Controllable Hydrosilylation and Dehydrogenative Silylation of Alkenes Catalyzed by a Manganese Alkyl Complex

We report additive-free Mn-(I)-catalyzed controllable hydrosilylation (HS) and dehydrogenative silylation (DS) of terminal alkenes. The precatalyst is the well-defined Mn-(I) alkyl complex fac-[Mn-(PC-iPr)-(CO)₃(CH₂CH₂CH₃)]. The reaction regioselectively yields either hydrosilylated alkenes or (E)-alkenyl silanes together with the corresponding alkanes in an approximately 1:1 ratio. The former are obtained selectively with primary and secondary silanes under neat conditions, while the latter are exclusively formed with more bulky tertiary silanes in THF as solvent. The reactions proceed with catalyst loadings of 1 mol % at 85 °C and a reaction time of 24 h. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate that undergoes Si-H bond cleavage of silane forming the active 16e^- Mn-(I) hydride complex [Mn-(PC-iPr)-(CO)₂(H)] together with a liberated siloxane Si-O-Si species. [Mn-(PC-iPr)-(CO)₂(H)] is the active catalyst in the DS cycle as well as the key intermediate for the formation of the silyl complex [Mn-(PC-iPr)-(CO)₂(silyl)], which is the key intermediate for the HS pathway. The DS pathway requires an alkene as a sacrificial hydrogen acceptor. Mechanistic insights are provided based on experimental data and DFT calculations.