A catalytic method for the enantioselective and C4-selective functionalization of pyridine derivatives is yet to be developed. Herein, we report an efficient method for the asymmetric β-pyridylations of enals that involve N-heterocyclic carbene (NHC) catalysis with excellent control over enantioselectivity and pyridyl C4-selectivity. The key strategy for precise stereocontrol involves enhancing interactions between the chiral NHC-bound homoenolate and pyridinium salt in the presence of hexafluorobenzene, which effectively differentiates the two faces of the homoenolate radical. Room temperature is sufficient for this transformation, and reaction efficiency is further accelerated by photo-mediation. This methodology exhibits broad functional group tolerance and enables facile access to a diverse range of enantioenriched β-pyridyl carbonyl compounds under mild and metal-free conditions.
A one-pot umpolung method for the ring-opening pyridylation
of
unstrained cyclic amines was developed using N-amidopyridinium
salts. This process involves the formation of electron donor–acceptor
complexes between bromide and N-amidopyridinium salts,
ultimately leading to the functionalization of pyridines. This protocol
is compatible with a range of 5- or 6-membered cyclic amines and pyridines,
thereby providing a powerful synthon for preparing C4-functionalized
pyridines under visible-light conditions in the absence of an external
photocatalyst.
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