Photocatalytic oxidation of toluene in an NMR tube

It is shown that NMR is a very effective method for monitoring the semiconductor photocatalytic oxidation of toluene to benzaldehyde and benzoic acid and that this is achieved in situ in an NMR tube on a much smaller scale than commonly used in the literature, using either a dispersion or film of the semiconductor. Maximum percentage conversions to benzaldehyde and benzoic acid of 27% (after 45 min) and 41% (after 75 min), respectively are reported using a titania film coated NMR tube. Similar percentage conversion values using a dispersion of the semiconductor in an NMR tube were obtained but over a slightly longer time scale (60 min c.f. 45 min). On a much larger scale (100 mL instead of 1.5 mL), the yields were also similar but much slower to reach (ca. 10 times). The marked reduction in photocatalytic efficiency using a dispersion on a large scale is probably due to the increased degree of light scattering and poor light penetration compared with the translucent titania film coating or dispersion in an NMR tube. The benefits and use of this titania film/NMR approach as a rapid screening method in semiconductor photocatalytic organic synthesis are discussed.