Photocatalysis in an NMR tube: Carbon–carbon coupling of phenoxyacetic acid with N-substituted maleimides

The semiconductor-sensitised photocatalytic (SPC) carbon–carbon coupling of phenoxyacetic acid (PAA) and N-methylmaleimide (NMI) or maleimide (MI) has recently been reported and is studied in more detail. Irradiations are performed on a much smaller scale than commonly used, i.e. 1 mL of reaction solution in a sol–gel titania coated NMR tube, and NMR is used to monitor the progress of the various photoreactions. Use of an NMR tube as the photoreactor allows much faster reaction times (ca. 10 times faster than the 40 mL scale reaction), and helps avoid the more usual need for sample-taking and pre-treatment, before analysis by NMR, HPLC etc. The photochemistries and photocatalytic processes associated with the individual reactants (i.e. PAA and NMI or MI) reveal the production of significant products which are largely absent in the SPC-sensitised coupling reaction. Thus, when both PAA and NMI or MI are irradiated together, in the study of the coupling reaction the two main products formed are: 1-methyl-3-(phenoxymethyl)pyrrolidine-2,5-dione (the adduct product), and 2-methyl-3a,4-dihydrochromeno[3,4-c]pyrrole-1,3(2H,9bH)-dione (the cyclic product), which are formed in reasonable yields up to 67% and 15% respectively after only 15 min (I = 3.5 mW cm−2 per photoreactor hemisphere). Despite the well-known photochemical feature of NMI to form dimers efficiently, this work demonstrates that the background photochemistry and photocatalysis associated with the individual reactants, PAA and NMI, in the photocatalysed coupling reaction are minimal due to the faster SPC kinetics for the coupling reaction, and the UV-filter effect of the sol–gel titania coating. The additional formation of organic polymeric material in all the photocatalytic processes studied is discussed briefly.