PirateDocBrown
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I had a conversation about cleaving alkenes.
I was speaking to a fellow amateur chemist about the oxidative cleaving of alkenes, yielding ketones and aldehydes. I had done this much as a
professional, sometimes where the products needed to be analyzed for structure, where I used an ozone generator. But for synthesis, a strong oxidizer,
such as KMnO4, would be employed. Naturally, this was not always a possibility, as the compound could have other parts that could be destroyed by such
conditions, which was where this conversation was leading.
Eventually, he suggested halogen addition across the double bond, and then lysis by means of a base, i.e. nucleophilic attack by a hydroxyl group on
the sigma bond between the vicinal halides. I could think of many reasons this might not work, the least of which is the possibility of nucleophilic
substitution on one of the carbons, releasing a proton.
Has anyone ever heard of this reaction? Does it ever work? Or could he be confused?
Does anyone know of any other alternative route to oxidative lysis of an alkene?
[Edited on 12/11/17 by PirateDocBrown]
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clearly_not_atara
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The reaction does occur for some aromatic alkenes, but in the only case I know, the substrate is a diene, which might explain the unusual
transformation. That comes from this old Hive reference:
https://www.erowid.org/archive/rhodium/chemistry/3base/piper...
which has the worst citations ever (actually most citations on Rhodium's Archive are pretty bad) but the key step appears to come from here:
http://www.sciencedirect.com/science/article/pii/S1876081308...
I also want to say that KMnO4 is a much more selective oxidant when used under PTC in stoichiometric proportion. This is described by Ogino &
Mochizuki in a 1979 article using triethylbenzylammonium chloride in DCM to selectively convert alkenes to diols or aldehydes
Attachment: ogino1979.pdf (5MB)
This file has been downloaded 349 times
[Edited on 04-20-1969 by clearly_not_atara]
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PirateDocBrown
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I didn't know there was so much on Erowid/Hive/Rhodium, however badly attributed. I thought it was mostly meth cooks swapping stories, and asking dumb
questions, like the ones we see here.
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Cryolite.
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There is a surprising amount of good chemistry hidden throughout Rhodium and the old Hive. For example, here: https://the-hive.archive.erowid.org/forum/showflat.pl?Cat=&a... is a very clever synthesis of deschloronorketamine via copper-catalyzed carbene
insertion into cyclopentanone. It's definitely worth looking around the archives if you have the time.
But on the topic of alkene cleavage,it seems to me that as soon as you can get a diol, periodate cleavage is straightforward (and periodate is not
difficult to make). So another direction to take this question is: are there good simple conditions for dihydroxylating an alkene in an amateur
setting? Epoxide formation + hydrolysis is natural of course, but there is also iodine + silver acetate for this transformation. Thoughts?
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PirateDocBrown
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I have considered periodate, but it would have the same drawbacks as MnO4-, i.e., oxidative attack on other parts of the target molecule. I'm thinking
epoxide formation conditions could be similar. But I2 + AgOAc, hmm. That has possibilities.
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CuReUS
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Quote: Originally posted by PirateDocBrown  | | Eventually, he suggested halogen addition across the double bond, and then lysis by means of a base, i.e. nucleophilic attack by a hydroxyl group on
the sigma bond between the vicinal halides. |
Even I am confused as to how this reaction occurs.I can understand the formation of a diketone or a keto-aldehyde,but how does the C-C bond break
? | Quote: Originally posted by PirateDocBrown | | I have considered periodate, but it would have the same drawbacks as MnO4-, i.e., oxidative attack on other parts of the target molecule. I'm thinking
epoxide formation conditions could be similar. |
you only need cold KMnO4 to convert an alkene to glycol,I don't think it would affect anything else.
Once you have the glycol,you could cleave it into ketones or aldehydes using lead tetraacetate
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zed
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Shulgin uses an odd method to cleave an alkene in order to produce myristicin aldehyde.
Involves nitration, followed by a reverse-aldol type reaction.
"A solution of 50 g isomyristicin in 300 mL dry acetone containing 24 g pyridine was vigorously stirred and cooled to 0 °C with an ice bath. To this
there was added 54 g tetranitromethane which had been pre-cooled to 0 °C. Stirring was continued for exactly 2 min, and then the reaction was
quenched by the addition of a cold solution of 16.8 g KOH in 300 mL H2O. Stirring was continued until the temperature had again been lowered to near 0
°C. The product was removed by filtration. Extraction of the filtrate with CH2Cl2 and removal of the solvent provided additional nitrostryrene, for a
combined yield of 50.7 g with a mp of 103 °C due to the presence of a small amount of free myristicinaldehyde. A recrystallization from MeOH produced
1-(3-methoxy-4,5-methylenedioxyphenyl)-2-nitropropene with a mp of 109-110 °C. This material was completely adequate for the above-described
reduction to MMDA. The conversion of this nitropropene to myristicinaldehyde is an alternative to the lengthy synthesis given above), and can be used
in the preparation of LOPHOPHINE.
A mixture of 50 g 1-(3-methoxy-4,5-methylenedioxyphenyl)-2-nitropropene and 26 g racemic a-methylbenzylamine was heated on the steam bath. The mixture
gradually formed a clear solution with the steady evolution of nitroethane. When the reaction became quiet, there was added a mixture of 20 mL
concentrated HCl in 100 mL H2O. The reaction mixture dissolved completely, and as the temperature continued to rise there was the abrupt
solidification as the formed myristicinaldehyde crystallized out. This product was removed by filtration and, when combined with a second crop
obtained by the hexane extraction of the filtrate, gave 36.9 g of myristicinaldehyde. The mp of 128-129 °C was raised to 133-134 °C by
recrystallization from hexane."
Alternately, it may be possible to avoid the use of Tetranitromethane for nitration.... https://www.erowid.org/archive/rhodium/chemistry/nitryliodid...
The clever part, if your substrate permits it, is distilling off the low boiling nitroalkene, while capturing the aldehyde fragment as a high boiling
imine.
Wish I had invented that.
[Edited on 13-12-2017 by zed]
[Edited on 13-12-2017 by zed]
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clearly_not_atara
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The alkene nitration can also be performed by the addition of N2O3 across the double bond, which forms a dimeric "pseudonitrosite", this then being
thermally cleaved to release H2N2O2 and the nitroalkene. However, the rxn is only regioselective for aryl alkenes. The pseudonitrosite is generally
agreed by... certain people to be more practical than nitryl iodide and safer than tetranitromethane.
https://erowid.org/archive/rhodium/chemistry/pseudonitrosite...
[Edited on 13-12-2017 by clearly_not_atara]
[Edited on 04-20-1969 by clearly_not_atara]
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