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.