Xiong, Yao, Muller, Kaupp, Driess. From silicon(II)-based dioxygen activation to adducts of elusive dioxasiliranes and sila-ureas stable at room temperature. Nature Chemistry 2010, 2, 577-580. DOI: 10.1038/nchem.666
Metal-free syntheses are currently under intense investigation as potentially cheaper and less-polluting alternatives to reactions involving stoichiometric or catalytic metals. In the Driess group’s recent contribution to Nature Chemistry, they report a silylene compound that activates O2, a biologically and industrially relevant reaction that tends to be associated with transition metals. In addition, they were able to isolate and crystallographically characterize the first dioxasilirane, the silicon-based analogue of the more familiar dioxirane reagents. Importantly, Driess’s dioxasilirane is capable of intramolecular O-atom transfer with the resulting silanone having the shortest silicon-oxygen bond reported to date.
The Driess synthesis is shown above and is notable for a few reasons. First, they use a modified diketiminate ligand that has been deprotonated to make a strongly π-donating, dianionic ancillary ligand to coordinate the silicon atom. The synthesis of the silylene is interesting in itself (shown below), as its precursor was prepared by an unexpected dehydrohalogenation of a putative (diketiminato)SiBr3 in the presence of tmeda followed by KC8 reduction (DOI: 10.1021/ja062928i). The similarity to N-heterocyclic carbenes is immediately obvious, with the nitrogen atoms reducing the electrophilicity of the silicon atom of 1a and 1b by π-donation.
Second, the reaction with O2 proceeds in good yield (79% and 69% from 1a and 1b, respectively) to provide the novel dioxasiliranes, which are stable at room temperature. This is in sharp contrast to the previously reported dioxasiliranes, which were spectroscopically characterized below -233ºC in argon matrices. The stability of 2a and 2b should thus enable for the first time a careful study of dioxasilirane reactivity, especially in relation to the analogous dioxiranes.
Third, they employ a bulky carbene ligand to stabilize the silicon center. The stabilization is likely both electronic and steric, providing electron donation to the electrophilic silicon of 1–3 as well as steric protection for the polar Si=O bond of 3. In addition the carbene serves as an oxygen atom acceptor in their intramolecular O-atom transfer reaction.
Last, the Si=O double bond is notable as being the shortest reported Si-O bond at 1.532(2) Å. However, the crystal structure of the silanone 3 reveals the geometry at Si to be clearly pyramidal (O-Si-O angle of 112.2º) implying the contribution of the ylide-like resonance form shown below.
For this chemistry to be ultimately useful, dioxasiliranes will need to be capable of intermolecular O-atom transfer to substrates which remain unbound (or transiently bound) to the silicon center. Furthermore, the silicon-based reagent would have to be an improvement on the common dioxiranes, which are based on carbon and prepared from abundant materials (e.g., dimethyldioxirane is prepared from acetone).