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

The dimeric β-diketiminate manganese hydride compound, [( ^2,6-iPr2Ph BDI)Mn(μ-H)] ₂ , was prepared by treating [( ^2,6-iPr2Ph BDI)Mn(μ-Cl)] ₂ with NaEt ₃ BH. 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-iPr2Ph BDI)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-iPr2Ph BDI)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-iPr2Ph BDI)Mn(μ-H)] ₂ -catalyzed olefin hydrosilylation, 4- tert -butylstyrene was added to [( ^2,6-iPr2Ph BDI)Mn(μ-H)] ₂ and conversion to the monomeric Mn alkyl complex, ( ^2,6-iPr2Ph BDI)Mn(CH(CH ₃ )(4- ^t BuPh)), 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.