Ce(IV) reagents have been extensively utilized in organic synthesis, however, their use in intramolecular C-C bond-forming reactions is very limited. In our recent work exploring the utility of Ce(IV) reagents in the oxidations with 2,4-diketones, we found that solvents played an important role in determining the chemoselectivity of the reactions. In an initial experiment, 6-phenyl-hexane-2,4-dione was treated with 2 equiv of Ceric Ammonium Nitrate (CAN) in both acetonitrile and MeOH at room temperature. The reaction in acetonitrile produced 3-phenyl-propionic acid exclusively, whereas, the same reaction in MeOH afforded the expected Friedel-Crafts cyclization product. This solvent dependent chemoselectivity was found for a number of substrates and results showed that cyclized products or carboxylic acids were obtained as the major or only products in MeOH or acetonitrile respectively. While the formation of carboxylic acids through fragmentation is known, they normally require high temperature and an acidic medium. Considering the advantage of neutral and mild reaction conditions with CAN, the scope of this new acid preparation method was studied. Compatibilities of functional groups were also investigated. Mechanistic studies were carried out with Stopped-Flow Spectroscopy and React IR. Initial results showed that nitrate group participated in the carboxylic acid formation. Further mechanistic studies indicate that the solvent dependent chemoselectivities may arise from the stabilities of the radical cations formed between 2,4-diketones and CAN in these two solvents.
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