Reversible C(sp3)-Si Oxidative Addition of Unsupported Organosilanes: Effects of Silicon Substituents on Kinetics and Thermodynamics.

The intermolecular oxidative addition of unactivated C(sp3)-Si bonds is reported for a family of organosilanes at a cationic pincer-supported iridium complex. To our knowledge, no examples of oxidative addition to give analogous unsupported (alkyl)metal silyl complexes have been previously reported. The generality of this transformation is excellent, with successful examples demonstrated for tetraorganosilanes, mono- and poly alkoxysilanes, and two siloxysilanes. Oxidative addition is found to be completely reversible, with the product of reductive elimination being subject to trapping by triethylsilane. The successful isolation of these metal silyl complexes has allowed for an in-depth kinetic analysis of C(sp3)-Si reductive elimination, a process with strong implications in both catalytic C-H silylation and olefin hydrosilylation. The apparent order of reactivity is SiMe3 > SiMe2(CF3) > SiMe2OSiMe3 > SiMe2OSiMe2OSiMe3 > SiMe2(OMe) > SiMe2(OEt) > SiMe(OMe)2. A DFT analysis of the oxidative addition products shows that the thermodynamic stability of the (alkyl)metal silyl complexes span a range of ca. 10 kcal·mol-1, which relate closely with the experimentally determined rates of C(sp3)-Si reductive elimination and trapping, though a clear kinetic distinction exists between methoxy- and siloxysilyl complexes.