The Photochemistry of Heterocycles

This chapter discusses the photochemistry of heterocyclic systems. The formation of heterocycles, both by photoaddition and photocyclization, is rapidly becoming a preferable method of synthesis. The chapter elaborates the mechanisms of photochemical reactions. The increase in energy in a molecule on absorption of UV light is sufficient to bring about bond cleavage. As a result, fragmentation and rearrangement of the molecule can occur. The effect on heterocycles is discussed and the transformations are classified based on ring size. Pyrazolines, heterocyclic dienes, and heteroaromatic compounds are also discussed. Photoaddition to heterocycles can be done by 1, 2-cycloaddition, miscellaneous photoadditions, addition of water or an alcohol, and dimerization of heterocycles. The synthesis by photoaddition involves 1,2-cycloaddition and miscellaneous additions, whereas, the synthesis by photocyclization involves oxidative photocyclization and heterocyclic analogs of phenanthrene. Also, the photoaddition to heterocycles occurs by 1,4-cycloaddition, cyclization of halogen-containing compounds, and miscellaneous photocyclizations. The chapter also provides for a description of photooxidation of heterocycles.