Highly Enantioselective Allylic C–H Alkylation of Terminal Olefins with Pyrazol-5-ones Enabled by Cooperative Catalysis of Palladium Complex and Brønsted Acid

A highly
enantioselective allylic C–H alkylation reaction
of allylarenes with pyrazol-5-ones has been established by the cooperative
catalysis of a chiral palladium complex and chiral Brønsted acid
to afford a wide spectrum of functionalized chiral N-heterocycles
with an all-carbon quaternary stereogenic center in high yields and
with high levels of enantioselectivity (up to 96% ee), wherein the
chiral ligand and phosphoric acid showed synergistic effect on the
control of stereoselectivity. In addition, a palladium-catalyzed asymmetric
allylic C–H alkylation of 1,4-pentadienes with pyrazol-5-ones
has been realized to furnish highly functionalized pyrazol-5-ones
in high enantioselectivities. In this case, the chiral ligand controls
the stereoselectivity while the achiral Bronsted acid, 2-fluorobenzoic
acid, turns out to be a better cocatalyst than the chiral phosphoric
acid. The installation of electron-deficient substituents at 3,3′-positions
of binaphthyl backbone of chiral phosphoramidites is actually beneficial
to the allylic C–H oxidation due to their survival in the presence
of quinone derivative oxidants. These allylic C–H alkylation
reactions undergo smoothly under mild conditions and tolerate a wide
range of substrates. The resultant highly functionalized chiral pyrazol-5-ones
have been applied to the preparation of more structurally diverse
heterocycles by classical transformations.