CIDNP Investication On The Mechanism Of Some Free Radical Reactions

Photoinduced electron transfer reaction is one of the most promising front fields in organic photochemistry. It created a brand new field - ion radicals chemistry. And Chemically Induced Dynamic Nuclear Polarization (CIDNP) is one of the most powerful means to study free radical reactions and structures of the intermediates.Firstly in this present paper, we use CIDNP technology to study the mechanistic aspects of photoinduced cycloaddition reactions of two benzoquinone derivatives with four arylacetylenes and the photoinduced H-abstraction reactions between l-acetylindole-2, 3-didone and different electron and hydrogen donors.CIDNP investigation on the mechanism of photocycloadditions of 2-chloro-5-methoxybenzoquinone and 5-methoxybenzoquinone with four arylacetylenes testified that: The structure of the reactants and the free energy change of electron transfer reaction could effect the reaction to anappreciable degree. Reactions between n-7t* triplet excited 2-chloro-5-methoxybenzoquinone and these arylacetylenes have characteristics of Paterno- Biichi Reaction. In nonpolar solvent, benzene, 1,4-biradical intermediates are formed directly, which lead to the observation of strong CIDNP effects. However, in polar solvent, acetonitrile, the reducing of the stability of the excited quinone and the drastic solvent polarity effect on the reaction, which made further dissociation of the triplet ion-radical pairs, and thus the suppressed intermolecular coupling leading to the cycladdition, could all contribute to the lack of polarization.The relatively weaker acceptor 5-methoxybenzoquinone can only react with benzoylpheylacetylene at its TI-TI excited state to give CIDNP effect.The reactions between l-acetylindole-2, 3-didone with phenols and piperonal are kind of free radical reactions. The hydrogen is abstracted by the oxygen atom in the carbonyl group of the excited triplet state of l-acetylindole-2,3-didone, which made the formation of the neutral free-radical pairs. Then, by back hydrogen transfer or random reaction, the polarized products can be observed to give CIDNP effects.