Studies on the carbenium-iminium ions derived from N-methylmorpholine-N-oxide (NMMO)

Two carbenium-iminium ions, an exo-centered species 2 and a ring-centered form 3, are derived from the widely used oxidant and cellulose solvent N-methylmorpholine-N-oxide (1) by heterolytic degradation. 3 rearranges into 2 in the presence of water, in an endothermic, bimolecular reaction involving a highly organized transition state, which is the first example of a carbenium-iminium ion interconversion. The reaction mechanism was investigated by a combined approach consisting of trapping reactions, isotopic labeling, kinetic studies, and computations on the DFT level.