Benzophenone-sensitized cyclodimerization of cinnamic acid in hydrotalcite interlayers : A study of the sensitization behavior

Abstract

Aromatic ketone-sensitized cyclodimerization was studied in the presence of an anion exchange, clay, hydrotalcite. Uv irradiation of the clay suspension coadsorbing trans-cinnamate and 4-benzoylbenzoate ions resulted in the efficient formation of a syn-head-to-head dimer. The ratio of the cis/dimer was significantly affected by the introduction of heavy atom substituents of the sensitizers in high concentrations but not in lower concentrations. Dimerization was revealed to be 10 times more efficient than E - Z isomerization by extrapolating to infinitely smaller concentrations of the sensitizers. The sensitization mechanism through the formation of an exciplex between the excited singlet 4-benzoylbenzoate and trans-cinnamate is presented on the basis of the efficient formation and stereoselectivity of the cyclodimer. A fluorescent probe, the 2-naphthoate ion, intercalated in the clay interlayers was shown to increase its fluorescence intensity in the presence of a sensitizer, implying the validity of a singlet sensitization mechanism via an exciplex formation. The turnover number for the energy transfer exceeded 40 per sensitizer molecule, thus the cyclodimerization was found to proceed catalytically with the sensitizers.