Regioselective access to di- and trisubstituted pyridines via a metal-oxidant-solvent-free domino reaction involving 3-chloropropiophenones

A remarkable metal-oxidant-solvent- and base-free domino route for regioselective access to a wide range of 2,4-di- and 2,3,4/6-trisubstituted pyridines including carbo- and heterocyclic fused pyridines is reported. This [3C + 2C + 1N] cyclization reaction occurs between 3-chloropropiophenones (3C units), enolizable acyclic/cyclic ketones (2C sources) and NH4OAc as a robust N source under neat conditions under an open atmosphere, producing new C=C and C=N-C bonds in highly chemo- and regioselective manners. Interestingly, this eco-friendly method has many positive features: excellent functional group tolerance, broad substrate scope, good to excellent regioselectivities, promising yields, no-unwanted products, neutral reaction conditions and appropriateness for large-scale synthesis. Mechanism studies reveal that the in situ generated β-amino ketone from 3-chloropropiophenone and an ammonium salt undergoes C=N bond formation with a ketone followed by an intramolecular cyclization process (C=C bond), which are the decisive steps for pyridine synthesis.