Visible-Light-Accelerated Pd-Catalyzed Cascade Addition/Cyclization of Arylboronic Acids to γ- and β‑Ketodinitriles for the Construction of 3‑Cyanopyridines and 3‑Cyanopyrrole Analogues

The one-pot synthetic strategies
for 2,4,6-triarylnicotinonitriles
and 2,5-diaryl-1H-pyrrole-3-carbonitriles have been
accomplished via a Pd-catalyzed coupling of arylboronic acid with
2-(3-oxo-1,3-diarylpropyl)­malononitrile and 2-(2-oxo-2-arylethyl)­malononitrile,
respectively, under mild reaction conditions, followed by intramolecular
cyclization of an intermediate formed after the regeneration of the
catalyst under acidic reaction conditions. The cascade reactions proceed
in 1,2-dichloroethane solvent under visible-light irradiation, and
the active catalyst is generated in situ in the presence of catalytic
amounts of Pd­(OAc)2 and 2,2′-bipyridine. The active
Pd catalyst undergoes photoexcitation by the virtue of metal-to-ligand
charge transfer (MLCT), and subsequent redox trans-metalation occurs
with arylboronic acid, thus obviating the necessity of any exogenous
photosensitizer. The targeted products, composed of a new C–C,
a C–N, a CN, and two new CC bonds, were isolated
in good yields.