Unexpected High Temperature Behaviour of 2,2′‐Diethynylbiphenyl in the Gas Phase – a Precursor for Acephenanthrylene Instead of Pyrene

Flash Vacuum Thermolysis (FVT) of 2,2′‐diethynylbiphenyl (6) gave, instead of pyrene (5), acephenanthrylene (10) and fluoranthene (11) as major products. The unequivocal identification of 9‐ethynylphenanthrene (12) at T ⩽ 800 °C suggests that 12 is the initial stable product derived from 6. It is documented that under high‐temperature conditions in the gas phase compound 12 is efficiently converted into 10, which subsequently rearranges to 11. The formation of 12 from 6 is rationalized by invoking the transient formation of cyclobuta[l]phenanthrene (13) by intramolecular cyclization of the ethynyl moieties of 6 followed by a retro‐carbene CH insertion and a 1,2‐H shift. This interpretation is supported by the results of independent FVT of 2,2′‐bis(1‐chloroethenyl)biphenyl (15).