Singlet Oxygen and Organic Photochemistry

There are two main chemical reactions of singlet oxygen that are of interest here. The first is cycloaddition in a process that resembles a Diels-Alder reaction. 

For a detailed discussion of the structure of singlet oxygen, see Kasha, M. in Primer, A. A., Ed. Singlet O2, Vol. I CRC Press Boca Raton, FL, 1985 pp. 1-11. 

The second is allylic oxidation, which can also be visualized as an ene reaction. The resulting allyl hydroperoxide can then be reduced to an allyl alcohol. 

A practical example of the use of singlet oxygen is provided by the synthesis of rose oxide (131) from /3-citronellol (129, equation 12.84). The sensitizer was rose bengal, a dye that can be used for singlet oxygen reactions induced by sunlight. The product of the ene reaction was reduced with Na2S03 to a diol (130), which then underwent spontaneous cyclization to 131 in a cold, acidic solution.  

The mechanism was characterized as a two-step process without an intermediate because two saddle points occur consecutively on the potential energy surface. Singleton, D. A. Hang, C. Szymanski, M. J. Meyer, M. P. Leach, A. G. Kuwata, K. T. Chen, J. S. Greer, A. Foote, C. S. Houk, K. N. /. Am. Cheni. Soc. 2003, 125,1319. 

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