Sunlight induced functionalisation reactions of heteroaromatic bases with aldehydes in the presence of TiO2: a hypothesis on the mechanism

In previous studies we reported a new photocatalytic system involving polycrystalline TiO2 for the selective functionalisation of heteroaromatic bases with ethers and amides. In order to extend the applications of this new reaction and to better understand the mechanism involved, we have examined aromatic and aliphatic aldehydes as acyl radical sources for the nucleophilic addition to protonated N-heteroarenes in acetonitrile as the solvent and TiO2/H2O2 as the photocatalytic system. Acyl radicals may undergo decarbonylation to yield the corresponding alkyl radicals. Acyl/alkyl derivative ratios depend on the nature of the aldehydes, and present a different distribution from that obtained in corresponding redox reactions. Indeed, decarbonylation of primary aldehydes occurs in our system in significant amounts. For instance, the acyl/alkyl ratio in the case of the pivaloyl radical is 0.03 versus 1.2 for the redox reaction carried out under otherwise identical conditions of temperature and base concentration. Different polycrystalline TiO2 samples were used and some differences in yields and product distribution were found. A mechanism is proposed on the basis of results obtained for which oxidation of the intermediate adducts is considered to occur on the surface of the photocatalyst TiO2 either via direct involvement of valence band holes or indirectly via photogenerated OH radicals, whereas the additional oxidants H2O2 and O2 (air) scavenge the photoelectrons produced.