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

Abstract

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.