Amidate Complexes of Tantalum and Niobium for the Hydroaminoalkylation of Unactivated Alkenes

A series
of mono­(amidate) Ta and Nb complexes with varying steric
and electronic properties were synthesized. These complexes were screened
as precatalysts for the hydroaminoalkylation of alkenes with secondary
amines. Sterically demanding mono­(amidate) Ta complexes were determined
to be the most effective precatalysts. Isotopic labeling and kinetic
studies were undertaken in an effort to elucidate the mechanism. The
reaction was shown to be dependent upon catalyst and alkene concentration
while being zero order in amine concentration. Mechanistic probes
for radical species support a two-electron mechanism. Bis­(amidate)
species of Ta and Nb were also synthesized, with metallaaziridine
formation observed for both metals. Insertion of acetonitrile into
the reactive M–C bond yielded a representative five-membered
metallacycle. Off-cycle equilibria and catalyst dormant states have
been identified as areas for future catalyst development efforts.