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Bromination of asarone
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Author Topic: Bromination of asarone
Sonson
Junior Member posted 06-29-98 04:33 PM
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I don't know if this subject has been discussed before...don't think so.
Well, it's all about addig HBr to asarone. It's well known that adding HBr to safrole under ionic conditions yields 2-bromo product, but in the case of asarone, which has the doublebond on the benzylic carbon, I found the theory a bit more tricky.
I'm not sure if adding HBr the free radical way to asarone yields 2-bromo...anybody?
Osmium
Member posted 06-30-98 04:23 AM
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Asarone is acid-sensitive. It dimerizes, even with much weaker acids than HBr.
Piglet
Member posted 06-30-98 05:38 AM
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Dimer, D'oh! You know, I always wondered if H2O2 (antimarkonikov addition) would allow acids to add the 'right' way, if you get my meaning. I wonder if the presence of peroxide has any effect? I did wonder if HOBr might add in an interesting way...
Just throwing in ideas,
Piglet 
assholium
Member posted 06-30-98 09:52 AM
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>benzylic carbon, I found the theory a bit more tricky.
>I'm not sure if adding HBr the free radical way to
>asarone yields 2-bromo...anybody?
Hm. How about this:
react asarone with N-bromosuccinimide in wet DMSO (quant.)
treat this bromohydrine (1-hydroxy-2-bromo-propane) with alch. ammonia (90-95%)
remove benzylic hydroxyl with you favorite route - f.e. Li/NH3, Pd/HCOONH4, etc... (85-90%)
I think that overall conversion asarone -> isopropylamine would be near 80%....
Details? Rhodium page, PPA synthesis
Sonson
Junior Member posted 06-30-98 04:50 PM
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Osmium: Ah, that's too bad eh? Do you know which way it polymerizes? Around the styrenecarbons or what? There should be some slick way of making 2-bromo asarone (sorry assholium...don't like the idea of removing that hydroxyl!)
Well, for one second forget asarone and think about isosafrole. If this substrate isn't acid sensitive would free radical addition of HBr yield the 2-bromo? It's obvious that this has to do with the stability of the carbocation intermediate...
Ritter
Member posted 06-30-98 08:44 PM
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I thought I'd throw my own two cents here about asarone and its acid sensitive nature. Many moons ago a friend of a fiend tried to make the 1,2 dibromo compound in hopes of thermally decomposing it into 2-bromo asarone. There is a synth for this rctn. w/ other propenyl benzenes in C.A. somewhere, dont have my notes on hand so can't give a citing. Well, anyway the reaction went like this: an ethereal solution of the propenyl benzene (isosafrole was dreamed of) was stirred up with some aq. 48% HBr in an ice bath and an equimolar(to the propenyl benzene) amount of N-bromosuccinimide was slowly added at a rate that kept the temp below 5'C. The ether was then sepped off, dried w/MgSO4 and evaporated to yield a quantitative amount of crude 1,2 dibromo compound as a greenish thick oil. The crude compound was then dissolved in a nonpolar solvent such as toluene and passed through a short column of silica. The silica absorbed all colored matter and filtrate was clear. After toluene was evaporated the pure 1,2 dibromo compound was left in about 90% yield as a clear oil. When this oil was heated to 120'C much gaseous HBr was driven off and B-bromosafrole was formed in perfect yield once again. This was a sweet synth for small amounts of bromosaf but scaling up could be expensive compared to the ways commonly circulating now. Oh, yeah- the best way to thermally decompose the molecule was determined to be heating it to the reflux point of hardware store xylene.
When this synth was dreamed about with asarone, as soon as the oil was put into contact w/ aq HBr it turned to black tar. What a nightmare. The C.A. citing gives details for every common propenyl benzene except asarone, now I know why.
Later,
Ritter
Osmium
Member posted 07-01-98 04:33 AM
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Sonson: I once saw a picture of the structure of that dimer. It was not symmetric, meaning that there was no cyclobutene in the middle. I guess it reacts via some cationic mechanism (H+ adds to double bond, and the shit then dimerizes). I've even seen poly(anethole) in the Aldrich catalogue, so that means that cationic polymerisation is indeed possible.
Ritter: I have some problems with your procedure. Thermally cleaving one HBr? doesn't that produce an alkene???
Ritter
Member posted 07-01-98 10:59 AM
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Osmium:
I'd think the same thing. The only thing I can do is dig up the ref later this week to prove it. Since the 1,2 dibromocompound is not a solid as it is in most other propenyl benzenes, they had problems characterizing it (ie. giving a mp.) This is when they go on to say that heating it from 120'C to 130'C thermally debrominates it to 2-bromosafrole, and it does indeed produce 2-bromosaf!
Osmium
Member posted 07-02-98 10:27 AM
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Strange. Really strange. But hopefully they know what they are talking about.
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