An electrochemically driven two-phase SN2 reaction

We demonstrate an S(N)2 reaction driven either electrochemically or by phase-transfer catalysis and show how to use electrochemical measurements of ion transfer across the aqueous-organic interface to understand the details of the process. The model reaction is the formation of 2-hydroxy-3-methylbutyric acid from 2-bromo-3-methylbutyric acid in a two-phase catalytic reaction involving the electrochemically driven transfer of iodide and hydroxide from aqueous to organic phase, coupled to transfer of bromide in the other direction and an S(N)2 reaction in the organic phase. Voltammetry confirmed the catalytic function of iodide in this reaction. The system is modelled as an ECE scheme across the liquid-liquid interface. Simulations are compared with the experimental voltammetry, and show that the substitution reactions are not irreversible processes; rather they reach equilibrium on the electrochemical timescale (seconds). The estimated equilibrium constants are: RBr + OH(-) = ROH + Br(-) K approximate to 10(4) RI + OH(-) = ROH + I(-) K approximate to 10(6) A synthetic procedure using alkylammonium ions as phase-transfer catalysts to fix the potential difference across the interface confirmed the products predicted by the electrochemical measurements. (C) 2001 Published by Elsevier Science BN.