Endoperoxide formation of helianthrene with triplet molecular oxygen. A spin-forbidden reaction

Article abstract of DOI:10.1021/ja00038a007

Dibenzo[A,o]perylene (helianthrene = HEL), known to be a very reactive singlet molecular oxygen (¹O₂, ¹Δ_g) acceptor, also reacts with molecular oxygen in its triplet ground state (³O₂,³∑^-_g) to form helianthrene endoperoxide (HELPO). The thermal endoperoxide formation was studied in several solvents at room temperature. In nonpolar solvents such as toluene and carbon disulfide the temperature-dependent equilibrium, HEL + ³O₂ ? HELPO, is established by the thermolysis of HELPO even at low temperatures. Kinetic parameters for the HELPO formation in toluene are ΔH^? = 10.2 ± 1.0 kcal mol^-1 and ΔS^? = -39 ± 5 eu, indicating that ¹O₂ is not involved in the reaction pathway. This is the first example of aromatic endoperoxide formation by reaction with molecular oxygen in its ground state.