1.1- Diarylethylenes, photooxygenation

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

Fig. 34 Mechanism of product formation in the Wagnerova Type II photooxygenations of 1,1 -diarylethylenes.

FT photooxygenation of 1,1-diarylethylenes in the presence of electron acceptors such as cyanoaromatics, Lewis acids, and dyes occurs efficiently to yield 1,2-dioxanes as the main product in addition to diarylketones as side products. The yield of the 1,2-dioxane derivatives proved to be dependent on aryl substitution, solvent polarity, the electron acceptor, and the excitation wavelength (Scheme 39). 

Gollnick, K., Schnatterer, A., and Utschick, G., Substituent-dependent electron-transfer-induced photooxygenation of 1,1-diarylethylenes,/. Org. Chem., 58, 6049,1993. 

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