Synthesis of nitrogen heterocycles by intramolecular cyclization of nitro-olefins attached to five-membered heterocycles, catalyzed by palladium complexes and with carbon monoxide as the reductant

Research in our unit recently showed that reductive cyclization of beta-nitrostyrenes catalyzed by palladium/phenanthroline complexes and with CO as a reductant affords indoles in good yields. The reaction proceeds by the activation of an aryl C-H bond. We decided to extend such cyclization reaction to other heterocyclic systems, although it is known that the activation of a C-H bond of electron-rich five member heterocycles is a more difficult reaction. Nitro olefins attached to 5-membered heterocyclic compounds were prepared by the Henry reaction and fully characterized by 1H-NMR, 13C-NMR and elemental and mass analyses. Subsequent reductive cyclization catalysed by palladium-phenanthroline complexes under CO pressure afforded a 10 pi-aromatic compounds, isoelectronic with pentalenyl dianion, and containing a pyrrole ring fused to another 5-membered heterocyclic compound. Compound was chosen as a model compound for the optimization of the experimental conditions. Among many tested ligands 4,7-dimethoxyphenanthroline showed good results for both conversion and selectivity. The optimization of temperature, CO pressure, nature of the base, type of ligand, reaction time and solvent are in progress.