johnames
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De-Tert-Butylation of 4-t-butylacetophenone
Hello,
I am having trouble understanding the mechanism of de-tert-butylation specifically the de-tert-butylation of 4-t-butylacetophenone to acetophenone.
Does this follow the same mechanism as Boc deprotection? I haven't found anything on the mechanism de-tert-butylation but have found various papers
suggesting strong Lewis acids are a sufficient catalyst. I have also seen reports of aluminum chloride and various acid activated clays.
Can anyone help me understand the mechanism of de-tert-butylation or provide references for the de-tert-butylation of an aromatic aldehyde?
Thanks
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UnintentionalChaos
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It's a retro friedel-crafts alkylation. Same mechanism, but backward. IIRC, you usually add an activated aromatic ring to the reaction mix to behave
as a t-butyl cation trap.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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johnames
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So a common Boc deprotection would work fine with added anisole?
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UnintentionalChaos
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Not even close. t-BOC looks nothing like a tert-butylbenzene and the mechanisms are wildly different. I found a mechanism for a friedel crafts
alkylation and reaction conditions for a friedel-crafts dealkylation using simple google searches. I'm sure you can manage to do the same now that you
know what to look for.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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johnames
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Thanks. Looks like FeCl3 in toluene should do the trick. Toluene should accept t-butyl so no need for the anisole.
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Nicodem
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You can check a review paper on the use of the t-butyl protecting group for positions on the aromatic rings: M. Tashiro, Synthesis, 1979,
921-936. I'm sure you will find plenty of examples with further references there.
I would rather use some acid other than FeCl3 for this reaction. In most cases it tends to give dirtier products than AlCl3 in Friedel-Crafts
acylations, so I would guess it would give a dirtier product also in a retro-Friedel crafts alkylation of an acylbenzene.
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johnames
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Use another acid as in use AlCl3 instead? I also found a few examples using sulfuric acid as well as phosphoric acid (I use 85% phosphoric for Boc
deprotections). Maybe acetic acid as solvent, anisole as receiver and sulfuric acid or phosphoric acid would do the trick.
[Edited on 13-3-2011 by johnames]
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spirocycle
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^it is not t-BOC deprotection.
you cant compare the conditions indiscriminantly
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johnames
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I understand that, but I found examples of dealkylation of benzene using 85% phosphoric acid. I was just commenting that I also use 85% phosphoric
acid for Boc deprotections. Example of dealkylation in phosphoric acid can be found in the reference Nicodem posted, don't have access to the article
though so can't post it.
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spirocycle
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ok gotcha, misunderstanding
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johnames
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I finally found the review Nicodem is talking about, it is attached. Looks like AlCl3 in toluene/xylene is common, efficient, and high yielding. I
haven't found much on why FeCl3 leads to dirtier products. Can you expand on this Nicodem?
Attachment: s-1979-28876.pdf (1.7MB)
This file has been downloaded 530 times
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johnames
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Going to do side by side runs of FeCl3/toluene and sulphuric acid/anisole/acetic acid first. If nothing works out I will try the standby
AlCl3/toluene. FeCl3/toluene will be washed with cold water and then vacuum distilled to yield final product. Sulphuric acid/anisole/acetic acid run
will be extracted with CH2Cl2, washed, and vacuum distilled to yield final product.
Should I post results in this thread or the Organic Chemistry section? Any tips/advice would be appreciated.
[Edited on 14-3-2011 by johnames]
[Edited on 14-3-2011 by johnames]
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Nicodem
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Not much to expand. Like I said, my limited experience with FeCl3, based on few uses, is in that it leads to dirtier products in acylations when
compared to AlCl3. It does not mean that this is generally true, or that it is true also for the dealkylation you want to use it for.
Otherwise, your proposed parallel runs sound reasonable to me. Though sulfuric acid in acetic acid efficiently catalyses the aldol condensations, so
you might expect some troubles with these kind of side products given that your substrate is a ketone.
Quote: Originally posted by johnames  | | Should I post results in this thread or the Organic Chemistry section? Any tips/advice would be appreciated. |
If you are going to do such comparative tests and are willing to write a short report with some minimal analytical data and used references, then the
Prepublication section is the place to post experimental results. I'm sure there are chemists who will find your experience valuable. On the other
extreme, if you don't feel like it, or are in a lazy mood, you can always just post a comment on your results in this thread. Anything will be
appreciated, though the more you give the more it is appreciated.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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