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(eta(1)sigma-NEtMe)(2)(=(NBu)-Bu-t)]. A molecular-level characterization of the surface organometallic Ta species was done with the help of characterization techniques including in situ infrared, H-1 and C-13 solid-state NMR (including double and triple quanta sequencing), and X-ray absorption spectroscopies. These were complemented by the state-of-the-art DNP-SENS N-15 characterization. Several catalytic intermediates have been isolated, in particular the 4-membered metallacycle ring intermediate resulting from the anti-Markovnikov addition of the alkyne to the surface tantalum imido. A mechanism is proposed that is on the isolation of all intermediates. A density functional theory (DFT) calculation has confirmed all the elementary steps and intermediates that were fully characterized.