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-zeta 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 -> Sc (Ar-O-CNS -> Ar-CO-NS) are good candidates for experimental observation with activation energies around 30 kcal/mol