ChemInform Abstract: Radical Ions and Photochemical Charge Transfer Phenomena. Part 16. Photoreactions of Biacetyl with Electron‐Rich Olefins. An Extended Mechanism.

Abstract

077ChemInform Abstract Products of cycloaddition and reduction are obtained by photolysis of (I) and (II) with quantum yields which increase with solvent polarity. The formation of reduction products connected with the presence of allylic hydrogen is enhanced with increasing oxidation potential. The photochemical and thermal reactions of (IX) with (II) are totally complementary, representing an example of a photochemically induced "Umpolung" of the reactivity of a 1,2-diketone. While (Id) and (II) react in a "clean" S1 reaction, the conversion with (IX) is a "clean" T1 process. The formation of (XI) and (XII) in addition to (X) in the photolysis in a polar solvent (MeCN) indicates photoionization (ESR of the radical cation derived from (Id)). It is concluded that an exciplex is formed reversibly which can be deactivated by electron transfer (ionic photodissociation), by product formation, and by decay to the ground state.