FireLion3
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OrganoZinc compounds as an alternative to OrganoMagnesium compounds?
https://en.wikipedia.org/wiki/Organozinc_compound
I stumbled upon the idea that Zinc can react with halides in a similar fashion to how Magnesium does, forming nucleophilic organometallic reagents
with halides. OrganoZinc Halides like this are typically quite nucleophilic (though not as much as Mg I would imagine), but most amazingly of all,
tolerant of water. A resulting OrganoZinc halide can react in many of the typical ways that a grignard reagent would react, are far less exothermic,
and can often be run at room temperature. The acidic work up is the same.
I was specifically interested in the Barbier Reaction:
https://en.wikipedia.org/wiki/Organozinc_compound#Barbier_Re...
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The Barbier reaction involves nucleophilic addition of a carbanion equivalent to a carbonyl. The conversion is similar to the Grignard reaction. The
organozinc reagent is generated via an oxidative addition into the alkyl halide. The reaction produces a primary, secondary, or tertiary alcohol via a
1,2-addition. The Barbier reaction is advantageous because it is a one-pot process: the organozinc reagent is generated in the presence of the
carbonyl substrate. Organozinc reagents are also less water sensitive, thus this reaction can be conducted in water. Similar to the Grignard
reaction, a schlenk equilibrium applies, in which the more reactive dialkylzinc can be formed.
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Some Grignard reactions are not high yielding because the intermediates can be especially sensitive to decomposition. Has anyone experimented with
these as alternatives to Grignards? They seem especially attractive for a large number of reasons, and elemental zinc is much easier to obtain than
magnesium. The lack of extreme exothermic reactions and tolerance of water allows for a great deal of synthetic ease over the traditional grignards.
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Metacelsus
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I've been interested in the Barbier reaction (possibly for benzylation; I have more benzyl bromide than I need) for some time. I have done a Grignard
reaction (triphenylmethanol synthesis) once, with moderate success. However, it was a real pain to set up.
The Barbier reaction usually is applied to allyl halides, but I suspect that a benzyl halide would work as well.
Now that I'm thinking more about it, I might try this.
I also wonder what would happen if I used a nitrile instead of a ketone. In the Grignard reaction, nitriles form iminium salts which are hydrolyzed
upon workup to ketones. If water is already present (as in the Barbier reaction), the hydrolysis might take place, and the ketone formed might react
again.
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FireLion3
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I am interested in the reactions with nitriles as well. I've never done a grignard with a nitrile but I have read that they can be low yielding
regardless of water (under 40%), and I'm not sure what the reason for that would is. If Organozinc halides are tolerant of water, then the resultant
iminium salt might also retain some of that zinc-related water tolerance. Just an assumption but it could stand to reason. This kinds of reactions are
not not limited to organozinc compounds, I don't think, but can be applied to organocopper compounds, and other compounds involving oxidative addition
to a metal.
[Edited on 23-7-2014 by FireLion3]
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