Intramolecular Aminoalkene Hydroamination Catalyzed by Magnesium Complexes Containing Multidentate Phenoxyamine Ligands

The magnesium complexes L 2 Mg i Pr (L 2 = 4- tert -butyl-6-(triphenylsilyl)-2-[bis((3-(dimethylamino)propyl)amino)methyl]phenoxyl) and L 3 Mg i Pr (L 3 = 4- tert -butyl-6-(triphenylsilyl)-2-[benzyl((3-(dimethylamino)propyl)amino)methyl]phenoxyl) supported by potentially tetradentate and tridentate triphenylsilyl-substituted phenoxyamine ligands have been prepared and fully characterized. The X-ray crystallographic analysis of L 2 Mg i Pr confirmed a monomeric structure in which only one of the amine side arms is bound to the four-coordinate magnesium atom. The free and coordinated side arms in L 2 Mg i Pr undergo an exchange process at 25 °C in solution, while the phenoxydiamine complex L 3 Mg i Pr, on the other hand, shows no sign of fluxionality. Both complexes, as well as L 1 Mg i Pr (L 1 = 4,6-di- tert -butyl-2-[bis((3-(dimethylamino)propyl)amino)methyl]phenoxyl), were shown to be competent catalysts in the cyclization of aminoalkenes. L 2 Mg i Pr exhibited the best catalytic activity, and both triphenylsilyl-substituted complexes display zero-order rate dependence on substrate concentration and first-order rate dependence on catalyst concentration, whereas the sterically less hindered complex L 1 Mg i Pr exhibits second-order rate dependence on substrate concentration. No Schlenk-type ligand redistributions were observed, and the catalytically active magnesium species was stable after prolonged heating to 120 °C, according to an NMR spectroscopic study.