New Insight into the Reaction of Singlet Oxygen with Sulfur-Containing Cyclic Alkenes: Dye-Sensitized Photooxygenation of 5,6-Dihydro-1,4-dithiins

The reaction of 3-methyl-5,6-dihydro-1,4-dithiins with singlet oxygen affords dicarbonyl compounds
and/or ring-contracted ketosulfoxides, the latter regio- and stereoselectively, depending on the nature of
the substituent at C-2 and on the reaction conditions. In competition with normal fragmentation, the
intermediate dioxetanes, derived from [2 + 2] cycloaddition of singlet oxygen to the double bond, undergo
an intramolecular oxygen transfer to the sulfur-1 atom, leading to labile epoxide intermediates. The latter
convert to cis- and trans-ketosulfoxides through a non-concerted S-4 migration. This pathway is promoted
by the electron-withdrawing group at C-2 and, for monosubstituted amide, by the solvent basicity.
S-Oxidation of dithiins is insignificant, except for the monosubstituted amide derivative or in the presence
of protic species, and occurs selectively at the S-1 atom.

3-甲基-5,6-二氢-1,4-二噻英与单线态氧的反应可生成二羰基化合物和/或缩环酮亚砜，后者的生成具有区域及立体选择性，其选择性取决于C-2位取代基的性质与反应条件。在与常规碎裂过程的竞争中，由单线态氧与双键发生[2+2]环加成生成的中间体二氧杂环丁烷，会发生分子内氧向S-1原子的转移，从而得到不稳定的环氧化物中间体。该环氧化物中间体可通过非协同的S-4迁移转化为顺式和反式酮亚砜。此反应路径可被C-2位的吸电子基团所促进，对于单取代酰胺底物而言，还可受溶剂碱性的调控。二噻英的硫氧化反应通常并不显著，仅在单取代酰胺衍生物存在或质子性物种存在的条件下才会发生，且该氧化过程会选择性地在S-1原子处进行。