Delocalized Sulfur Radical Cations

Sulfur-centered radical cations derived from aromatic thioethers (Ar-S-Ar) have been investigated much less extensively. An important feature of one-electron oxidation of aromatic thioethers is the lack of dimeric radical sulfur radical cations (ArS.. S-Ar) because of the spin delocalization onto the aromatic ring. Oxidation of thioanisole (Ar-S-CHj) by OH radicals was studied using pulse radiolysis. At neutral pH, OH addition led to the prompt formation of monomeric sulfur radical cations and hydroxycyclohexadienyl radicals (see Scheme 8). 

The latter radicals decay into products which do not include the corresponding radical cations with the delocalized positive charge on the aromatic ring. At low pH, OH addition, both to the thioether functionality and to the aromatic ring, led promptly only to the monomeric sulfur radical cations. These observations were rationalized in terms of the highly unstable nature of Ph -S-CHg radical... 

In the photochemical one-electron oxidation of aromatic sulfides, dimer radical cations were formed in rapid equilibrium with monomeric radical cation (59). The complex formation of a- and tt-types has been shown to be sensitive to the steric and electronic influence of substituent. For the case of jo-(methylthio)anisole the formation of TT-type dimer was shown to be reduced due to steric hindrance of two methyl groups. No formation of dimer radical cation was observed for jo-(methoxy)thioanisole and diphenyl disulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom. Density-functional calculations supported the experimental results. The intramolecular formation of similar radical... 

The structures of compounds 1 and 2 as well as their radical cations were geometry-optimized at the PM3 level. The spin density distribution in the radical cations was shown to be delocalized over the entire molecules. For 2" this result is in agreement with ESR measurements, while in contrast, electron spin resonance (ESR) measurements showed the spin population to be higher on the sulfur atoms in 1... 

In addition to 7r-delocalization of alkyl aryl sulfide radical cations, other possibilities exist for stabilization of the radical cation if there are electron-rich groups with unpaired electrons ortho to the sulfur atom. Because of the importance of these possibilities, they will be discussed in detail. The radical cation of 49a may prefer a two-center, three-electron (2c, 3e) bond between the two sulfur atoms as illustrated in 50 rather than 7r-delocalization. 

Oxidation takes place at sulfur but charge and spin are highly delocalized. There will be no further elaboration on this transient here since this review is primarily devoted to radiation chemical contributions and aliphatic systems. But it should be mentioned that a wealth of information is available on thianthrene radical cations, particularly in the work of Shine [110]. Further examples for these species, including many thianthrene derivatives, have recently been reviewed by Glass [111], and informative reference may also be found in an article by Russell and Law [112]. 

The reactions of electrogenerated cation radicals of diarylsulfldes are mainly orbital-controlled and at this level the electronic structure of their frontier orbitals (HOMO-SOMO) has very interesting synthetic consequences. The 3p orbitals of sulfur are conjugated with only one aromatic ring even if there are two aryls bound to sulfur. Therefore, only one ring can be activated electrochemically. The degree of the charge delocalization in the ArS moiety of a cation radical on the one hand, and the availability of p- and o-positions for the substitution on the other, determine quite different reactivity of such species. 

A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

The structure of the thianthrene cation radical is shown in structure 1 as Th +. Its electron spin and charge are fully delocalized (2), yet reactions with nucleophiles occur principally at one of the sulfur atoms (3). Therefore,... 

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