Mechanism of 1,2-shift through pi-complex transition state

A systematic calculation of the potential curves or surfaces for 1,2-shift has been realized by using MNDO or other models in MOPAC programs. By refer ring to the previous authors' viewpoints, the 1,1,2-shift can be divided in to two categories. I,2-electron-deficient shift is that the electronic conf iguration of the atom which accepts the migrating group is a cation or an e lectron-deficient atom, and 1,2-anion shift is the one that the accepted at om of the migration group is a negative ion, in terms of the experimental f acts and the calculation of the potential surfaces, in electron-deficient s hift such as Beckmann or Baeyer-Villiger rearrangement, the migration occur s through a transition complex formed between the pi -bond and the cation o r electron-deficient migrating group, but in anion shift such as Wittig or Stevens rearrangement, the electron pair in pi -orbit excites at first to p i* orbit, and then the migration occurs through the new formed complex betw een the anion migration group and the vacant pi orbit. The above mechanisms explain reasonably the intramolecular properties, the configuration retent ions of the migration group, and the corresponding migratory aptitudes of t he two type 1,2-shifts. The partial and less important free radical reactio n of 1,2-anion shift has been explained by the pi -complex mechanism too.