Imido-alkyne coupling in titanium complexes: New insights into the alkyne hydroamination reaction

Titanium imido complexes [Ti(NR)(N2ArN py)(L)] (N2ArNpy = MeC(2-C 5H4N)(CH2NAr)2, Ar = 4-C 6H4-Me or 3,5-C6H3Me2) have been prepared from the corresponding bis(amide) complexes [Ti(N 2ArNpy)-(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. © 2007 American Chemical Society