Synthesis and Reactivity of Phosphine-Quinolinolato Rhodium Complexes: Intermediacy of Vinylidene and (Amino)carbene Complexes in the Catalytic Hydroamination of Terminal Alkynes

We report here on the syntheses of
rhodium­(I) complexes bearing a phosphine-quinolinolate ligand and
the isolation of two classes of important intermediates in the anti-Markovnikov
hydroamination of terminal alkynes with secondary amines: vinylidene-bridged
dirhodium complexes and (amino)­carbene complexes. We first prepared
various rhodium­(I) complexes bearing a phosphine-quinolinolate ligand
and found that some of these complexes had catalytic activity for
the anti-Markovnikov hydroamination of terminal alkynes
with secondary amines. The reaction of stoichiometric amounts of a
(PNO)Rh complex with terminal alkynes provided vinylidene-bridged
dirhodium complexes. When the resulting vinylidene complexes were
reacted with secondary amines, (amino)­carbene complexes were generated,
which gave the hydroamination product upon heating with an additive.
These results suggest that this anti-Markovnikov hydroamination catalyzed
by (PNO)Rh complexes proceeds via vinylidene and (amino)­carbene complexes.