Catalytic Intramolecular Hydroamination with a Bifunctional Iridium Pyrazolato Complex: Substrate Scope and Mechanistic Elucidation

Catalytic intramolecular cyclization of nonactivated
aminoalkene with functional group compatibility provides an atom-economical
and concise route to valuable nitrogen-containing heterocycles yet
remains a challenge. In this paper, we report the detailed substrate
scope and mechanism of catalytic intramolecular hydroamination with
a half-sandwich-type iridium pyrazolato complex we have recently developed.
This metal–ligand bifunctional catalyst promoted the hydroamination
of various primary and secondary aminoalkenes at mild temperatures
(50–110 °C) without side reactions such as oxidative amination.
Cyclization of secondary aminoalkenes containing ester, cyano, bromo,
and hydroxy groups occurred with maintenance of these functional groups,
while the reactions of aminoalkenes bearing allylic substituents proceeded
with a perfect diastereoselectivity. Catalyst optimization revealed
that the proton-responsive functional group at the position β
to the metal is crucial to efficient catalytic turnover. Kinetic analysis
indicated a highly ordered transition state associated with N–H
bond cleavage in the rate-determining step. On the basis of these
data along with the stoichiometric reactions and DFT calculations,
we propose an unprecedented metal–ligand cooperating mechanism,
in which cyclization occurs through syn addition
of the amino group to the coordinated olefin bond with the aid of
the Brønsted basic pyrazolato ligand.