1,4-Dihydropyridyl complexes of magnesium: synthesis by pyridine insertion into the magnesium–silicon bond of triphenylsilyls and catalytic pyridine hydrofunctionalization

Abstract

Magnesium bis(triphenylsilyl) [Mg(SiPh₃)₂(THF)₂]·THF (1) reacted with a stoichiometric amount of pyridine to give the magnesium 4-(triphenylsilyl)dihydropyridyl complex [Mg(NC₅H₅-4-SiPh₃)₂(THF)₃] (2). Using an excess of pyridine, a mixture of magnesium dihydropyridyl [Mg(NC₅H₆)₂(py)₄] (3) and 4-(triphenylsilyl)pyridine was formed. Complex 3 underwent exchange with pyridine-d₅ at 25 °C to give [Mg(NC₅D₅H-4)₂(py-d₅)₄] (3-HD). Analogous reactions with Me₃TACD-supported magnesium triphenylsilyls [(Me₃TACD)Mg(SiPh₃)] (4) and [(Me₃TACD·AlEt₃)Mg(SiPh₃)] (6) ((Me₃TACD)H = Me₃[12]aneN₄: 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) with pyridine gave [(Me₃TACD)Mg(NC₅H₅-4-SiPh₃)] (5), [(Me₃TACD·AlEt₃)Mg(NC₅H₅-4-SiPh₃)] (7) and a mixture of [(Me₃TACD)Mg(NC₅H₆)] (8) and 4-(triphenylsilyl)pyridine. Complex 8 is also formed by reacting 3 with (Me₃TACD)H and underwent exchange with pyridine-d₅ at higher temperatures. The activation energy for the exchange is about 25 kJ mol⁻¹ higher than that for the exchange reaction of 3 to 3-HD. Complexes 2, 3, 3-HD, 5, 7 and 8 were characterized by NMR spectroscopy and 3, 5 and 8 by single crystal structure analysis. Complex 3 was found to be slightly active in the hydrosilylation of pyridine using phenylsilane, whereas complex 8 showed no activity. Both complexes 3 and 8 were active in the hydroboration of pyridine with pinacolborane.