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

Abstract

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.