Silicon-assisted synthesis of thioaldehydes and trapping

A solution of the aldehyde (1 mmol), the diene (1.5 mmol) and bis(trimethylsilyl) sulfide (2 mmol) in acetonitrile (0.5 ml) was treated at room temperature with a solution of CoC12.6H20 (0.2 mmol) in acetonitrile (2.5 ml). Progress of the reaction was monitored by GC/MS analysis. The reaction mixture was quenched with saturated ammonium chloride, extracted with ether and dried over sodium sulfate, and the solvent removed under vacuum. The crude reaction product was then purified by column chromatography or TLC on silica gel. For R1 = Ph a 94% yield was secured. 

The reaction proved to be highly chemoselective, allowing thionation of aldehydes in the presence of other carbonyl groups. As a noticeable exception acylsilanes were converted to thioacylsilanes [123], 

The use of CF3S03SiMe3 as a catalyst allowed the thionation of ketones as well as aldehydes. Stereochemical control of the Diels-Alder adducts could be achieved by varying the molar ratio of the sulfurating agent. 

A similar direct conversion of aldehydes to thioaldehydes has been achieved [124] using Me3SiSSiMe3 and a catalytic amount of n-BuLi. 

Photolvsis of phenacyl sulfides [125] and the base-mediated 1,2 elimination leaction of sulfenyl derivatives [126] are other general methods of preparation of these reactive species. 

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