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)­TiX<sub>2</sub> (ONO = pyridine-2,6-bis­(4,6-di-<i>tert</i>-butylphenolate); X = Bn, NMe<sub>2</sub>) has been synthesized and crystallographically characterized. The (ONO)­TiX<sub>2</sub> (X = Bn, NMe<sub>2</sub>, X<sub>2</sub> = 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 = HNMe<sub>2</sub>, py; R = Ph, <sup><i>t</i></sup>Bu), 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)­TiBn<sub>2</sub> (<b>1</b>) promotes the cyclotrimerization of electron-rich alkynes, likely via a catalytically active Ti<sup>II</sup> species that is generated in situ from <b>1</b>. Depending on reaction conditions, these Ti<sup>II</sup> species are proposed to be generated through Ti benzylidene or imido intermediates. A formally Ti<sup>II</sup> complex, (ONO)­Ti<sup>II</sup>(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)­(HNMe<sub>2</sub>) (<b>7</b>), has been prepared and structurally characterized.