Author Topic: "Wet" Lewis Acid reactions  (Read 872 times)

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Ziqquratu

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"Wet" Lewis Acid reactions
« on: September 19, 2003, 11:14:00 AM »
Lewis-Acid Catalyzed Organic Reactions in Water. The Case of AlCl3, TiCl4, and SnCl4 Believed To Be Unusable in Aqueous Medium
Fringuelli, F., Pizzo, F. and Vaccaro, L., J. Org. Chem., 2001, 66, 4719-4722

Abstract:
Classical Lewis acids such as AlCl3, TiCl4, and SnCl4, believed to be unusable as catalysts in aqueous medium, efficiently catalyzed regio- and stereoselective azidolysis and iodolysis of alpha,beta-epoxycarboxylic acids in water at pH 4.0 and 1.5, respectively. The concept of water-tolerant metal-salt is re-examined in direct relationship to the aqua ion hydrolysis constant.


The two reactions tested in the paper were azidolysis and iodolysis of alpha,beta-epoxycarboxylic acids.  The whole idea is to make sure that the pH of the solution is kept below the pK1,1, which is determined using the equation:
K1,1=[M(OH)(x-1)+][H+]/[Mx+]
For all of us who could care less about maths, they generously provide us with a number of these values:

Cu2+ = 8.0
Al3+ = 4.97-5.5
Ti4+ = -0.3
In3+ = 3.0-4.0
Sn4+ = -0.57

Thus, to use the above ions as a lewis acid, they must simply be used in a solution with a pH lower than the above mentioned values - thus, for example, AlCl3 works perfectly in moderately acidic solution, while Cu(NO3)2 would work in a neutral solution.  It would depend on the substrate being used, as always.  A minor drawback is that a higher amount of catalyst is required than would normally be used under anhydrous conditions, but since for the most part they are reasonably cheap or easily made, then this isn't a great issue... and in the rest of the cases the catalyst can probably be recovered anyway.