Journal of Photosciences

Recent Advances in Di-p-methane Processes. Novel Reactions of 1,4-Unsaturated Compounds Promoted by Triplet Sensitization and Photoelectron Transfer

Volume 9(2002), 9-20 page

ÀúÀÚ: Diego Armesto, Maria J. Ortiz and Antonia R. Agarrabeitia

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Keywords: Di-p-methane rearrangements  Single electron transfer  Radical-cations  Radical-anions  Cyclopropanes  Vinylaziridines


Recent studies on the photoreactivity of 1,4-unsaturated systems have changed some ideas that were firmly established in this area of research for many years. Thus, we have described the first examples of 2-aza-di-p-
methane (2-ADPM) rearrangements promoted by triplet-sensitization and by single electron transfer (SET) using electron-acceptor sensitizers. These reactions afford N-vinylaziridine and cyclopropylimine photoproducts in the
first examples of di-p-methane processes that yield three-membered ring heterocycles. 1-Aza-1,4-dienes also undergo SET-promoted 1-aza-di-p-methane (1-ADPM) rearrangements via radical-cation intermediates using
electron acceptor sensitizers. In some cases, alternative cyclizations yielding different carbocycles and heterocycles have been observed. The 1-ADPM and di-p-methane (DPM) reactions also occur via radical-anion intermediates on irradiation using electron donor sensitizers. On the other hand, the photoreactivity reported for b,g-unsaturated aldehydes for many years was decarbonylation to the corresponding alkenes. However, our studies demonstrate that these compounds undergo the oxa-di-p-methane (ODPM) rearrangement with high chemical and quantum efficiency. A comparison of the photochemical reactivity of b,g-unsaturated aldehydes and corresponding methyl ketones has shown that the ketones do not undergo the ODPM rearrangement while the corresponding aldehydes are reactive by this pathway. Monosubstituted b,g-unsaturated aldehydes at C-2 undergo the ODPM rearrangement yielding the corresponding cyclopropane carbaldehydes diastereoselectively. Finally, we have described the first
examples of reactions, similar to the well know Norrish Type I process, which take place in the triplet excited state of b,g-unsaturated carbonyl compounds by excitation of the C-C double bond instead of the carbonyl group.