Abstract
In situ EPR spectroscopy at cryogenic temperatures has been used to observe and identify paramagnetic species produced when titania is irradiated in the presence of reactants used in the photocatalytic alkylation of maleimide with t-butyl carboxylic acid or phenoxyacetic acid. It is shown that maleimide acts as an acceptor of conduction band electrons. Valence band holes oxidise t-butyl carboxylic acid to the t-butyl radical and phenoxyacetic acid to the phenoxyacetic acid radical cation. In the presence of maleimide, the phenoxymethyl radical is formed from phenoxyacetic acid. The relevance of these observations to the mechanisms of titania photocatalyst-promoted alkylation of alkenes is discussed.
Highlights
Semiconductor photocatalysis (SPC) is widely used in environmental applications for the destruction of organic compounds in solution or the gas phase [1], there is growing interest in applying the method to productive synthetic chemistry
We recently reported the first successful application of this strategy to achieve alkylation and cyclisation reactions when a variety of carboxylic acid precursors reacted with electron-deficient alkenes in the presence of titania photocatalysts [6]
Their observation supports the reaction mechanisms postulated of radicals generated from the carboxylic acid precursors attacking the double bonds of electron-deficient alkenes, but does not address the issue of how radicals are generated at the photocatalyst surface
Summary
Semiconductor photocatalysis (SPC) is widely used in environmental applications for the destruction of organic compounds in solution or the gas phase [1], there is growing interest in applying the method to productive synthetic chemistry. The isotropic nature of the EPR signals observed from these radicals at room temperature suggested that they may be present in solution rather than on the titania photocatalyst surface Their observation supports the reaction mechanisms postulated of radicals generated from the carboxylic acid precursors attacking the double bonds of electron-deficient alkenes, but does not address the issue of how radicals are generated at the photocatalyst surface. EPR spectroscopy detects the initial products of the trapping of the valence band holes and conduction band electrons by adsorbed reactants during UV irradiation of the photocatalyst. We show that this provides further insight into the mechanisms of these reactions
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