Fragmentation Reactions

Fragmentations as defined by Grob are heterolytic bond-cleavage reactions in which a molecule breaks into three (or more) fragments derived from an electrofuge, middle part, and nucleofuge. Concerted bond-cleavage events involve parallel alignment of the cleaving bonds in well-ordered transition states. This chapter provides first an introductory overview of fragmentation reactions in organic synthesis, then a historical perspective on their development, and a mechanistic framework for interpreting and predicting fragmentation behavior. Synthetic applications are organized according to what arises from the middle part (middle part→alkenes, alkynes, or allenes), and are presented based on the fragmentation results in a skeletal degradation, ring opening, or ring expansion. The Beckmann fragmentation (middle part→nitrile) is presented separately. Practical considerations such as conformational requirements and appropriate choice of nucleofuge for various applications in synthesis are discussed in context and are summarized at the end.