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(=N^tBu)]. A molecular-level characterization of the surface organometallic Ta species was done with the help of characterization techniques including in situ infrared, ¹H and ¹³C 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 ¹⁵N 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.