A Cobalt Mediated Nitrene Transfer aza -Wittig Cascade Reaction To Access 1,3,4-Oxadiazole Scaffolds.

A cobalt(II) mediated three-component synthesis of 5-substituted- N -sulfonyl-1,3,4-oxadiazol-2(3 H )-imines using sulfonyl azides, N -isocyaniminotriphenylphosphorane (NIITP), and carboxylic acids has been developed. This one-pot tandem reaction starts with a nitrene transfer to NIITP, followed by addition of the carboxylic acid to the in situ formed carbodiimide and subsequent intramolecular aza -Wittig reaction. Both the steric constraints of carboxylic acid and the stoichiometry of the employed cobalt salt determine the selectivity toward the two products, i.e. 5-substituted- N -sulfonyl-1,3,4-oxadiazol-2(3 H )-imine versus 5-substituted-4-tosyl-2,4-dihydro-3 H -1,2,4-triazol-3-one.