Imido-Alkyne Coupling in Titanium Complexes: New Insights into the Alkyne Hydroamination Reaction

Titanium imido complexes [Ti(NR)(N2ArNpy)(L)] (N2ArNpy = MeC(2-C5H4N)(CH2NAr)2, Ar = 4-C6H4Me or 3,5-C6H3Me2) have been prepared from the corresponding bis(amide) complexes [Ti(N2ArNpy)(NMe2)2]. The reaction chemistry of the imido ligand has been investigated with aryl acetylenes, affording the {2 + 2} cycloaddition products [Ti(N2ArNpy){κ2-N(tBu)CHCAr‘}] (Ar‘ = Ph and 4-C6H4Me) which represent a catalytic intermediate in the anti-Markovnikov hydroamination of terminal alkynes. Reaction of these azatitanacyclobutene complexes with tert-butylamine completes the catalytic cycle, affording trans-cinnamyl(tert-butyl)amine; conversely, reaction with a second equivalent of alkyne afforded a second insertion product, an azatitanacyclohexadiene [Ti(N2ArNpy){κ2-N(tBu)CHC(Ar‘)C(Ar‘) = CH}], two examples of which have been characterized by X-ray crystallography. Studies into the catalytic hydroamination of phenyl acetylene with tert-butylamine have been performed and indicate that the formation of an azatitanacyclohexadiene complex represents a deactivation pathway in this catalytic reaction.