4 + 1 Radical annulations with isonitriles: a simple route to cyclopenta-fused quinolines

Abstract

Sunlamp irradiation of 1-substituted 5-iodo-1-pentynes, 5 equiv of phenyl isocyanide, and 1.5 equiv of hexamethylditin in tert-butylbenzene (0.01-0.025 M) at 150 o C produces 9-substituted 2,3-dihydro-1H-cyclopenta [b] quinolines in 36-70% yields. A mechanistic proposal for this first example of a 4+1 radical annulation includes the following: (1) radical addition to an isonitrile, (2) cyclization of the resulting imidoyl radical to the alkyne, (3) addition of the so-formed vinyl radical to the aromatic ring, and (4) rearomatization. When substituted isonitriles (p-F p-OMe, m-F) are employed, the major unrearranged products are accompanied by 7-30% or rearranged products. Independent generation of a proposed intermediate suggests that the rearranged products arise by initial closure of the vinyl radical to form a five-membered ring, followed by C-N bond cleavage and recyclization