[lutesium]

Dear Sir/Ma'am

What's the mechanism that BBr3 uses to de(alkyl)methylate ethers???


Lutesium...

[lutesium]

Any references for this???

[weiguxp]

have you checked organic-chemistry.org or wikichem.net?

[AWK]

Lutesium
All mechanisms you need are apparantly analogous.
Lewis acid attacs an electron pait of ether oxygen, then decomposition occurs with moving halide to methyl (or alkyl) group
ROR1 + BBr3 = ROBBr2 + R1Br and so on

Forget about separate mechanism for each reaction. There are only about 40 mechanisms for all organic reactions!

[lutesium]

I didn't know that!!! Is it really true that there's only 40 types of mechanism for all of the organic reactions???


Lutesium...

[lutesium]

Ok but where is the hydrogen source???


Lutesium...

[zq]

see http://www.alsnotebook.com/demethylation.html

[agrobert]

I didn't know that!!! Is it really true that there's only 40 types of mechanism for all of the organic reactions???
Lutesium...

Lutesium-

AWK is using this as an expression to convince you that it is unnecessary to memorize every reaction mechanism for all named reactions.  Your mechanistic understanding should come from a foundation of standard organic reactions (acid/base, substitution, elimination, addition-elimination, carbonyl, reduction-oxidation, organometallic and radical chemistry).  Indeed BBr₃ demethylation is a tricky one both on paper and in the lab.  The link provided by ppm is useful.  You should recognize the affinity of boron for oxygen thus being the reason why this reagent is standard for methyl ethers.

[lutesium]

Is BBr3 or AlBr3 a better candidate for demethylation??? Being more electropositive may help the Al counterpart be more effective or not??? Does electropositivity play a role in it???


Lutesium...

[AWK]

You can demethylate many  thousands ethers. The choose of reagent depends on many factors. For more complex compounds chemists are mainly interested in selectivity of breaking ether bond in the presence of other gropups, even other ether groups.
Note there are much more reagents for this purpose, and there are many modifications of parent  reagents.
It is impossible to aswer for your question generally.

[lutesium]

You're right AWK but as far as I know the mildest one is Prydinium HCl and the harshest one is HI or HBr are these right???

PS: I would appreciate if you can list the ether cleaving agents!!!

Lutesium...

[zq]

Check out this review: Recent advances in ether dealkylation, Weissman, S. A. et al Tetrahedron 2005, 61, 7833-7863.

[lutesium]

Thanks a million for your concern but I want to learn how can we choose the appropiate ether cleaving agent??? By experimenting??? That's not a logicial way in my sight!!! Is there a general fomula for it???


Lutesium...

[kalacrow]

Hello folks

I do believe that what L is trying to get at is a method to cleave an aryl methyl ether, such as eugenol, which are known to be notoriously difficult to do for an amateur.

He probably found the reference to Demethylating Eugenol to Hydroxychavicol (a great anti oxidant) via BBr3, which unfortunately needs to be done under a temp of -78.. let alone the fact that you are using BBr3 to begin with!

It's a useful thing to know how to do without having to use all sorts of toxic things or complicated equipment, but I have also found little in the way of references in the literature, except those using.. toxic things and complicated setups. Perhaps because it just has not yet been thought of to be simplified for industry.

Ordinary, common lewis acids, like the AL complexes (AlCl3, AlI3, etc) apparently work badly, not at all, or with very poor yields even with a PTC.

So, if those of you with better knowledge than I of the mechanisms at work here (Im really just a tinkerer!) could put a little thought into it, I'm sure we'd all appreciate the knowledge!

[kalacrow]

Quick addition: Now that Ive actually focussed my attention on this, I am finding some interesting references to thiols being used for this process.

Now.. I don't know about you, but this sounds absolutely nasty:

"a solution of sodium thioethoxide prepared from 287 mg 60% NaOH/mineral oil and 0.56 ml of ethanethiol in 10 ml of dimethylformamide. "

I also found a reference to a novel process that I haven't looked at yet. Those of you with University access should have no trouble getting the published article:

http://www.springerlink.com/content/ku154711580741r3/

Odorless thiols sounds enticing.

I would still like to hear if there is a novel, simpler way to cleave aryl methyl ethers, even in poor yield, that doesn't involve Boron or thiols. Thanks in advance!