Synthesis of Group 4 Metal Complexes Stabilized by an Amine-Bridged Bis(phenolato) Ligand and Their Catalytic Behavior in Intermolecular Hydroamination Reactions

Zirconium
and titanium complexes 1 and 2, bearing
an amine-bridged bis­(phenolato) ligand, have been synthesized
and characterized. Although 1 and 2 were
inactive in catalyzing intermolecular hydroamination reactions, cationic
complexes generated in situ from treatment of 1 and 2 with borate [Ph3C]­[B­(C6F5)4], respectively, were found to be highly active. In
general, excellent yields (up to >99%) and 100% regioselectivity
for
a broad range of terminal alkynes and anilines were observed within
a reaction time of 1 h. Reactions with internal alkynes of moderate
sterics also led to good yields and moderate regioselectivity. A kinetic
study was also conducted, which provided some insights into the mechanism
of hydroamination reactions.