Sulfur orbital interactions with

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. 

If this interpretation is correct, it must mean (Scheme 14, first four entries) that S behaves like CH2 and not like 0. In other words, the extended filled p-orbitals oh sulfur must interact with the axial Hfs (or C-H bonding electrons) of the ring much like the C-H bonds of the methyl group do. 

Cycloaddition reactions also have important applications for acyclic chalcogen-nitrogen species. Extensive studies have been carried out on the cycloaddition chemistry of [NSa]" which, unlike [NOa]", undergoes quantitative, cycloaddition reactions with unsaturated molecules such as alkenes, alkynes and nitriles (Section 5.3.2). ° The frontier orbital interactions involved in the cycloaddition of [NSa]" and alkynes are illustrated in Fig. 4.13. The HOMO ( Tn) and LUMO ( r ) of the sulfur-nitrogen species are of the correct symmetry to interact with the LUMO (tt ) and HOMO (tt) of a typical alkyne, respectively. Although both... 

Similar considerations apply to the thiirene oxide system (18), since in this case too the sulfur s 3d-orbitals have the potential of interacting with the 2p-orbitals of both the adjacent carbon and oxygen atoms. Such an interaction should facilitate some kind of conjugation of the carbon-carbon double-bond -electrons with the formally unoccupied 3d-orbitals, which in turn would give rise to Hiickel-type stabilization associated with cyclic array of 4n + 2 (n = 0) 7t-electrons. 

An enormous amount of work has been done in this wide field and a number of excellent reviews on different aspects of sulfur electrochemistry has been published [1-7], so here we confine our attention to some principal reactions and interesting apphcations of both anodic and cathodic activation of sulfur-containing molecules. Compared to other chalco-genides, sulfur has frontier orbitals that have volume, symmetry, and energy more suitable for efficient interaction with adjacent carbon atoms. The ionization of molecular sulfur requires about 10 eV. Conjugation of the pz orbitals of sulfur with a 7T-system lowers the ionization potential by ca. 2 eV. For this reason, compounds of divalent sulfur undergo oxidation rather easily often giving rise to cation radicals or dications. The stability of this species is in line with the... 

---