Rearrangements and Interconversions of Heteroatom-substituted Isocyanates, Isothiocyanates, Nitrile Oxides, and Nitrile Sulfides, RX-NCY and RY-CNX

Isocyanates and isothiocyanates of the type RX-NCY (X and Y = O or S) and the isomeric nitrile oxides and nitrile sulfides RY-CNX are highly reactive compounds. A number of potential 1,4-shifts of substituent groups of the type R-Y-CNX → R-X-N═C═Y, 1,3-shifts R-C(═Y)-N═X → R-X-N═C═Y, and 1,2-shifts R-C(═Y)-N═X → R-Y-CNX have been evaluated computationally. The results obtained for the relatively new functional MPW1K and the well-established B3LYP, together with a triple-ζ quality basis set, are very similar. The 1,3- and 1,4-halogen shifts in the title compounds are usually highly exothermic and possess low activation barriers. 1,3-Aryl shifts are feasible for for 5e → 6e (Ar-CO-NSO(2) → Ar-SO(2)-NCO) with activation barriers of less than 40 kcal/mol. Additionally, several 1,3- and 1,4-hydrogen shifts and the 1,4-methyl-shift in methoxynitrile sulfide MeO-CNS to methylsulfenyl isocyanate MeS-NCO (4c → 6c) are potentially feasible. The 1,2-shift reactions 4b → 5b (HO-NCS → H-CS-NO) and 4c → 5c (Ar-O-CNS→ Ar-CO-NS) are good candidates for experimental observation with activation energies around 30 kcal/mol.