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 ¹Ha 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. (Ni)triles and tribulations: 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) produces unusual pyrroloarenes 2, which have unusual structures (see scheme) and exhibit interesting physical properties. The diluted solutions of these pyrroloarenes are strongly fluorescent, even under ambient light.