Oxidation of sulfides plays a significant role in numerous fields such as wastewater treatment, chemical warfare agent disposal, fossil fuels desulfurization and preparation of various pharmaceuticals. Therefore, the tremendous effort has been made to develop chemoselective transformation of sulfides to sulfoxides. Most common synthetic approaches rely either on hazardous oxidizing agents like peroxides and hypervalent iodine reagents, require expensive and toxic transition metals as catalysts or photosensitizers such as Rose Bengal, Eosin Y, Bodipy, etc. Nevertheless, poor chemoselectivity and low catalytic activity still limit their prospective application [1].
Visible-light mediated metal-free heterogenous catalysts have been extensively pursued in hopes of enabling more efficient and environmentally sustainable chemical transformations. Hence, we propose unmodified fullerene soot and fullerene C60 nanodispersion as a low-cost heterogenous photocatalysts for chemoselective sulfides oxidation to sulfoxides in ethanol without further overoxidation to sulfones. Since C60 is well-known good electron acceptor and is easily excited by blue light, the photooxidation can take place by two different mechanisms: either by excited C60 state generating singlet oxygen or by an electron-transfer mechanism employing triplet oxygen.
The fullerene soot possesses the huge advantage amongst other reusable catalyst because it can be easily recovered by simple filtration and reused many times without losing its intrinsic properties. Additionally, it does not require any modification or catalyst immobilization onto heterogenous supports such as resins or insoluble polymers, it is available from multiple suppliers and is very cheap. In addition, larger scale reactions were carried out in excellent yields using a flow reactor with a non-immobilized heterogenous fullerene soot.
The wider applicability of fullerene nanodispersion as a heterogenous photocatalyst was successfully demonstrated in radical cyclization, boronic acid oxidation and imine formation reactions [2].