New mechanism for the intermolecular hydroamination of alkynes: Catalysis by dinuclear ruthenium complexes with a rigid dicyclopentadienyl ligand

The dinuclear ruthenium complex {(eta(5)-C5H3)(2)(SiMe2)(2)]Ru-2(CO)(3)(C2H4)H+BF4- (1) catalyzes the intermolecular hydroamination of arylalkynes (e.g., phenylacetylene) with arylamines (e.g., p-toluidine) to give imines (e.g., (Ph(Me)C=N(p-MeC6H4)). Although the catalyst has a limited lifetime (up to 6 turnovers), details of the reaction mechanism have been elucidated by isolation and characterization (X-ray and/or NMR) of six of the seven intermediates and reaction-terminating species. The mechanism is fundamentally different from all previously proposed mechanisms for alkyne hydroamination. The rigid doubly bridged bis((dimethyl-silyl)cyclopentadienyl) ligand is an important feature of the catalyst that facilitates this new type of hydroamination mechanism.