Photoinduced Cycloadditions with Olefins

The oxidation of a large variety of thiocarbonyl compounds to the corresponding S-oxides has been studied by Zwanenburg.40 Like normal thiones, thioacylsilanes 

The geometry of the silyl sulfines 30 was determined by LIS measurements. During chromatographic purification on silica gel, a partial protiodesilylation to the corresponding thioaldehydes 5-oxides was observed (vide infra). 

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The photoinduced [2 + 2] cycloaddition of carbonyl acceptors with electron-rich olefins leads to oxetanes (Paterno-Biichi reaction) with high regio- and stereoselectivities (equation 25). 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

In the context of photoinduced [2 -I- 2] cycloadditions, the reaction of an olefin with a carbonyl center, known as the Patemo-Biichi reaction, has to be mentioned. The resulting oxetanes are thereby produced with high regioselectivity and stereoselectivity. 

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