Olefins, photoinduced cycloaddition

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. 

Examples of photoinduced addition reactions of double bonds (a) acyclic addition of methanol, (b) cycloaddition of an olefin... 

The described problem was encountered in investigations of cis-trans isomeriza-tions and cycloadditions of donor olefins D and acceptor olefins A in acetonitrile. All polarizations, both of the cycloadducts and of the starting and isomerized olefins, could be traced to radical ion pairs D" + A" formed by photoinduced electron transfer. As, however, exciplexes are frequently discussed as percursors to the products in such systems,and CIDNP does not respond to exciplexes because no diffusive separation is possible, the question as to the relative contributions of the radical ion and exciplex pathways arose. To answer it, we employed photoinduced electron transfer sensitization (PET-sensitization). 

Cycloadditions of carbonyl compounds to olefins generally involve exci-plexes and biradicals. (Cf. Section 7.4.4.) While normal olefins frequently yield a number of products, photoinduced electron transfer may be utilized in the case of electron-rich olefins to influence the regioselectivity. Thus, irradiation of the ketene acetal 165 and biacetyl (164) yields exclusively the oxetane 167. Since the radical cation 166 could be trapped, electron transfer... 

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