Rh(III)-Catalyzed Oxidant-Free Dehydrogenative Mizoroki–Heck-Type Reaction: Identification of Off-Cycle Processes Potentially Limiting Catalytic Turnover

The Rh(III)-catalyzed dehydrogenative Mizoroki–Heck-type
reaction (DMHR) has been extensively studied owing to its diverse
synthetic applications. This transformation generally requires stoichiometric
amounts of an external oxidant, H2 acceptor, or internal
oxidizing group for efficient catalytic turnover. By contrast, the
oxidant-free DMHR involving the release of H2 gas is an
ideal method for minimizing waste; however, it remains underdeveloped.
Here, we investigated the Rh(III)-catalyzed DMHR of 4-amino-2-quinolones
under oxidant-free conditions. A combined experimental and computational
study revealed that an off-cycle resting species, formed via the migratory
insertion of an alkenylation product into a rhodium hydride species,
was involved in the inhibition of catalytic turnover. The highly efficient
oxidant-free DMHR of 4-amino-2-quinolones was achieved by excluding
the coupling product from the reaction system to suppress the formation
of the off-cycle resting species. Furthermore, we revisited the Rh(III)-catalyzed
DMHR of aromatic substrates bearing different types of directing groups
and demonstrated that substrates bearing representative directing
groups such as amides, carbamates, carboxylic acids, 1-naphthol, indoles,
pyridines, pyrimidines, and pyrazoles underwent alkenylation in the
absence of an oxidizing agent.