Sulfur valence-shell expansion

The structures of selenaaromatic compounds are closely related to those of analogous sulfur compounds. The best known is selenophene (1). As for thiophene, the idea can be offered in terms of resonance theory (Fig. 3). This means that outer valence shell resonance structures la-d may be hybridized into an aromatic sextet. In these cases, selenium acts as an electron acceptor and negative charge is localized on the selenium atom. In contrast, structures le-h have octet formulation not involving valence shell expansion. Selenium acts as electron donor and in these cases positive charge is localized on the selenium atom (Fig. 3). 

In contrast to furans, photoinduced ring-degenerate rearrangements are largely represented in the thiophene series. Reported results have been often rationalized in terms of different competing mechanisms such as the zwitterion-tricyclic route, which takes advantage of the valence shell expansion of a sulfur atom and involves... 

The elements with a lone pair of electrons and very rich redox chemistry, have certainly been well exploited in ionic reactions. We decided to study the radical chain reactions based on acyl derivatives of thiohydroxamates and incorporating compounds of group V or VI elements in a unique fashion. In these very highly disciplined and efficient chain reactions (if properly designed and administered) the basic concept is one of valence shell expansion.11 The general philosophy of this series of reactions developed by us is depicted in Scheme 31. A simple illustration of this concept is the use of sulfur dioxide as a radical trap in conjunction with an o-acyl thiohydroxamate leading to the formation of S-pyridyl alkylthiosulfonates 117.84 For practical reasons this reaction is carried out at —10 °C in a mixture of dichloromethane and liquid S02. The sulfur atom undergoes a valence shell expansion from S(IV) to S(VI). 

CASE 3 VALENCE SHELL EXPANSION Lewis diagrams become more complex in compounds formed from elements in the third and subsequent periods. For example, sulfur forms some compounds that are readily described by Lewis diagrams that give closed shells to all atoms. An example is hydrogen snlfide (H2S),... 

Exception 3. Beyond the second row, the simple Lewis model is not strictly apphed, and elements may be surrounded by more than eight valence electrons, a feature referred to as valence-shell expansion. For example, phosphorus and sulfur (as relatives of nitrogen and oxygen) are trivalent and divalent, respectively, and we can readily formulate Lewis octet structures for their derivatives. But they also form stable compounds of higher valency, among them the familiar phosphoric and sulfuric acids. Some examples of octet and expanded-octet molecules containing these elements are shown below. 

In some cases, to draw octet Lewis structures, charge separation is necessary that is, guideline 1 takes precedence over guideline 3. An example is carbon monoxide. Other examples are phosphoric and sulfuric acids, although valence-shell expansion allows the formulation of expanded octet structures (see also Section 1-4 and guideline 1). 

Valence-shell expansion of sulfur accounts for the special reactivity of thiols and sulfides... 

Isolable Oxysulfuranes. Sulfuranes, as described by Musher (1969), are compounds of nlfurlIV) in which four ligands are attached 10 sulfur and have in common with tare-gas compounds such as XeF, an electronic structure involving a formal expansion of the valence shell of the central atom from 8 to 10 electrons, Martin and Perozzi (1976) pointed out that the incorporation of oxygen ligands makes possible a wide range of new structural types that illustrate structure-reactivity relationships in a particularly illuminating way. 

An alternative explanation considered for the thiophene rearrangement is that an intermediate of type 154, arising by expansion of the valence shell of the sulfur atom, may be involved.135 136 E. F. Ullman and B. Singh, J. Am. Chem. Soc. 88, 1844 (1966). 

There is another reasonable structure, 1-11, for dimethyl sulfoxide, which corresponds to an expansion of the valence shell of sulfur to accommodate 10 electrons. Note that our calculation of electron demand counted eight electrons for sulfur. The 10-electron sulfur has an electron demand of 10 and... 

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