Flash vacuum thermolysis of acenaphtho[1,2-a]acenaphthylene, fluoranthene, benzo[k]- and benzo[j]fluoranthene - Homolytic scission of carbon-carbon single bonds of internally fused cyclopenta moieties at T ≥ 1100°C

Flash vacuum thermolysis (FVT, 1000 °C ≥ T ≥ 1200 °C) of acenaphtho[1,2-a]acenaphthylene (3, C₂₂H₁₂) gave the C₂₂H₁₂ cyclopenta-fused polycyclic aromatic hydrocarbon (CP-PAH) acenaphtho[1,2- e]acenaphthylene (4), cyclopenta[cd]perylene (5) and cyclopenta[def]benzo[hi]chrysene (6). Whereas the formation of 4 is explained by a ring contraction/ring expansion rearrangement of 3, the identification of 5 and 6 suggests that 3 also undergoes homolytic scission of a five- membered ring's carbon-carbon single bond furnishing the transient diradical intermediate 7. Ring closure of 7 to form 8 after rotation around the carbon- carbon single bond of the intact five-membered ring followed by hydrogen shifts will give 6. The latter can rearrange subsequently into 5 by ring contraction/ring expansion. The structural assignment of 4 and 5 was supported by independent FVT of 6,12-bis(1-chloroethenyl)chrysene (14) and 3- (1-chloroethenyl)perylene (23), respectively. FVT of 14 (900-1200 °C) gave in a consecutive process 6,12-bis(ethynyl)chrysene (15), 9- ethynylbenz[j]acephenanthrylene (16) and bis(cyclopenta[hi,qr])chrysene (17). Although at T ≥ 900 °C 17 selectively rearranges into 4 by ring contraction/ring expansion, at 1200 °C the latter is converted into 5 presumably via a diradical intermediate obtained by homolytic scission of a single carbon-carbon bond of a five-membered ring. FVT of 23 gave in situ 3- ethynylperylene (25), which at 1000 °C is nearly quantitatively converted into 5. The propensity of internal cyclopenta moieties to undergo homolytic scission of a five-membered ring's carbon-carbon single bond was corroborated by independent FVT of benzo[k]- (11) and benzo[j]fluoranthene (12). Previously unknown thermal pathways to important (CP)-PAH combustion effluents are disclosed at T≥ 1000 °C.