Sulfoxide Radical Cations

Sulfoxides and Sulfonyls 5.1. Aryl sulfoxides radical cations... 

Aromatic sulfoxide radical cations have been generated by pulse radiolysis from the parent sulfoxides (Chart 11)7 ... 

They are characterized by an intense absorption band in the UV region ( 300 nm) and a broad, less-intense band in the visible region (from 500 to 1000 nm) whose position depends on the nature of the ring substituents H/550 nm, OCHj/OOO nm, CH3/67O nm, Br/660 nm, and CN/560 nm. At low radiation doses the radical cations decayed by first-order kinetics with the decay rate increasing with pH. The decay involves a nucleophilic attack of H2O or OH on the positively charged sulfur atom resulting in the formation of [ArSO(OH)CH3], as suggested earlier for dialkyl sulfoxide radical cations. ... 

DFT calculations were carried out for aromatic sulfoxide radical cations showing that in the lowest energy conformers the S-O bond is almost coplanar with the aromatic ring. In all radical cations the major fraction of charge and spin density is localized on the -S(=0)-Me... 

Bacciochi E, Del Giacco T, Gerini MF, Lanzalunga O. (2006) Aryl sulfoxide radical cations. Generation, spectral properties, and theoretical calculations. JPhys Chem A 110 9940-9948. 

Another relatively stable sulfur-chlorine three-electron bonded radical appears to be R2S(0) C1 [136]. Its formation can be achieved via one-electron oxidation of a sulfoxide to the sulfoxide radical cation, R2S(0), in the presence of chloride ions (at pH < 4-5) reaction (68)... 

The three highest occupied orbitals of sulfoxides are the lone pairs ns and n0, as well as the 7iso bond210. The 1,3-dithietane 1-oxide adds a lone-pair ionization and destabilizes the n0 and nso radical-cation states compared with thietane oxide. According to a hyperconjugative MO model, the ns+ combination in 1,3-dithietane is destabilized by about leV relative to the basis orbital energy a(ns) due to the combination with the... 

Rao and Symons49 studied the formation of radicals in y-radiolysis of dilute solutions of dimethyl sulfoxide in fluorotrichloromethane. By ESR studies they found the radical cation (CH3)2SOt whose ESR spectrum show considerable g anisotropy and small methyl proton hyperfine coupling. 

II. ESR SPECTROMETRY OF RADICAL ANIONS AND RADICAL CATIONS OF SULFOXIDES AND SULFONES... 

TABLE 6. ESR parameters for radical cations arising from sulfoxides or sulfones... 

As seen before, the radical cation of dimethyl sulfoxide (CH3)2SO has been detected by ESR spectroscopy among other radicals when DMSO glasses at 77 K are submitted to y-irradiation28. It has also been reported in pulse radiolysis experiments30 (Table 6). Constant current electrochemical oxidation of bis(dialkylamino)sulfoxides (R2N)2SO gives rise to radical cations which have been detected by ESR spectroscopy33. 

Ce4+ is a versatile one-electron oxidizing agent (E° = - 1.71 eV in HC10466 capable of oxidizing sulfoxides. Rao and coworkers66 have described the oxidation of dimethyl sulfoxide to dimethyl sulfone by Ce4+ cation in perchloric acid and proposed a SET mechanism. In the first step DMSO rapidly replaces a molecule of water in the coordination sphere of the metal (Ce v has a coordination number of 8). An intramolecular electron transfer leads to the production of a cation which is subsequently converted into sulfone by reaction with water. The formation of radicals was confirmed by polymerization of acrylonitrile added to the medium. We have written a plausible mechanism for the process (Scheme 8), but there is no compelling experimental data concerning the inner versus outer sphere character of the reaction between HzO and the radical cation of DMSO. 

Oxidation of thiophene with Fenton-like reagents produces 2-hydroxythiophene of which the 2(570 One isomer is the most stable (Eq. 1) <96JCR(S)242>. In contrast, methyltrioxorhenium (Vn) catalyzed hydrogen peroxide oxidation of thiophene and its derivatives forms first the sulfoxide and ultimately the sulfone derivatives <96107211>. Anodic oxidation of aminated dibenzothiophene produces stable radical cation salts <96BSF597>. Reduction of dihalothiophene at carbon cathodes produces the first example of an electrochemical halogen dance reaction (Eq. 2) <96JOC8074>. 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

ESR experiments employing in situ photolytic decomposition of the peroxydisulfate anion (S20g ) have been carried out to study the reaction of S04 with aliphatic sulfoxides. In the case of dimethyl sulfoxide three radicals are detected ( CHj, CH3 S02, CH2 S(0)CH3), the proportion being pH-dependent. The reaction is assumed to proceed via an initially formed radical cation (not detected) which would be rapidly hydrated to give an intermediate identical with that generated by OH addition on the sulfoxide. Such a process parallels the rapid hydration of radical cations formed from thiophene in their reactions with SO/ and... 

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... 

The redox characteristics, using linear sweep and cyclic voltammetry, of a series of (Z)-6-arylidene-2-phenyl-2,3-dihydrothiazolo[2,3-r][l,2,4]triazol-5(6//)-ones 155 (Figure 24) have been investigated in different dry solvents (acetonitrile, 1,2-dichloroethane, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO)) at platinum and gold electrodes. It was concluded that these compounds lose one electron forming the radical cation, which loses a proton to form the radical. The radical dimerizes to yield the bis-compound which is still electroactive and undergoes further oxidation in one irreversible two-electron process to form the diradical dication on the newly formed C-C bond <2001MI3>. 

A type I one-electron photo-oxidation of methionine-methionine-containing peptides by triplet carboxybenzophenone in air-saturated aqueous solution has been reported the S+ radical cation that is formed then reacts with the other Met-S to form an S-S three-electron complex which reacts with superoxide radical anion before hydrolysis to Met(=0)-Met(=0) bis-sulfoxide. Alternatively, cyclization of the A-terminal NH2 on to the S can occur to give a three-electron S-N complex which can react with superoxide radical anion to give a cyclic sulfonium intermediate. 

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

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