From silicon(II)-based dioxygen activation to adducts of elusive dioxasiliranes and sila-ureas stable at room temperature.

Dioxygen activation for the subsequent oxygenation of organic substrates that involves cheap and environmentally friendly chemical elements is at the cutting edge of chemical research. As silicon is a non-toxic and highly oxophilic element, the use of silylenes could be attractive for facile dioxygen activation to give dioxasiliranes with a SiO(2)-peroxo ring as versatile oxo-transfer reagents. However, the latter are elusive species, and have been generated and studied only in argon matrices at -233 degrees C. Recently, it was demonstrated that unstable silicon species can be isolated by applying the concept of donor-acceptor stabilization. We now report the first synthesis and crystallographic characterization of dioxasiliranes stabilized by N-heterocyclic carbenes that feature a three-membered SiO(2)-peroxide ring, isolable at room temperature. Unexpectedly, these can undergo internal oxygen transfer in toluene solution at ambient temperature to give a unique complex of cyclic sila-urea with C=O --> Si=O interaction and the shortest Si=O double-bond distance reported to date.