N-heterocyclic Superelectrophiles And Evidence For Single Electron Transferrnchemistry

摘要

Since the pioneering work of Olah in the 1970s, superelec-trophilic chemistry has been an active area of research. Superelectrophiles have been the basis for many unusual synthetic transformations, for example leading to functionalized hydrocarbons, heterocycles, and even polymers. Among the many superelectrophiles described in the literature, the proto-nitronium ion (HNO_2~(2+)) is typical. When nitronium ion (NO_2~+) salts are dissolved in excess Lewis acids or Bronsted superacids, the resulting species exhibit greatly enhanced electrophilic reactivities. In the case of Bronsted acids, this is considered to be the result of protosolvation of the nitronium ion (partial or complete proton transfer to the oxygen). As a result of protonation, the ion develops an increasing positive charge, and in the limiting case it may form the dicationic protonitronium ion. The superelectrophilic nitronium ion is capable of reacting with exceptionally weak nucleophiles, such as alkanes and strongly deactivated arenes. Besides synthetic studies, the protonitronium ion has been studied computationally, and in accord with calculations, it has been observed as a kinetically stable species in gas-phase experiments.