Synthetic applications of photoinduced electron transfer decarboxylation reactions

Photoinduced electron transfer (PET) decarboxylation of alkyl carboxylates in water leads to primary, secondary or tertiary carbon radicals which unde rgo C-C coupling reactions either in an intra- or intermolecular fashion. I ntramolecular coupling gives rise to heterocyclic ring systems (lactams, la ctones, cyclopeptides, cyclic ethers, crown ethers) with ring sizes from 5 to 28 and a broad variety of functionalities. Intermolecular coupling gives Grignard-type adducts (but with different chemo- and regioselectivities as for carbanion reactions). Yields of these reactions are mostly high, dilut ion conditions are not necessary, quantum yields are in the range of 0.5-0. 6, and chemo- as well as regioselectivities are excellent. The electron-acc epting chromophores which have been investigated are the imides of phthalic , maleic, quinolinic and trimellitic acid. Selectivity can be further contr olled by two simple tricks: Only triplet excited accepters are active in these electron-transfer reacti ons, thus sensitization excludes other (homolytic) pathways; Donor groups (SR, NR2, aryl, OR, C=C) other than the carboxylate can be act ivated by lowering the pH (and thus protonating the COO-donor). In intramolecular reactions, substrate concentration up to 0.1 M were appli ed successfully and the reactions were completed in 10-12 h for multigram c onversions.