1.5- Dithiacyclooctane cation

The molecular geometry, which allows optimal p orbital interaction to yield a three-electron bond, presumes an orientation of p orbitals belonging to each sulfur atom along the S S axis. This is the case of the chair-boat conformer of the 1,5-dithiacyclooctane cation-radical, the first structure in Scheme 3.22.In the 1,3-dithiacyclopentane cation-radical, the sulfurp orbitals are aligned almost perpendicular to the ring plane, and this prevents stabilization by the transannular interaction between the two sulfur atoms in the cycle. This unreal structure (the second structure in brackets in Scheme 3.22) cannot exist. However, the cation-radical of bis(2-methyl-1,3-dithianyl)methanol (the third structure in Scheme 3.22) was predicted to exist Li and Kutateladze (2003) calculated this structure as the most stable because it differs by a special orbital pattern from the structure in brackets. 

Musker29 carried out a systematic study of the oxidation of several cyclic 22 and acyclic 23 bis-sulfides using nitrosonium salts. Several unstable dications were characterized as sulfoxides 24. These oxidations proceed through stepwise transfer of two electrons from a bis-sulfide to the nitrosonium cation and the intermediate formation of the corresponding radical cation. Radical cations of 1,5-dithiacyclooctane 11 and 1,5-dithiacyclononane are sufficiently stable to be isolated as individual compounds (Scheme 8).50... 

Addition of 1,5-dithiacyclooctane to zeolite CaY in the presence of molecular oxygen results in spontaneous oxidation to mono- and bis-sulfoxides through formation of the corresponding radical cation characterized by ESR and diffuse reflectance of UV-Vis spectroscopy.51... 

In 1999, Zhou and Clennan [76] reported a type IIaRH oxidation of 1,5-dithiacyclooctane, 26, in CaY. They suggested (Fig. 24) that 26 was activated by an electron transfer to give a radical cation, which subsequently reacted with oxygen to give a peroxysulfonium cation radical intermediate. This scenario is supported by the ease of oxidation of 26 [77] and by the reported propensity of... 

Figure B3.2. 1,5-Dithiacyclooctane (a) staggered conformation (b) folded conformation, (c) Interaction diagram in answer to question 3(a) for the bonding in the radical cation.

Transannular cation radicals with the intramolecular sulfur-sulfur bond of the 2ct-1ct type generated from medium-ring disulfides like 1,5-dithiacyclooctane are an exception in terms of their stability, although they are not resistant to water (Musker 1980). ESR and resonance Raman spectroscopy studies revealed the existence of the 1,5-dithia-cyclooctane cation radical, with substantial bonding between the sulfur atoms (T. Brown et al. 1981 Tamaoki et al. 1989). Computations confirmed this statement and pointed out that the chair-boat conformer has the lowest energy as compared to other possible conformers (Stowasser et al. 1999). 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

Intramolecular S. S (2a/la ) three-electron bonded radical cations The steric parameter becomes even more pronounced, and in fact dominant, for radical cations with intramolecular sulfur-sulfur coupling. Most examples, studied by radiation chemical (but also other) methods, have been observed in the oxidation of cyclic and open chain dithia compounds.84,85,117- 23 xhe two extreme situations with respect to the most (left) and least favorable (right) orbital alignment are depicted in Figure 3. The former is closest realized upon one-electron oxidation of 1,5-dithiacyclooctane, 6, and was first described in conventional ESR work by W. K. Musker.l 19-121 jt exhibits an optical absorption with 400 nm and, incidentally, consti-... 

A molecular geometry which allows optimal p-orbital interaction to yield a 2a a bond between the two sulfur atoms is shown on the left hand side of Figure 3. In reality such a configuration seems most closely approached in the radical cation obtained upon one-electron oxidation of 1,5-dithiacyclooctane, 7, and was first described in ESR work by Musker and co-workers [103, 105, 106]. It exhibits an optical absorption with 400 nm and constitutes the most blue-shifted example known (except for the all-hydrogen substituted (H2S SH2) with Amax at 370 nm). Stabilization of 7 is facilitated by the establishment of two five-membered rings on either side of the transannular S S bridge. 

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