The processes involved in the conversion of triphenylene , C18H12, into cyclopent [hi] acephenanthrylene, C18H10, under flash vacuum pyrolytic conditions at 900-1100°C have been investigated by pyrolysing triphenylene-1,2- and -2,3-dicarboxylic anhydrides and diallyl triphenylene-1,3- and -1,4-dicarboxylates to give the corresponding didehydrotriphenylenes in the gas phase. These didehydro intermediates are converted into mixtures of cyclopent [hi] acephenanthrylene and triphenylene in different yields and proportions. Pyrolysis of 9,10-diethynylphenanthrene. C18H10, yields cyclopent [hi] acephenanthrylene in good yield. Pyrolysis of 1-nitrotriphenylene and allyl triphenylene-2-carboxylate to give the triphenylen-1-yl and -2-yl radicals leads to formation of the same products. Mechanisms involving radical rearrangements (C18H11 species) and benzyne-cyclopentadienylidenecarbene and ethyne-ethenylidene rearrangements (C18H10 species) are discussed.
Flash vacuum pyrolysis (f.v.p.) of [2,3-13C2]triphenylene-2,3-dicarboxylic anhydride (c. 22·5% 13C2) at 950°C gave a pyrolysate which was analysed by 13C n.m.r. spectroscopy. The principal product was [2,3-13C2]triphenylene. The second major product was a 1 : 2 : 1 mixture of [4,7- 13C2]-, [4,6-13C2]- and [5,6-13C2]-cyclopent[hi]acephenanthrylene.