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 TiII species
that is generated in situ from 1. Depending on reaction
conditions, these TiII species are proposed to be generated
through Ti benzylidene or imido intermediates. A formally TiII complex, (ONO)­TiII(η2-C2H4)­(HNMe2) (7), has been prepared and
structurally characterized.