Transannular and intermolecular interactions between chalcogen atoms: syntheses and properties of dichalcogena dications and trichalcogena hypervalent dications

Abstract

Intramolecular (transannular) and intermolecular through-space interactions between chalcogen atoms have been described. This account includes the following three topics. (1) Various types of bis-chalcogenides and their mono-oxides, which have the two chalcogen atoms in close proximity in a molecule, give the dichalcogena dications upon the oxidation with two equivalents of NOBF₄ or treatment with one equivalent of triflic anhydride (Tf₂O). The stability of the dichalcogena dications increases in the order S<Se<Te. The dichalcogena dications with flexible scaffolds cause dealkylation via an S_N1 type reaction to form chalcogenachalcogenonium salts and alkyl cations. (2) The reaction of 2,6-bis[(phenylthio- or phenylseleno)methyl]phenyl phenyl telluride or the corresponding Teoxide with NOBF₄ or Tf₂O affords the positively-charged hypervalent species, dicationic telluranes. Dicationic telluranes can be prepared by the remote oxidation through π-conjugation. The stable sulfenium cation can be isolated by taking advantage of 2,6-bis[(dimethylamino)methyl]phenyl ligand. (3) The remote oxygen migration and Pummerer reactions of p-bis(methylthio)-aromatic mono-oxides are discussed in light of an intermolecular through-space interaction between sulfur atoms, and a cyclic bis(dithia dication) dimer is proposed as an intermediate.

Introduction

Transannular interaction is a well-known phenomenon in organic chemistry [1]. When two chalcogen atoms are arranged appropriately in juxtaposition, a repulsive force between the chalcogen atoms dominates at the neutral state. However, upon removal of two electrons from the chalcogen atoms, an attractive force operates between the chalcogen atoms to create a new σ-bond which is called dichalcogena dication: a two-center two-electron (2c-2e) bond [2]. Furthermore, when a lone pair of electrons attached to a third chalcogen atom participates transannularly in the formation of a dichalcogena dication, a new dicationic species is formed in which the central chalcogen atom is tetracoordinated hypervalently with the two neighboring chalcogen atoms as the apical ligands: a 3-center 4-electron (3c-4e) bond [3]. The simple molecular orbital representation of these bond formations is shown in Fig. 1.

The stability and reactivity of dichalcogena dications and trichalcogena hypervalent dications largely depend on the nature of chalcogen atoms and the conformational flexibility of the scaffold. Dichalcogena dications bearing a rigid conformation such as a cyclic system (1,5-dithiacyclooctane and dibenzodithiocin) are stable and can be isolated, and have been studied extensively [4]. On the other hand, dithia dications generated from an acyclic system have not been studied well because of their instability. We have found the facile dealkylation of dichalcogena dications with a flexible conformation to give chalcogenachalcogenonium salts and alkyl cations. Trichalcogena hypervalent dications with a rigid system have also been studied intensively in our laboratory [5]. We have succeeded in the isolation and full characterization of the trichalcogena hypervalent dications with tellurium atom at the central position, which are generated from an acyclic system. This review describes our recent advances in new types of dichalcogena dications and trichalcogena hypervalent dications. We also propose a cyclic bis(dithia dication) dimer via an intermolecular through-space interaction between sulfur atoms.