Nickel-Catalyzed, Cascade Cycloadditions of 1-Ethynyl-8-halonaphthalenes with Nitriles: Synthesis, Structure, and Physical Properties of New Pyrroloarenes

The reaction of 1-ethynyl-8-halonaphthalenes 1 with nitriles in the presence of the catalytic system [NiBr2(dppe)]/Zn (dppe=1,2-bis(diphenylphosphino)ethane) is found to produce unusual pyrroloarenes 2. The carbon nitrogen triple bond in nitrile is activated twice, and five new bonds are formed in a one-pot transformation, which causes a pyrrole and two six-membered rings to be generated simultaneously. The scope and limitations of this reaction are examined. Similarly, alkyl-bridged diynes also furnish the corresponding polycycles. Diaryl-substituted cycloadducts 2 (R-1 = Ar) are fluxional, because of the restriction in rotation of the aryl groups. The rotational barrier is studied by performing H-1 NMR experiments at various temperatures. The structures of several compounds are determined by X-ray crystallographic analysis. The photophysical and electrochemical properties of the pyrroloarenes are also investigated.