Flash vacuum thermolysis of 5-(1-chloroethenyl)acenaphthene. A short synthesis of pyracylene and its behaviour under high temperature conditions

Flash vacuum thermolysis (FVT) of 5-(1-chloroethenyl)acenaphthene (4) has been found to give rise to pyracylene (1). Pure 1 can be isolated from the 1100 °C pyrolysate by recrystallization at –20 °C. The temperature conversion data reveal that 1 decomposes and rearranges at T⩽ 1000 °C; acenaphthylene (11), 1-ethynyl-(23) and 3-ethynyl-acenaphthylene (24) have been identified. The formation of 23 and 24 indicates that 1 rearranges to the transient cyclopent[bc]acenaphthylene (20)via a single ring-contraction–ring-expansion mechanism under FVT conditions. The experimental data are supported by semiempirical AM1 calculations of the Cl4H8 potential energy surface.