Cyclopent[a]iondene (Benzopentalene) and Pentalene: Pyrolytic Formation From 3-Phenyl- and 3-Vinyl-phthalic Anhydrides

Pyrolysis of 3-phenylphthalic anhydride at 900°/0.02mm gives cyclopent [a]indene, characterized by 1H n.m.r. at -70° and by formation of a cyclopentadiene adduct and of a dimer at room temperature. Pyrolysis of 3,4-, 3,5- and 3,6-diphenylphthalic anhydrides also gives much cyclopent [a]indene by loss of the additional phenyl group. 3,4-Diphenylphthalic anhydride forms triphenylene as the major product. The pyrolysate from 3-vinylphthalic anhydride contains the known dimer of pentalene , and phenylacetylene. Pyrolyses of 3-(1-naphthyl) phthalic anhydride and of 4-phenylphenanthrene-2,3-dicarboxylic anhydride are described.