75640-15-2Relevant articles and documents
Photocyclization of aryl halides. Part I. 5-(2-Halogenophenyl)-1,3-diphenylpyrazoles; homolytic fission assisted by radical complexation
Grimshaw, James,Silva, A. Prasanna de
, p. 1880 - 1888 (2007/10/02)
The photocyclization of 5-(2-halogenophenyl)-1,3-diphenylpyrazoles proceeds efficiently with the exclusion of solvent participation hydrogen abstraction reactions.In a given solvent, the quantum yields of cyclization (Φ) of Cl, Br, and I compounds show no correlation with carbon-halogen bond dissociation energies.In a series of solvents, Φ varies smoothly with solvent viscosity and is independent of solvent polarity.The Φ values are independent of triplet quenchers, air, intensity and wavelength of radiation, and reactant concentration.Photocyclization can be triplet sensitized at up to 80percent of the direct reaction rate and the reactant triplet energies are about 65 kcal mol-1 while DC-Br is 81 and DC-Cl is 95 kcal mol-1.A mechanism of homolytic fission assisted by radical complexation is proposed to accomodate the results.Photocyclization of the H- and MeO-substituted analogues proceeds inefficiently and with Φ 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.