Title

Electrochemical synthesis in microemulsions using cobalt complex catalysts dissolved or immobilized on electrodes

Date of Completion

January 2000

Keywords

Chemistry, Analytical|Chemistry, Biochemistry|Chemistry, Organic

Degree

Ph.D.

Abstract

Microemulsions of water, oil and surfactant are attractive as media for electrochemical synthesis due to their low toxicity and good solvent properties. ^ The first part of this Thesis documents successful stereoselective cyclization of 2-(4-bromobutyl)-2-cyclohexen-1-one to trans-1-decalone mediated by an electrochemically generated cobalt(I) complex in microemulsions. Selective formation of trans-1-decalone is attributed to equilibration of isomers via keto-enol tautomerization catalyzed by hydroxide ions formed by co-electrolysis of water. ^ The remainder of this Thesis focuses on using catalytic films in microemulsions for synthesis. Catalytic films of poly-L-lysine (PLL) and the cobalt corrin vitamin B12 hexacarboxylic acid [B12(COOH)6] were constructed onto oxidized carbon electrodes. Covalent bonds from electrode to PLL and PLL to B12(COOH)6 provided good stability in microemulsions. Films gave reversible electron transfer for the Co(II)/Co(I) redox couple and exhibited characteristic voltammetric features of surface-confined electrochemistry. Films also demonstrated excellent catalytic activity in microemulsions for reduction of organohalides. ^ Electrochemical and catalytic properties of the PLL-B12(COOH) 6 films were controlled by microemulsion composition. Segregation of SDS into micelles within cationic films results in charge reversal in the films. Catalytic turnover rates for reduction of dibromocyclohexane to cyclohexene mediated by the films in SDS microemulsions were controlled by the difference between the reduction potential of the reactant and the catalyst E o in the film. Thus, reaction rate in these systems is controlled by the intrinsic activation free energy. The catalyst Eo was controlled by microemulsion composition via interactions of surfactant and salt with the polymer network. ^ PLL-B12(COOH)6 films were constructed onto carbon surface layer by layer. Quartz crystal microbalance showed regular and reproducible layer formation. Electrochemical and catalytic properties of the Co IIL/CoIL redox couple in these films were investigated. These films obeyed theoretical predictions of a maximum in voltammetric catalytic efficiency as thickness increased for the reduction of 1,2-dibromocyclohexane in a microemulsion. In films with less than optimum thickness, kinetic control of the chemical reaction between CoIL and DBCH predominated. As film thickness was increased, electron and reactant mass transport became limiting factors. Application of these catalytic films in mediated carbon-carbon bond forming reactions is reported. ^