Dynamic aspects of hypervalent compounds effected by the formation of three center-four electron bond in heteroatoms

Hypervalent bond (three center-four electron bond: 3c-4e) is the central feature of the structure and reactivity of hypervalent compounds. The SSS bond of thiathiophthene, the so-called no-bond resonance compound, is realized as one of the examples of 3c-4e bond centered at the sulfur atom (10-S-3). A variety of reactions such as cycloaddition-elimination, bond-switching rearrangement, and bond-switching equilibration of sulfur-containing heterocycles are exemplified to proceed through the presence of hypervalent 10-S-3 species. In the transition state of edge inversion, it is predicted that a vacant p-type orbital is perpendicular to the square planar species. Antimony(III) and bismuth(III) compounds bearing a bidentate Martin ligand and a phenyl, or 2-dimethylaminomethylphenyl, or 2,6-bis(dimethylaminomethyl)phenyl group were prepared. The rates of inversion of these compounds at the central atom of Sb or Bi were accelerated significantly by the coordination of the dimethylamino group and also of pyridine from the solvent, where 3c-4e bond is formed by the coordination to stabilize the transition state. Finally, the presence of rapid bond-switching equilibration between NB···N bond of 1,8-bis(dimethylamino)-9-(substituted)borylanthracenes was shown to proceed through 10-B-5 species at the transition state. It is concluded that the bond-switching equilibration takes place as a piston rod in the range of 1.32 A. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:161–175, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20326