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.