Hypervalent Heterocycles

Abstract

This chapter provides an overview of recent literature on heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term “hypervalent” has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used in synthetic works despite criticism from theoretical chemists. Typical hypervalent heterocycles include polycoordinated 10-electron or 12-electron centers with distorted trigonal-bipyramidal or pseudooctahedral geometry, respectively. In general, heterocyclic compounds of elements with double bonds are not classified as hypervalent molecules because of the ylidic, zwitterionic nature of such bonds resulting in the classical 8-electron species. Despite the lack of aromatic conjugation, hypervalent heterocycles often have a considerably higher thermal stability compared to their acyclic analogs, which is especially important in the chemistry of polyvalent bromine and iodine. This review is centered mainly on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as, boron, silicon, phosphorus, sulfur, selenium, bromine, and iodine, with emphasis on structural and synthetic aspects of their chemistry.