On Oxalyl Halide Isothiocyanates Part I: Syntheses and Isomerism

The reaction of oxalyl bromide with one equivalent of ammonium thiocyanate affords oxalyl bromide isothiocyanate as a bright yellow liquid with three signals in its C NMR spectrum (159.9, 153.6, and 153.3 ppm). In contrast, the analogous reaction starting from oxalyl chloride affords a mixure of oxalyl chloride isothiocyanate (162.3, 153.7, and 153.3 ppm) and its constitutional isomer 2‐chlorothiazole‐4,5‐dione (184.4, 180.9, and 166.9 ppm). The isomerization of both bromo and chloro species is solvent dependent, with low‐polarity solvents showing exclusively the oxalyl isomer, whereas for increasingly polar solvents both isomers are observed. Crystal structures of oxalyl bromide isothiocyanate and 2‐chlorothiazole‐4,5‐dione are reported. Oxalyl chloride isothiocyanate could not be crystallized but solidifies as a glassy solid.