smaerd
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Ullmann Reaction Questions
http://www.organic-chemistry.org/namedreactions/ullmann-reac...
According to this it states that, a Cu(i) salt/cation catalyzes a reaction in a basic media, a nucleophile and and aryl halide.
I'm having a hard-time finding out further information about this reaction in practice verses on paper. Most examples of it use it as a aryl coupling
reaction, such as in:
I'm assuming this is done in water, with a weak nucleophillic base(Potassium Carbonate?). Can anyone confirm or deny this?
Also is this likely to work on an aryl alkyl halide? If not is there any reaction where a nucleophile will substitute with the halogen in an aryl
alkyl halide?
Thanks again scimad!
[Edited on 10-7-2011 by smaerd]
[Edited on 11-7-2011 by smaerd]
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Nicodem
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Quote: Originally posted by smaerd  | | I'm having a hard-time finding out further information about this reaction in practice verses on paper. Most examples of it use it as a aryl coupling
reaction |
You are mixing up two different reactions: the Ullmann biaryl synthesis and the Ullmann condensation.
| Quote: | | I'm assuming this is done in water, with a weak nucleophillic base(Potassium Carbonate?). Can anyone confirm or deny this? |
If you mean the Ullmann condensation, then yes, some substrates can be used in the presence of water or even in water as solvent, but generally these
substrates must be aryl halides that have internal ligand sites, like 2-iodobenzoic acid and such. But why assuming? Rather check the literature on
examples related to what you want to achieve. Obviously, as always, you first need to understand the reaction mechanism in order to understand how
applicable is the literature example to your particular reagent combination.
The Ullmann biaryl synthesis does not work in the presence of water, protic solvents and generally fails on substrates having acidic functional
groups.
| Quote: | | Also is this likely to work on an aryl alkyl halide? If not is there any reaction where a nucleophile will substitute with the halogen in an aryl
alkyl halide? |
No, alkyl halides can not undergo the Ullmann condensation, and obviously can not undergo Ullmann biaryl synthesis (as already the name implies).
Check the mechanisms of both reactions to understand why not.
However, primary alkyl halides can easily undergo SN2 based reactions, provided the elimination reaction represents no obstacle. Obviously, without
you specifying what you are talking about, nobody can give you a solution.
…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)
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vulture
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| Quote: |
I'm assuming this is done in water, with a weak nucleophillic base(Potassium Carbonate?). Can anyone confirm or deny this? |
No base required for biaryl formation. Solvent of choice is usually DMF or something else polar aprotic and high boiling. Ullmann biaryl synthesis is
messy, requires forcing conditions and generally gives low yields unless for activated substrates. If possible, it is replaced by Pd- or Ni-catalyzed
reactions.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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