First, a series of thiol-based molecules were investigated as potential molecular catalysts for the isomerization of maleate to fumarate. Preliminary investigations showed the geometrical isomerization of maleate to fumarate using excess amounts of thiols (Fig. ). However, the reactions were less than stoichiometric, with the thiol-based molecules being consumed during the reactions. Even the best result achieved with N-acetyl L-cys- teine methyl ester (4) yielded almost stoichiometric conversion. This was primarily due to the formation of adducts between thiols and maleate as the main products (Figs. ), which accounted for the total thiol balance (Fig. ). This is in good agreement with the previous computational studies where the 1,2-adducts are thermodynamically the most stable among reactants, products, and intermediates generated via the ionic processes. The use of 2,2'-azobis(isobutyronitrile) (AIBN) in organic solvents led to a moderate acceleration in the conversion (Table , entry 3), but was not effective.