Singlet oxygen reaction intermediates exciplexes

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

Gorman and coworkers found ° that the reactions of singlet oxygen with a variety of substrates have negative activation enthalpies and concluded that a reversible formed exciplex is the intermediate. They pointed out that none of the experiments clearly distinguishes between formation of a perepoxide or a geometrically similar exciplex. 

The papers on simple alkenes [151] and enol ethers [152] discussed above brought the possibility of exciplex intermediacy to the attention of singlet oxygen chemists and since then several papers concerned with reactions involving allylic hydroperoxide, dioxetane and/or endoperoxide formation have invoked their participation [157-165], some as the sole intermediate, some as precursors to perepoxides, open zwitterions or biradicals. The molecules shown in structures 43-51, and relatives thereof, have been... 

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