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Hiperwalentne związki siarki , selenu i telluru. Część 3. Sulfuran Martina , sulfurany 10-S-5 i 12-S-6

Zając A.

Wiadomości Chemiczne

2012

[Z] 66, 9-10

893-907

In this part of the review one of the most famous 10-S-4 sulfurane called Martin sulfurane together with sulfuranes 10-S-5 and persulfuranes 12-S-6 will be presented. Martin sulfurane has been well known for a relatively long time but it is still useful in organic synthesis as a dehydrating, coupling and oxidizing agent. Its use in selected substitution reactions will be also described. Next, synthesis and selected properties of sulfuranes 10-S-5, mainly sulfurane oxides, and persulfuranes 12-S-6, will be shown. Besides synthetic methods, the review is focused on the investigations of the stability and isomerization of these types of compounds, which is particularly interesting for persulfuranes, because of the presence of three 3c-4e bonds.

Hiperwalentne związki siarki, selenu i telluru. Część 1. Charakterystyka ogólna

Zając A.

Wiadomości Chemiczne

2012

[Z] 66, 5-6

529-541

The goal of this four-part review is a presentation of the results of recent studies on the properties and chemistry of hypervalent sulfur, selenium and tellurium compounds. The term “hypervalency” has been known since 1969 when Musher used it to describe molecules bearing heteroatoms which formally did not fulfill the octet rule. This violation was explained by the postulate concerning the existence of a 3-center 4-electron bond between a hypervalent heteroatom and two axial electronegative ligands. The bond is the a combination of two ligand orbitals and a pz orbital of a central heteroatom which results in the formation of three molecular orbitals. The distances between the central atom and these two ligands are longer than the length of the typical sp2 bonds, such as equatorial ones. Moreover, the effective electron density is shifted from the central atom towards the axial ligands what results in the fulfillment of the octet rule of this atom. The geometry of this system is trigonal bipyramid (Fig. 2), except from compounds having three 3-center 4-electron (3c-4e) bonds which have tetragonal bipyramid geometry (Fig. 3). The term “geometry” includes positions of ligands and lone electron pairs. The stability of hypervalent compounds is affected by a few factors: electronegativity of ligands, formation of five-membered cyclic structures involving the central atom and the number of electron shells of the central atom. Martin proposed three-symbol notation N-X-L for these structures, which was further modified (Tab. 1). Hypervalent compounds can isomerize according to various mechanisms: Berry pseudorotation (Scheme 1), turnstile rotation (Scheme 2), cuneal inversion (Scheme 3), lever mechanism (Scheme 4), or Bailar twist (Scheme 5). Furthermore, hypervalent structures of 10-X-4 and 10-X-5 type with trigonal bipyramid geometry, C1 or C2 symmetry and at least three different ligands can exist as optically active species (Tab. 2, Fig. 5–7, Scheme 6), especially the "spiro" ones, which are resistant to isomerization. In 1977 Martin and Balthazor proposed extended Cahn-Ingold-Prelog convention for description of the absolute configuration of chiral hypervalent compounds (Fig. 5).