Contrasting Reactivity of CS₂ with Cyclic vs. Acyclic Amidines

The interaction between carbon dioxide (CO₂) and amidines such as 1,8-diazabicyclo[5.4.0]undecane (DBU) has been extensively studied, but the reaction of isovalent CS₂ with such bases has been largely ignored, apart from a single crystallography report. Acyclic acetamidines are cleaved by CS₂ at room temperature to give an isothiocyanate and a thioacetamide. Because the pathway to that cleavage involves a rotation that is difficult for cyclic amidines, the reaction of CS₂ with cyclic amidines produces an entirely different product: a cyclic carbamic carboxylic trithioanhydride structure. The path to that product involves sp³ C-H activation leading to the formation of a new C-C bond at a carbon α to the central carbon of the amidine group. Alkylation and ring-opening of the cyclic carbamic carboxylic trithioanhydride has also been demonstrated under ambient conditions.