No abstract

Consideration of the relationship between the mechanisms and orientation effects of the nitro group in nucleophilic photosubstitutions of nitroaromatics leads to the proposal of two rules, making use of frontier molecular orbital theory, for interpreting the regioselectivities and determining the reaction mechanisms. The first rule is a summary of previously proposed interpretations, concerning photoreactions accompanying the direct addition of a nucleophile to an excited nitroaromatic substrate. In this case, the regioselectivity is governed by the HOMO of the substrate. The second rule regards reactions involving photo-induced electron transfer, where regioselectivity is LUMO-controlled. The application of these rules is successfully demonstrated for various types of compounds.

Photo-Smiles rearrangement reaction of N-[ω-(p-nitrophenoxy)alkyl]aniline was studied by measuring the absorption spectra of transient species. Two intermediates A and B were observed and were assigned to a solvated ion pair and the Meisenheimer complex, respectively. The reaction scheme, starting material→intramolecular charge transfer on excitation→A→B→final product, was derived from the relative yields and lifetimes of A and B as well as from the effects of solvent polarity, of triethylamine and oxygen, and of the alkyl chain length on each step of reaction.

3-Nitrenopheny1)methylene (I), a non-Kekule molecule with quintet ground state, has been generated photochemically from (3-azidopheny1)diazomethane in organic matrix at cryogenic temperatures.

Orientation rules for photochemical nucleophilic substitutions are deduced from intramolecular photoreactions of some bichromophoric chain molecules, p-O2NC6H4O(CH2)nNHC6H5. The lower homologues undergo photochemical nucleophilic substitutions which can be classified as a Smiles-type photorearrangement. The photochemistry of this type of bifunctional chain species depends upon the methylene chain length. When the chain length is sufficiently long, the photorearrangement is totally forbidden. Photoredox reactions are the only accessible reaction processes for the higher homologues. The photoredox reaction consisting of cage and escape pathways is modulated by an external magnetic field. The magnetic field effects on the end product distribution are due to changes in the intersystem crossing rate induced by hyperfine interaction within biradical intermediates.