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

Abstract

Titanium imido complexes [Ti(NR)(N 2 Ar N py )(L)] (N 2 Ar N py = MeC(2-C 5 H 4 N)(CH 2 NAr) 2, Ar = 4-C 6 H 4 Me or 3,5-C 6 H 3 Me 2 ) have been prepared from the corresponding bis(amide) complexes [Ti(N 2 Ar N py )(NMe 2 ) 2 ]. The reaction chemistry of the imido ligand has been investigated with aryl acetylenes, affording the {2 + 2} cycloaddition products [Ti(N 2 Ar N py ){κ 2 -N( t Bu)CH CAr‘}] (Ar‘ = Ph and 4-C 6 H 4 Me) 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(N 2 Ar N py ){κ 2 -N( t Bu)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.