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.