Alkyne hydroamination and trimerization with titanium bis(phenolate) pyridine complexes: Evidence for low-valent titanium intermediates and synthesis of an ethylene adduct of titanium(II)

A class of titanium precatalysts of the type (ONO)TiX₂ (ONO = pyridine-2,6-bis(4,6-di-tert-butylphenolate); X = Bn, NMe₂) has been synthesized and crystallographically characterized. The (ONO)TiX₂ (X = Bn, NMe₂, X₂ = NPh) complexes are highly active precatalysts for the hydroamination of internal alkynes with primary arylamines and some alkylamines. A class of titanium imido/ligand adducts, (ONO)Ti(L)(NR) (L = HNMe₂, py; R = Ph, ^tBu), have also been synthesized and characterized and provide structural analogues to intermediates on the purported catalytic cycle. Furthermore, these complexes exhibit unusual redox behavior. (ONO)TiBn₂ (1) promotes the cyclotrimerization of electron-rich alkynes, likely via a catalytically active Ti^II species that is generated in situ from 1. Depending on reaction conditions, these Ti^II species are proposed to be generated through Ti benzylidene or imido intermediates. A formally Ti^II complex, (ONO)Ti ^II(η²-C₂H₄)(HNMe₂) (7), has been prepared and structurally characterized.