(Z)-Tetrahydrothiophene and (Z)-tetrahydrothiopyran synthesis through nucleophilic substitution and intramolecular cycloaddition of alkynyl halides and EtOCS2K

Ruthenium(II)-Catalyzed Oxidative Annulation of Imidazo[1,5-a]quinolin-2-iums Salts and Internal Alkynes via C-H Bond Activation

Ruthenium(II)-catalyzed synthesis of π-conjugated fused imidazo[1,5-a]quinolin-2-ium derivatives have been achieved via C-H activation of quinoline-functionalized NHC (NHC=N-heterocyclic carbene) and oxidative coupling with internal alkynes. The reaction occurred with high efficiency, broad substrate scope, tolerates a wide range of functional groups and utilized into a gram-scale. Synthetic applications of the coupled product have been exemplified in the late-stage derivatization of various highly functionalized scaffolds. Moreover, most of the annulated products exhibit intense fluorescence and have potential applications in optoelectronic devices. Mechanistic studies have provided insights into the spectroscopic characterization of key five-membered ruthenacycle intermediate and Ru(0) sandwich species. Based on several control experiments, deuterium-kinetic isotope effect, and thermodynamic activation parameters the mechanistic finding demonstrated that fused imidazo-[1,5-a]quinolin-2-ium C(2)-H bond cleavage is the rate-determining step and ruling out the possibility of reductive elimination for controlling the rate of reaction.

Thiolation of Terminal Alkynes with Thiuram Disulfide Reagents Using Water as the Hydrogen Source: Stereoselective Synthesis of (Z)-Vinyl Sulfides

A stereoselective and environmentally friendly thiolation of terminal alkynes was reported. Thiuram disulfide reagents (tetramethylthiuram disulfide and tetraethylthiuram disulfide) that reacted with alkynes in dimethyl sulfoxide (DMSO)/H2O could give (Z)-vinyl sulfides in good yields (up to 88%). This protocol features broad substrate scope, good stereoselectivity, high atom economy, good yields, and is transition metal-free. Mechanistic studies revealed that water and DMSO served as hydrogen sources, which greatly highlighted the unique reactivity of this special reaction involving two H-atom donors.

Synthesis of Substituted Thiophenes through Dehydration and Heterocyclization of Alkynols

A protocol was described for obtaining a variety of substituted thiophenes with functional potential via metal-free dehydration and sulfur cyclization of alkynols with elemental sulfur (S₈) or EtOCS₂K in moderate-to-good yields. The method provides the base-free generation of a trisulfur radical anion (S₃^•-) and its addition to alkynes as an initiator. This research broadens the applications of S₃^•- in the synthesis of sulfur-containing heterocycles.

Access to Substituted Thiophenes through Xanthate-Mediated Vinyl C(sp2)-Br Bond Cleavage and Heterocyclization of Bromoenynes

An environmentally sustainable strategy for the chemoselective heterocyclization of bromoenynes through a transition-metal-free sulfuration/cyclization process is reported. Using inexpensive and safe EtOCS₂K as a thiol surrogate and tetrabutylphosphonium bromide and H₂O as a mixed solvent, the reaction provided a range of substituted thiophenes in moderate to good yields. In addition, 2,3,4,5-tetrasubstituted thiophenes were able to be prepared under mild reaction conditions by electrophilic heterocyclization with NH₄I and EtOCS₂K in good yields.

Access to 4-substituted isothiazoles through three-component cascade annulation and their application in C–H activation

The use of EtOCS₂k enabled the annulation of isopropene derivatives with NH₄I, affording various 4-substituted isothiazoles in good yields.