Catalytic Production of Isothiocyanates via a Mo(II)/Mo(IV) Cycle for the “Soft” Sulfur Oxidation of Isonitriles

In the presence of excess amounts of elemental sulfur, the dimolybdenum dinitrogen complex {Cp*Mo­[N­(ⁱPr)­C­(Ph)­N­(ⁱPr)]}₂(μ-N₂) (4; Cp* = η⁵-C₅Me₅) serves as a precatalyst for the production of isothiocyanates from isonitriles via highly efficient and atom-economical metal-mediated sulfur atom transfer (SAT) under mild conditions. Mechanistic and structural studies support a catalytic cycle for SAT involving initial formation of a Mo­(II) bis­(isonitrile) complex that then undergoes sulfination to generate a formal “side-bound” Mo­(IV) κ²-(C,S)-isothiocyanate as the key intermediate. This metal-catalyzed SAT process has further been employed for the “on-demand” production of isothiocyanates that are trapped in situ by benzhydrazides to provide thiosemicarbazides, which are useful precursors to biologically active thiadiazoles.