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.