Hypervalent sulfur atom

Crystalline trithia dication 151 contains a 3-ccntcrA-clcctron bond with a central hypervalent sulfur atom. It was shown to be sufficiently stable to be isolated and studied spectroscopically. One important feature is its boat-boat conformation resulting from the formation of the transannular bond between the three sulfur atoms <1988JA1280>. 

Transformation of the thiadiazolopyrimidine compound 138 to the fused dithiazole 140 also follows a fairly complicated pathway <2004JHC99>. When the derivative 138 is treated with carbon disulfide, a cyclization reminiscent of 1,3-dipolar cyclization takes place with the reagent acting as a dipolarophile to give a />OT-fused tricyclic intermediate containing a hypervalent sulfur atom 139. This intermediate can undergo isothiocyanate elimination to furnish 140. It is interesting to note that the sulfur atom of the thione moiety in the product is derived from carbon disulfide. 

Reduction of dinitrobenzothiadiazoles 280 with iron dust in acetic acid gave diamines 281 (Scheme 39). The reaction of diamines 281 with selenium dioxide gave [l,2,5]selenadiazo[4,5-c]-2,l,3-benzothiazole derivatives containing a hypervalent sulfur atom 82 and 83 in 40% and 82% yields, respectively <1997T10169>. 

This alkaline hydrolysis shows a rate term that is second order in hydroxide ion concentration, which is indicative of a stepwise mechanism involving a TBPI with a hypervalent sulfur atom. Reversible attack of the hydroxide ion on a /3-sultam generates a monoanionic TBPI-, which requires deprotonation by a second hydroxide ion before the intermediate can collapse to products. 

In applying an electron-pair bond model for the hypervalent molecules studied, the (apical, equatorial) and (equatorial, equatorial) isomers, namely 23-eq and 23b, respectively, are both local minima. This means that the tetracoordinated species are less strained than the pentacoordinated ones, due to less antibonding properties of the electron delocalization between the geminal ring bonds on the hypervalent sulfur atom. Yet, the more strained (eq, eq) isomers are, surprisingly, more stable than the (ap, eq) isomers in most of the hypervalent three-membered rings studied <2001PCA10711>. 

Sulfuranes with hypervalent sulfur atoms possess expanded valence shells, and consequently the molecules are relatively unstable. The central sulfur atom can, however, attain the normal stable octet of electrons by extruding a ligand or a pair of ligands in an elimination process. The former results in ligand coupling (Figure 8 and Schemes 14a and... 

In a more recent work, Kawashima reported the preparation and structural studies of a series of four-membered lO-S-4 and lO-S-5 heterocycles 51—54 (Figure 11) (2011PSS1046). These compounds containing a tetra-coordinated or pentacoordinated hypervalent sulfur atom together with other heteroatoms were isolated as thermally stable products and structurally... 

Figure 11 Four-membered heterocycles with a hypervalent sulfur atom.

In addition to halogen bonded complexes or ionic salts, it is also possible for sulfur and selenium electron donors to form complexes in which the electron donor atom inserts into the X2 bond, giving a hypervalent donor atom with a T-shaped geometry. It has been recently reported [147] that for dibromine and selenium, this type of complex is favored over halogen bonded complexes. While no purely halogen bonded complex is reported for dibromine, there is one complex (IRABEI) in which one selenium atom of each of several selenanthrene molecules in the asymmetric unit does insert into a Br2 bond, but for one of the molecules, the other selenium atom forms a halogen bond with a Br2 molecule to form a simple adduct (A). 

Generation of disulfonium dications involves stabilization of the cationic centers on the sulfur atoms through formation of a new S-S bond between the two sulfonium centers. When such an interaction involves more than two chalogen atoms, it leads to formation of new interesting types of dications, which contain hypervalent central atoms, such as sulfurane, selenurane or tellururane.94... 

The parent compound and a set of monosubstituted bis(acylamino)diarylspiro-X4-sulfanes (360 X = H, Me, MeO, Cl, NO2) undergo hydrolysis to the corresponding sulfoxides (361). The probable mechanism involves rate-determining cleavage of one of the S—N hypervalent bonds in the spiro ring with simultaneous proton transfer to the nitrogen atom. The hydroxide ion which is formed thereby then attacks the sulfur atom in a fast step to form a diaryl(acylamino)hydroxy-k4-sulfane (362), which is converted into the sulfoxide (361) (Scheme 47). ... 

H atoms (H ) tend to abstract hydrogen atoms from C-H bonds, particularly if these are activated by neighboring functionalities such as sulfur atoms. The sulfur atom in thioethers was not usually considered as the primary target of H atom despite some, mostly recent, reports showing that bimolecular homolytic substitutions with H atom might occur with thioethers. Because these sulfur atoms are hypervalent, they can form adducts with H atoms. Radical chemistry studies on 1,3,5-trithiane (1,3,5-TT), performed by pulse radiolysis,... 

In hypervalent sulfur compounds like (37) and (38), the apical bonds formed by the /r-orbitals are longer and more polar than the equatorial bonds. The special stability of many polycoordinated sulfur compounds is still concluded to be associated with the added orbital interaction with the energetically accessible 3J-orbitals. The discovery of sulfuranes has provided valuable insight into many nucleophilic substitution reactions on polycoordinate sulfur atoms and into ligand-coupling reactions which occur via a-sulfurane intermediates. 

The oxidation of thiols follows a completely different course as compared with the oxidation of alcohols, because the capacity of the sulfur atom to form hypervalent compounds allows it to become the site of oxidation. Thiols are readily oxidised to disulfides by mild oxidants such as atmospheric oxygen, halogens or iron(III) salts (Scheme 6). This type of reaction is unique to thiols and is not undergone by alcohols, it is a consequence of the lower bond strength of the S-H as compared with the O-H bond, so that thiols are oxidised at the weaker S-H bonds, whereas alcohols are preferentially oxidised at the weaker C-H bonds (Scheme 7). The mechanism of oxidation of thiols may be either radical or polar or both (Scheme 6). The polar mechanism probably involves transient sulfenic acid intermediates like (7) and (8). In contrast, thiols react with more powerful oxidants, like potassium permanganate, concentrated nitric acid or hydrogen peroxide, to yield the corresponding sulfonic acids (10). This oxidation probably proceeds via the relatively unstable sulfenic (7) and sulfinic acids (9), which are too susceptible to further oxidation to be isolated (Scheme 8). 

Sulfides are easily oxidised at the sulfur atom by sources of electrophilic oxygen in the oxidation process, sulfur accepts electrons in its J-orbitals, leading to hypervalent sulfur compounds. The first oxidation product is the sulfoxide (23), and this is further oxidised to the stable sulfone (24) (Scheme 18) sulfones have many applications in synthetic organic chemistry (see Chapter 10, p.195). 

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