Photocyclization of aryl halides. Part I. 5-(2-Halogenophenyl)-1,3-diphenylpyrazoles; homolytic fission assisted by radical complexation

The photocyclization of 5-(2-halogenophenyl)-1,3-diphenyl pyrazoles proceeds efficiently with the exclusion of solvent participation hydrogen abstraction reactions. In a given solvent, the quantum yields of cyclization [Formula: see text] of Cl, Br, and I compounds show no correlation with carbon–halogen bond dissociation energies. In a series of solvents, [Formula: see text] varies smoothly with solvent viscosity and is independent of solvent polarity. The [Formula: see text] values are independent of triplet quenchers, air, intensity and wavelength of radiation, and reactant concentration. Photocyclization can be triplet sensitized at up to 80% of the direct reaction rate and the reactant triplet energies are about 65 kcalmol−1 while DC—Br is 81 and DC—Cl is 95 kcalmol−1. A mechanism of homolytic fission assisted by radical complexation is proposed to accommodate the results. Photocyclization of the H- and MeO-substituted analogues proceeds inefficiently and with [Formula: see text] values dependent on solvent polarity. An electrocyclic mechanism is followed in these cases. The assisted homolysis model explains several anomalies in the field of aryl halide photochemistry.