Mechanism of photocyclization of substituted phenylbenzoquinones

Abstract

The mechanism of the photocyclization of phenylbenzoquinones substituted in meta position on the phenyl to 2-hydroxydibenzofuran derivatives was investigated by combined laser flash photolysis (LFP) and continuous irradiation measurements in various solvents. In any type of solvent, the triplet state of the quinone is converted by a thermally activated process to a cyclized intermediate X which, in polar and/or acidified solvents, rearranges to the final product by a sequence protonation/deprotonation, whereas in non-polar solvents, it reopens to the starting material. The triplet state of the quinone responsible for the cyclization is an nπ ∗ state located, depending on the substituent, a few kilojoules per mole above or below the ππ ∗ triplet state involved in dimerization but the rate of cyclization is not determined by the nature of the lowest triplet state because the energy splitting between the nπ ∗ and ππ ∗ triplet states is much smaller than the activation energy of the cyclization reaction. The meta methoxy substituted compound gives rise, in the excited singlet state, to an intramolecular charge transfer state stabilized by polar solvents so that the yield of formation of the triplet and, consequently, the cyclization quantum yield strongly decrease in these solvents. The two rotamers of this compound have, in their triplet state, quite different reactivities so that the cyclization is regiospecific. Substitution by an electron withdrawing group such as CF 3 has two major effects: (i) it raises the energy level of the ππ ∗ states so that the lowest triplet state becomes nπ ∗ and H-abstraction to alkanes and alcohols becomes an important reaction pathway and (ii) it makes possible an electron transfer from solvents with a low oxidation potential to the quinone triplet state, reducing strongly the triplet lifetime and, consequently, the quantum yield of cyclization. Thus, many factors influence the cyclization quantum yield measured under continuous irradiation as well as the competition between the various reaction pathways. That is why previous mechanistic studies based on measurements of this type on a series of compounds in various solvents are largely erroneous.