Synthesis of Thiourea and Thioamide S‑Oxides via SO Transfer from a Thiirane S‑Oxide onto N‑Heterocyclic Carbenes

Thiourea S-oxides are labile compounds that suffer from a quick overoxidation when directly prepared from thioureas. Therefore, we have developed an alternative route on the basis of the SO transfer from a thiirane S-oxide onto sterically encumbered N-heterocyclic carbenes, including imidazole- and imidazoline-ylidenes, and a cyclic alkyl amino carbene. Reaction conditions and DFT computations suggest a concerted SO transfer mechanism. Thiourea S-oxides are nucleophiles adding to dimethyl acetylenedicarboxylate under fission of the S–O bond.