Cyclic Ureate Tantalum Catalyst for Preferential Hydroaminoalkylation with Aliphatic Amines: Mechanistic Insights into Substrate Controlled Reactivity.

The efficient and catalytic amination of unactivated alkenes with simple secondary alkyl amines is preferentially achieved. A sterically accessible, N,O-chelated cyclic ureate tantalum catalyst was prepared and characterized by X-ray crystallography. This optimized catalyst can be used for the hydroaminoalkylation of 1-octene with a variety of aryl and alkyl amines, but notably enhanced catalytic activity can be realized with challenging N-alkyl secondary amine substrates. This catalyst offers turnover frequencies of up to 60 h-1, affording full conversion at 5 mol% catalyst loading in approximately 20 min with these nucleophilic amines. Mechanistic investigations, including kinetic isotope effect (KIE) studies, reveal that catalytic turnover is limited by protonolysis of the intermediate 5-membered azametallacycle. A Hammett kinetic analysis shows that catalytic turnover is promoted by electron rich amine substrates that enable catalytic turnover. This more active catalyst is shown to be effective for late stage drug modification.