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| - Singlet Oxygen Promoted Carbon−Heteroatom Bond Cleavage inDibenzyl Sulfides and Tertiary Dibenzylamines. Structural Effectsand the Role of Exciplexes
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| - The C−heteroatom cleavage reactions of substituted dibenzyl sulfides and substituted dibenzylcyclohexylamines promoted by singlet oxygen in MeCN have been investigated. In both systems, the cleavagereactions (leading to benzaldehyde and substituted benzaldehyde) were slightly favored by electron-withdrawing substituents with ρ values of +0.47 (sulfides) and +0.27 (amines). With dibenzyl sulfides,sulfones were also obtained whereas sulfoxide formation became negligible when the reactions werecarried out in the presence of a base. Through a careful product study for the oxidation of dibenzylsulfide, in the presence and in the absence of Ph2SO, it was established that sulfone and cleavage product(benzaldehyde) do not come by the same route (involving the persulfoxide and the hydroperoxysulfoniumylide) as required by the generally accepted mechanism (Scheme 1) for C−heteroatom cleavage reactionsof sulfides promoted by singlet oxygen. On this basis and in light of the similar structural effects notedabove it is suggested that dibenzyl sulfides and dibenzylamines form benzaldehydes by a very similarmechanism. The reaction with singlet oxygen leads to an exciplex that can undergo an intracomplexhydrogen atom transfer to produce a radical pair. With sulfides, collapse of the radical pair leads to anα-hydroperoxy sulfide than can give benzaldehyde by an intramolecular path as described in Scheme 3.With amines, the radical pair undergoes an electron-transfer reaction to form an iminium cation thathydrolyzes to benzaldehyde. From a kinetic study it has been established that the fraction of exciplexconverted to aldehyde is ca. 20% with sulfides and ca. 7% with amines.
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