Unexpected Radical Mechanism in a [4 + 1] Cycloaddition Reaction

9,10-Phenanthrenequinone monoimines (2,7-R-PQI, R = tBu, H, Br, NO2, 1a–d) undergo a [4+1] cycloaddition reaction with triphenylphosphine to give 2,3-dihydro-2,2,2-triphenylphenanthro[9,10-d]-1,3,2λ5-oxazaphospholes (3a–d). During the reaction, highly colored radicals are formed as intermediates, which were characterized by EPR and UV-vis spectroscopy. The formation rate and the rate of decay of these radicals were determined kinetically. These radicals exhibit high persistency and under inert conditions can be handled conveniently. Based on detailed kinetic and spectroscopic studies and DFT calculations, a plausible mechanism has been proposed.