Cycloaddition/Electrocyclic Ring Opening Sequence between Alkynyl Sulfides and Azodicarboxylates To Provide N,N-Dicarbamoyl 2-Iminothioimidates.

The α-oxidized thioimidates are useful bidentate ligands and are important motifs in pharmaceuticals, pesticides, and fungicides. Despite their broad utility, a direct route for their synthesis has been elusive. Herein, we describe a one-step synthesis of N,N-dicarbamoyl 2-iminothioimidates from easily accessible thioacetylenes and commercially available azodicarboxylates (20 examples, ≤99% yield). Additionally, the mechanism of the transformation was extensively explored by variable-temperature NMR, in situ IR, and quantum mechanical simulations. These experiments suggest that the reaction commences with a highly asynchronous [2 + 2] cycloaddition, which leads to a four-membered diazacyclobutene intermediate with a barrier consistent with the observed reaction rate. This intermediate was then isolated for subsequent kinetic measurements, which yielded an experimental barrier within 1 kcal/mol of the calculated barrier for a subsequent 4π electrocyclic ring opening leading to the observed iminothioimidate products. This method represents the first direct route to α-oxidized thioimidates from readily accessible starting materials.

Accessing the Rare Diazacyclobutene Motif.

A formal [2 + 2] cycloaddition of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) with electron-rich alkynyl sulfides and selenides is described. These investigations provide a convenient method to access diazacyclobutenes in good yield while tolerating a relatively broad substrate scope of thio-acetylenes. This method provides ready access to a unique and hitherto rarely accessible class of heterocycles. A combination of dynamic NMR, X-ray crystallography, and computation sheds light on the potential aromaticity of the scaffold.