Iron porphyrin and cysteine mediated reduction of ten polyhalogenated methanes in homogeneous aqueous solution: Product analyses and mechanistic considerations

Reductive dehalogenation reactions of polyhalogenated C-1- and C-2-compounds are presently of particular interest because of the potential applicability of such processes in the treatment of wastes as well as in remediation approaches to removing such compounds from contaminated soils and aquifers. In this context, it is not only important to know the reaction kinetics of a specific compound with relevant reductants but also the type of product(s) formed under given conditions. In this study we have identified reaction intermediates as well as the final products of the reduction of 10 polyhalogenated methanes (PHMs) by an iron porphyrin in the presence of cysteine. Cysteine was chosen for two reasons: (i) as bulk electron donor and (ii) as an aqueous organic compound exhibiting functional groups (i.e., -NH2, -SH) that may undergo reactions other than just hydrogen abstraction with reactive intermediates such as radicals and carbenes. The data obtained support our hypothesis postulated in an earlier kinetic study that the initial and rate-determining step in the reduction of PHMs by the iron porphyrin is a dissociative one-electron transfer. Furthermore, it is shown that in fast consecutive reactions involving primarily the mercapto group of cysteine, all compounds were completely dehalogenated. Except for the fluorine containing compounds, the carbon of a given PHM was quantitatively recovered as N-formylcysteine. In the case of the fluorinated compounds, carbene intermediates could be trapped, which reacted further to some unidentified product(s), possibly including carbon monoxide. Finally, it is shown that the reduction of tetrahalomethanes by cysteine as sole reductant leads predominantly to the formation of the corresponding haloforms, suggesting that, in this case, the reaction occurred primarily by an X-philic dissociative two-electron transfer. The results of this study offer an interesting perspective for a fast complete dehalogenation of PHMs by using a very reactive one-electron donor (i.e., a reactive iron species) in the presence of organic matter exhibiting reduced sulfur groups.