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

The dimeric beta-diketiminate manganese hydride compound, [(2,6-iPr2PhBDI)Mn(mu-H)](2), was prepared by treating [(2,6-iPr2PhBDI)Mn(mu-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(mu-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(mu-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(mu-H)](2)-catalyzed olefin hydrosilylation, 4-tert-butylstyrene was added to [(2,6-iPr2PhBDI)Mn(mu-H)](2) and conversion to the monomeric Mn alkyl complex, (2,6-iPr2PhBDI)Mn(CH(CH3)(4-(BuPh)-Bu-t)), 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.