Visible-Light Photocatalyzed Selective Synthesis of 1,2- or 1,3-Dithianes From Thietanes: An Experimental and Theoretical Study

Unlike other sulfur-containing heterocycles, the potential applications of functionalized dithianes have been less explored to date due to their challenging synthesis. To address this issue, we herein leverage the straightforward access to functionalized thietanes and describe a selective photochemical ring enlargement of the latter to 1,2- or 1,3-dithianes using thiocarbonyl substrates, which are in situ generated via a Norrish-type II fragmentation reaction. The library of dithianes obtained by this photocatalyzed domino sequence between diversely substituted thietanes and various thiocarbonyls revealed regioselectivity rules: excited thioketones lead exclusively to 1,2-dithianes, while excited thioaldehydes selectively yield 1,3-dithianes, where 1,2-dithianes serve as precursors to their regioisomers. The ring expansion reaction leading to 1,2-dithianes was also integrated into a triple photochemical cascade, where the thietane intermediates were obtained via a thia-Paternò^_Büchi reaction between Norrish-II thiocarbonyl products and various alkene partners. These experimental results were complemented with time-dependent density functional theory calculations, which support the complex chemical pathway, rationalize the observed regioselectivity, and further substantiate mechanistic proposals related to the photolysis of disulfide bridges in peptides.