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

Abstract

Can. J. Chem. 58, 1880(1980). The photocyclization of 5-(2-halogenopheny1)-1,3-diphenyl pyrazoles proceeds efficiently with the exclusion ofsolvent participation hydrogen abstraction reactions. In agiven solvent, the quantum yields of cyclization (4) of CI, Br, and I compounds show no correlation with carbon-halogen bond dissociation energies. In a series of solvents, 4 varies smoothly with solvent viscosity and is independent of solvent polarity. The 4 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 kcal mol-I while DC-Br is 81 and DcPcl is 95 kcal mol-'. 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 4 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. [Traduit par lejournal]

Synthesis of Paullone and Kenpaullone Derivatives by Photocyclization of 2‐(2‐Chloro‐1H‐indol‐3‐yl)‐N‐arylacetamides

“…Such a low‐energy pathway would also be reasonable to explain the photocyclizations of 1a – k , because the photoreactions were more efficient in the triplet states of 1a – k (Table 2) and the triplet states of the indole rings in 1a – k each have the diradical structure,7b which should be favorable for coupling of the C‐2 moiety in the 2‐chloroindole system with tethered N ‐aryl groups. Chlorine and bromine atoms are electron‐deficient species with high electron affinities,12 their complexation by aromatic molecules is well known, and anchimeric assistance to C–X bond cleavage by radical complexation has also been confirmed by dynamic methods13 and flash photolysis studies 11b,14. A plausible mechanism based on the above suggestions is shown in Scheme .…”

Intramolecular Photochemical Cross‐Coupling Reactions of 3‐Acyl‐2‐haloindoles and 2‐Chloropyrrole‐3‐carbaldehydes with Substituted Benzenes

“…We believe this factor to be a type of anchimeric assistance whereby the phenyl ring can, through its π‐cloud, complex the developing radical centres in the stretching CX bond and thus lower the transition state energy for reaction as depicted in Scheme . Chlorine and bromine atoms are electron‐deficient species with high electron affinities22 and their complexation by aromatic molecules is well known and the anchimeric assistance to CX bond cleavage by radical complexation has also proved by dynamic methods23a and flash photolysis studies 23b,c. The mechanism affords an explanation why the chloro‐compound cyclizes faster than the bromo‐compound even though D C−Cl > D C−Br , since the transition state 5 is reached earlier along the reaction coordinate for X=C1 than for X=Br due to the higher electron affinity of the chlorine atom.…”

Photoinduced Polymerization of Haloaniline Studied by Liquid Beam-Multiphoton Ionization-Mass Spectroscopy

“…[20][21][22][23] In an electronically excited state of the solute molecule, homolytic cleavage of a carbon-hydrogen bond, nucleophilic substitution, and hydrogen abstraction are known to proceed, while nucleophilic substitution is expected to occur through its ion as well because electrophilicity of the molecule increases with its ionization (abstraction of an electron). [24][25][26] In addition, haloaniline has two functional groups; the halogen atom is released upon photoexcitation, and the radical site is produced accordingly, while the NH 2 group is vulnerable by electrophilic attack. This feature predicts polymerization of haloaniline by the photoexcitation.…”