In an effort to develop more active catalysts for the intramolecular hydroamination of amino alkenes, we have investigated the application of sterically hindered mono(phosphines) as supporting ligands for platinum-catalyzed hydroamination. Here we demonstrate that mixtures of PtCl2 and di(tertbutyl)o-biphenyl phosphines such as P(t-Bu) 2[2-(2-NMe2C6H4)C6H 4] catalyze the hydroamination of 4-pentenyl and 5-hexenyl amines at 60-80 °C. This catalyst system tolerates a number of functional groups and catalyzes the cyclization of unsubstituted amino alkenes. © 2008 American Chemical Society.