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

The dimeric β-diketiminate manganese hydride compound, [(BDI)Mn(μ-H)], was prepared by treating [(BDI)Mn(μ-Cl)] with NaEtBH. 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, [(BDI)Mn(μ-H)] 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 [(BDI)Mn(μ-H)] 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 [(BDI)Mn(μ-H)]-catalyzed olefin hydrosilylation, 4--butylstyrene was added to [(BDI)Mn(μ-H)] and conversion to the monomeric Mn alkyl complex, (BDI)Mn(CH(CH)(4- 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.