α-Thio Carbocations (Thionium Ions) as Intermediates in Brønsted Acid-Catalyzed Reactions of Enone-Derived 1,3-Dithianes and 1,3-Dithiolanes

Abstract Evidence was collected for the intermediate formation of thionium ions in Brønsted acid-catalyzed [2 + 2] photocycloaddition and electrophilic addition reactions to enone dithianes and dithiolanes. Low-temperature NMR studies helped to elucidate the structure and configuration of the thionium ions and thus support previous and current results obtained by UV/Vis spectroscopy. Graphical Abstract Electronic supplementary material The online version of this article (10.1007/s11244-018-0905-6) contains supplementary material, which is available to authorized users.


General Information
All moisture and air sensitive reactions were carried out in flame-dried glassware under an argon atmosphere using standard Schlenk techniques.
Commercially available chemicals were used without further purification, if not further mentioned. For moisture sensitive reactions tetrahydrofuran (THF) and dichloromethane (CH2Cl2) were purified using a MBSPS 800 MBraun solvent purification system. The following columns were used: For NMR studies, Deutero dichloromethane-d2 (99.6 atom% D) was employed which was dried by filtration through a pad of activated basic aluminium oxide under argon atmosphere and was stored over molecular sieves (4 Å).
The following dry solvents are commercially available and were used without further purification: Methanol: Acros Organics, 99.8%, extra dry, over molecular sieve.

Analytical Methods
Thin layer chromatography (TLC) was performed on silica coated glass plates (Merck, silica 60 F254) with detection by UV-light (λ = 254 nm) and/or by staining with a potassium permanganate solution [KMnO4] followed by heat treatment.
Infrared spectra (IR) were recorded on a JASCO IR-4100 spectrometer or a Perkin Elmer Frontier IR-FTR spectrometer by ATR technique. The signal intensities are assigned using the following abbreviations: s (strong), m (medium), w (weak), br (broad).
The aqueous layer was acidified to a pH of 1 via dropwise addition of conc. aqueous HCl solution at 0 °C. The title compound 16 was obtained after extraction with CH2Cl2 (5x20 mL), drying over Na2SO4 and removal of volatiles in vacuo as a colourless oil in (2.6 g, 23mmol, 41%).   The obtained data matched that previously reported in the literature [2] S8
The solution was allowed to warm to room temperature and was stirred for four hours. After addition of water (60 mL), the aqueous layer was extracted with CH2Cl2 (3x45 mL). The combined organic layers were dried over Na2SO4, filtered and the solvent was removed in vacuo. The crude mesylate was used without further purification and was dissolved in acetone S9 (55 mL). After addition of lithium bromide (4.68 g, 53.9 mmol, 3.00 equiv), the mixture was stirred at 60 °C for 12 h. After cooling to room temperature, water (30 mL) was added and the aqueous layer was extracted with Et2O (3x30 mL). The combined organic layers were dried over Na2SO4, filtered and the solvent was removed in vacuo. After purification by column chromatography (SiO2, P), the title compound 18 was obtained as a colourless oil (1.65 g, 9.88 mmol, 55%).  The obtained data matched that previously reported in the literature. [4]