New Mechanism for the Intermolecular Hydroamination of Alkynes: Catalysis by Dinuclear Ruthenium Complexes with a Rigid Dicyclopentadienyl Ligand

The dinuclear ruthenium complex {(η5-C5H3)2(SiMe2)2}Ru2(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((dimethylsilyl)cyclopentadienyl) ligand is an important feature of the catalyst that facilitates this
new type of hydroamination mechanism.