A β-diketiminate manganese catalyst for alkene hydrosilylation: Substrate scope, silicone preparation, and mechanistic insight

The dimeric β-diketiminate manganese hydride compound, [(^2,6-iPr2PhBDI)Mn(μ-H)]₂, was prepared by treating [(^2,6-iPr2PhBDI)Mn(μ-Cl)]₂ with NaEt₃BH. This compound was characterized by single crystal X-ray diffraction and found to feature high-spin Mn centres that exhibit strong magnetic coupling by EPR spectroscopy. Once characterized, [(^2,6-iPr2PhBDI)Mn(μ-H)]₂ was found to mediate the hydrosilylation of a broad scope of alkenes at elevated temperature. Aliphatic alkenes were found to undergo anti-Markovnikov hydrosilylation, while the hydrosilylation of styrenes using [(^2,6-iPr2PhBDI)Mn(μ-H)]₂ afforded Markovnikov's product. Importantly, this catalyst has also been employed for the cross-linking of industrially-relevant silicones derived from vinyl-terminated poly(dimethylsiloxane) and 1,2,4-trivinylcyclohexane with catalyst loadings as low as 0.05 mol%. To gain a mechanistic understanding of [(^2,6-iPr2PhBDI)Mn(μ-H)]₂-catalyzed olefin hydrosilylation, 4-tert-butylstyrene was added to [(^2,6-iPr2PhBDI)Mn(μ-H)]₂ and conversion to the monomeric Mn alkyl complex, (^2,6-iPr2PhBDI)Mn(CH(CH₃)(4-^tBuPh)), was observed. Isolation of this secondary alkyl intermediate confirms that olefin insertion into the Mn-H bond dictates the observed regioselectivities. The importance of our mechanistic findings as they relate to recent advances in Mn hydrosilylation catalysis is described herein.