[km]

I have a question about the Negishi reaction.. Why it is not usually done under aquous conditions?
Thanks in advance.

[voidSetup]

I think the Negishi reaction utilizes organozinc comounds which are similar to Grignard reagents.  The carbon is more electronegative than the zinc/magnesium, and they are very sensitive to oxidation.  Protic solvents will cause them to decompose.

[voidSetup]

Hm I'm actually not so sure about this because the Barbier reaction works with water present, but it's one-pot.

[democanarchis]

I don't think there is an issue with organozinc reagents being water sensitive over the timeframe of the reaction, as mentioned the Barbier reaction works fine. The issue is that the organozinc species is typically generated in situ, usually from an organic halide via deprotonation with BuLi and then transmetallation with ZnCl₂. Obviously, BuLi and water don't play nice.

More generally, the temperature range during this kind of reaction rules out the use of water; formation of the organozinc reagent is typically carried out at sub zero temperatures.

That said, the limitations above are based primarily on being restricted due to the formation of the organozinc species. A brief lit search showed that Krasovskiy et al. [dx.doi.org/10.1021/ja906803t] published a Negishi type coupling between two aryl halides, using Zn powder in a completely aqueous solution to facilitate direct insertion as opposed to transmetallation.

The Suzuki reaction is well developed in respect to aqueous systems, so I imagine it's just a matter of time before the scope of aqueous Negishi couplings is expanded further.