A copper-catalyzed oxygenation of methylazaarenes was found to occur in the absence of both ligand and additive, and has been successfully employed for the synthesis of trifluoromethylazinylketols. This synthetic strategy incorporates aerobic oxidation and a trifluoromethylation in one-pot and provides a novel method for the trifluoromethylation of aliphatic CH bond.

A combination of palladium and silane has been well used for the catalytic desulfurative cross-coupling reactions under neutral conditions. Catalyzed by 10 mol-% of bis(triphenylphosphine) palladium(II) dichloride in dimethylformamide in the presence of 2 equiv. of triphenylsilane, the carbocyclizations of α-(3-butenoyl) ketene dithioacetals took place smoothly leading to 2-cyclopentenones in high yields. Thus, a new Heck-type cyclization based on C–S bond cleavage for the synthesis of spiro compounds by using silanes as the desulfurative agents has been established.

An annulation strategy based on ketene dithioacetals is described. Under very mild conditions, a number of pyrrolidinetriones 2 and piperidinetriones 3 have been synthesized by the cycloaddition reaction of α-carbamoyl ketene dithioacetals 1 with di-carboxylic acid dichlorides in good to excellent yields. Further application of this protocol is highlighted by the synthesis of a set of fused pyrimidine derivatives 4/5 from 2/3 and amidines.

The insertion of an aryne into a CS bond can suppress the addition of an S nucleophile to the aryne in the presence of palladium. Catalyzed by Pd(OAc)₂, a wide range of α-carbamoyl ketene dithioacetals readily react with arynes to selectively afford functionalized 2-quinolinones in high yields under neutral reaction conditions by a CS activation/aryne insertion/intramolecular coupling sequence. The attractive feature of the new strategy also lies in the versatile transformations of the alkythio-substituted quinolinone products.

The insertion of an aryne into a CS bond can suppress the addition of an S nucleophile to the aryne in the presence of palladium. Catalyzed by Pd(OAc)₂, a wide range of α-carbamoyl ketene dithioacetals readily react with arynes to selectively afford functionalized 2-quinolinones in high yields under neutral reaction conditions by a CS activation/aryne insertion/intramolecular coupling sequence. The attractive feature of the new strategy also lies in the versatile transformations of the alkythio-substituted quinolinone products.

A highly regioselective allylic substitution of quinone monoketals with α-oxoketene dithioacetals is achieved under the catalysis of only tin tetrachloride (1 mol%). The advantages of the reaction, including its simplicity, rapidity, low catalyst loading of inexpensive tin tetrachloride, mild conditions and; in particular, the regiospecificity, is proposed to be due to a pseudo-intramolecular process. This new synthetic method provides a facile [3+2] cycloaddition route to benzofurans and is highlighted by the synthesis of coumestans.

A novel catalytic alkylation of indoles with allylic alcohols has been developed. Catalyzed by sulfuric acid (10 mol-%), the reaction between indoles 2 and allylic alcohols 1 based on ketene dithioacetal affords polyfunctionalized indoles 3 in good to excellent yields with high regioselectivities under mild conditions. The catalytic carbon–carbon coupling reaction provides a facile method for the environmentally benign functionalization of indoles with the advantages of good regiochemistry, atom efficiency, easily available catalyst, good yields and the synthetic potential of the polyfunctionalized indole products. Thus, further transformation of the resulting indoles 3 into pyridoindolone derivatives 4 was investigated. The efficiency of this carbon–carbon coupling reaction of 1 with 2 under the catalysis of sulfuric acid is due to the efficient formation of the reactive intermediate I, which is a carbocation that is strongly stabilized by two alkylthio groups at the terminal of the carbon–carbon double bond. The extension of this catalytic strategy to the synthesis of β-naphthols 5 was also achieved.

The synthesis of polyfunctionalized 4H-chromenes 3 and dihydrocoumarins 4 has been developed from the same substrates. Catalyzed by copper(II) bromide (0.3 equiv.), the reactions of the easily available ketene dithioacetals 1 with 2-(hydroxymethyl)phenols 2 lead to 4H-chromenes 3 in high to excellent yields in dichloromethane solvent, whereas, 3,4-trans-disubstituted dihydrocoumarins 4 are obtained in high yields with high diastereoselectivities by prolonging the reaction time or changing the solvent from dichloromethane to acetonitrile.

A new application of copper(II) bromide-activated ketene dithioacetals as nucleophiles in organic chemistry has been developed. Under the cocatalysis of copper(II) bromide (2.0 mol%) and boron trifluoride etherate (10 mol%), the conjugate addition and sequential cyclization of α-electron-withdrawing group-substituted ketene dithioacetals with p-quinones in acetonitrile at room temperature gave a variety of benzofurans. This formal [3+2] cycloaddition provides a general method for catalytic synthesis of polyfunctionalized benzofurans with the advantages of readily available starting materials, cheap catalysts, mild reaction conditions, good yields and wide range of synthetic potential for the benzofuran products. Further transformations of the resulting benzofurans to 2-aminobenzofurans and benzofuro[2,3-d]pyrimidine derivatives are also investigated.

Hydrobromic acid was found to be a unique catalyst in CC bond-forming reactions with ketene dithioacetals. Distinctly different from other acids (including Lewis and Brønsted acids), the remarkable catalytic performance of hydrobromic acid in catalytic amounts was observed in the “acid”-catalyzed reactions of readily available functionalized ketene dithioacetals 1 with various electrophiles. Under the catalysis of 0.1 equivalents of hydrobromic acid, the reaction of 1 with carbonyl compounds 2 a–l gave polyfunctionalized penta-1,4-dienes 3 or conjugated dienes 4 in good to excellent yields. The reaction tolerated a broad range of substituents on both the ketene dithioacetals 1 and the carbonyl compounds 2. Application of this efficient CC bond-forming method generated coumarins 5 and benzofurans 7 under mild, metal-free conditions by hydrobromic acid-catalyzed reactions of 1 with salicylaldehydes 2 m–o and p-quinones 6 a–d, respectively. A new reactive species, a sulfur-stabilized carbonium ylide, formed depending on the nature of the counterion, and this was proposed as the key intermediate in the unique catalysis of hydrobromic acid.

A new catalytic CC bond-forming reaction has been developed. Catalyzed by cheap and commercially available copper(II) bromide (CuBr₂; 10 mol%), the reactions of α-electron-withdrawing group (EWG)-substituted ketene S,S-acetals with aldehydes or ketones in acetonitrile at room temperature gave a variety of densely functionalized coupling products in excellent yields (80–98%). Based on this catalytic process, coumarin derivatives were efficiently synthesized when salicylaldehydes were selected as the carbonyl components.