Mechanistic study of hydroamination of alkyne through tantalum-based silica-supported surface species

Selective hydroamination of terminal alkynes with primary aryl amines is catalyzed by an unprecedented well-defined silica-supported tantalum complex [(≡Si-O-)Ta(η1σ-NEtMe)2(=NtBu)]. A molecular-level characterization of the surface organometallic Ta species was done with the help of characterization tech-niques including as in situ infrared, in situ elemental microanal-ysis, 1H and 13C solid-state NMR (including double and triple quanta sequence), and X-ray absorption spectroscopies. These were complemented by the state-of-the-art DNP-SENS 15N characterization. Several catalytic intermediates have been isolated in particular the 4-membered metallacycle ring inter-mediate resulting from the anti Markovnikov addition of the alkyne to the surface tantalum imido. The mechanism proposed was based on the isolation of all intermediates. A DFT calcula-tion has confirmed all the elementary steps and intermediates that were fully characterized.