Synthetic applications of photoinduced electron transfer decarboxylation reactions

Abstract

Photoinduced electron transfer (PET) decarboxylation of alkyl carboxylates in water leads to primary, secondary or tertiary carbon radicals which undergo C-C coupling reactions either in an intra- or intermolecular fashion. Intramolecular coupling gives rise to heterocyclic ring systems (lactams, lactones, 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, dilution 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-accepting chromophores which have been investigated are the imides of phthalic, maleic, quinolinic and trimellitic acid. Selectivity can be further controlled by two simple tricks: - Only triplet excited acceptors are active in these electron-transfer reactions, thus sensitization excludes other (homolytic) pathways; - Donor groups (SR, NR2, aryl, OR, C=C) other than the carboxylate can be activated by lowering the pH (and thus protonating the COO-donor). In intramolecular reactions, substrate concentration up to 0.1 M were applied successfully and the reactions were completed in 10-12 h for multigram conversions.