Rose Bengal Functionalized Phase-Transfer Catalysts Promoting Photooxidations with Singlet Oxygen. Nucleophilic Displacements on Dioxetanic and Endoperoxidic Intermediates

Article abstract of DOI:10.1021/jo00392a002

Photooxidations with ¹O2 promoted by an anionic photosensitizer, such as Rose Bengal, can be readily performed in an aqueous-organic, two-phase system in the presence of catalytic amounts of quaternary onium salts that transfer the anion in the organic phase, This means that one can use low-polar solvents, which are unusual in reactions with this sensitizer.Soluble and silica gel immobilized onium salts were used as the phase-transfer catalysts.If a nucleophile is also present in the reaction mixture, it, too, is transported in the organic phase and reacts via S_N2, if the intermediate is of the dioxetanic or endoperoxidic type.With the heterogenized catalysts it is possible to carry out the reaction in two steps: the photooxidation itself and later the nucleophilic displacement.In addition to being productive with regard to the synthesis, this can clarify the photooxidation reaction pathway.The following organic substrates were photooxidized under PTC conditions: anthracene, 2,3-dimethyl-2-butene, 5,6-dihydro-2,3-diphenyl-p-dioxin, 1,3-cyclohexadiene, and indene.In the presence of N3^- as nucleophile, anthracene gives 10-imino-9(10H)-anthracenone while 5,6-dihydro-2,3-diphenyl-p-dioxin yields, selectively, two different azido derivatives according to the procedure employed.