Reactions at high pressure. Part 15. Rates, activation parameters, and a volume profile for retro-Diels–Alder reactions in the pyrrole series
Thermal decomposition of N-acylpyrrole adducts of N-phenylmaleimide by a [π2S+σ2S+σ2S] route occurs at temperatures below 70 °C. Rates and activation parameters for the thermolyses ot endo- and exo-10-benzoyl-4-phenyl-4,10-diazatricyclo[5.2.1.02,6]dec-8-ene-3,5-dione (6) and (7), exo-10-acetyl-4-phenyl-4,10-diazatricyclo[5.2.1.02,6]dec-8-ene-3,5-dione (8), and endo-4-phenyl-4-aza-10-oxatricyclo[18.104.22.168]dec-8-ene-3,5-dione (9) have been measured at various temperatures and pressures and in various solvents. It is confirmed that the volume of activation for thermolysis of (6) is negative, consistent with a transition state more compact than either reagents or products and not accountable for in terms of dipolar character.