Surface-mediated oxidations of sulfides, sulfoxides, and amines: Selectives and mechanisms

Previous studies in our laboratories have shown that the presence of silica gel or alumina can greatly facilitate certain reactions such as the hydrohalogenation of alkenes and alkynes. The application of similar surface-mediated techniques to the oxidation of sulfides and amines has now provided a selective, safe, and environmentally-sound procedure for the synthesis of sulfoxides and hydroxylamines, as well as the synthesis of sulfones from their sulfoxide counterparts.; In most cases these surface-mediated methods were found to be more selective than their solution-phase counterparts. In the case of secondary and primary amines these methods have produced the first convenient procedure for the selective oxidation to hydroxylamines using affordable, commercially available oxidants. These surface-mediated reactions also have the advantage of being environmentally-sound, using benign, easily recyclable adsorbents; that obviate the need for potentially toxic heavy-metal catalysts that are often required in solution-phase oxidations.; The mechanisms of these oxidations have been studied, and selective adsorption of the oxidants to different sites on the surface of silica gel has been discovered. This is apparently the first case in which discriminatory adsorptions have been considered for mechanistic models. The excellent selectivities observed for surface-mediated oxidations can also be explained by these mechanisms.; Recent application of these techniques to microwave-assisted oxidations has been studied. Although several reports in the literature concerning “solventless” microwave reactions have used silica gel and alumina, we are apparently the first to consider the role of the surface. Surface-mediated oxidations of sulfides using liquid and supercritical carbon dioxide, an environmentally advantageous solvent with easily varied physical properties, have also been studied.