Photocatalysis for Organic Synthesis

Inorganic semiconductors such as TiO2 are known to generate free radicals when irradiated with UV-visible light in the presence of suitable substrates. This project will explore the chemistry of such radicals with the particular objective of identifying and optimising free radical addition reactions which will be beneficial in organic synthesis. Organic synthesis driven by heterogeneous photocatalysis is environmentally and economically attractive, and has the potential to achieve higher selectivity to desired products than conventional routes. We propose to explore a wide range of free radical addition reactions initiated by the known photo-Kolbe reaction of carboxylic acids over titania surfaces. Reactions showing the most promise will be examined in more detail, using in particular in-situ EPR spectroscopy (to observe the initially generated free radicals), in-situ NMR spectroscopy (to identify intermediates and products), and time resolved optical spectroscopy (to observe short lived species) to determine the reaction pathways. Initial studies will be made with TiO2, but we will also explore improvements in performance by adding metals to enhance hole:electron separation, or nitrogen dopants to achieve visible light activation. Visible light activation will also be attempted with other semiconductors. A crucial component of the project is the design and construction of reactors for scaling up promising reactions to a scale attractive to the pharmaceutical industry. The project team has wide experience in photocatalysis, free radical chemistry, in-situ spectroscopic methods and photocatalytic reactor design and construction. Advice and assistance in selection of target reactions relevant to the pharmaceutical industry is provided by GlaxoSmithKline. A successful outcome of the project could bring about a paradigm shift in technologies for high value organic synthesis.

A highly active photocatalyst film can be coated onto the inside of an nmr tube to enable the rapid screen of photocatalysed organic synthetic reactions.