that's unrelated to those things.

Given response so far I will try DJF90's ideas. I'll also do a dual run with argentation chromatography from Ziqquratu's response.

I'll let you know how it turns out if you like.
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