CYCLOPENT[A]INDENE (BENZOPENTALENE) AND PENTALENE - PYROLYTIC FORMATION FROM 3-PHENYL-PHTHALIC AND 3-VINYL-PHTHALIC ANHYDRIDES

Pyrolysis of 3-phenylphthalic anhydride at 900 degrees/0.02 mm gives cyclopent[a]indene, characterized by H-1 n.m.r. at -70 degrees 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[alpha]indene by loss of the additional phenyl group. 3,4-Diphenylphthalic anhydride forms triphenylene as the major product. The pyrolysate from 5-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.