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)
posted on 2013-06-24, 00:00authored byIan A. Tonks, Josef C. Meier, John E. Bercaw
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.