C-S cleavage

If an electron acceptor is available in homogeneous solution, photochemical reaction can be observed. For example, when 2 is excited (X > 350 nm) in anhydrous dimethylsulfoxide (DMSO), methylation occurs, ultimately giving rise to 9,9-dimethyl-fluorene in >80% yield. By analogy with Tolbert s mechanism for photomethylation in DMSO (4), such a process may be initiated by electron transfer to DMSO to form a caged radical-radical anion pair from which subsequent C-S cleavage occurs (eqn 4). 

Cyclization of hydroxy dithioketals. Nicolaou et al.1 have used this reaction for construction of oxocenes. The most satisfactory method involves activation of the sulfur of the starting material (1) with AgC104 to produce an oxocene derivative (2), followed by C-S cleavage. This second step can be effected with (C6H5)3SnH (AIBN) or by oxidation to the corresponding sulfoxide or sulfone, which then is cleaved with... 

With sulfides as intermediates, alkenes can be used as precursors to organolithiums with regioselectivity in the formation of 56 determined by whether a radical11 or polar73 thiol addition is employed. Easy lithiation of phenyl benzyl sulfide 57 makes substituted benzyllithiums such as 58 readily available.73 Reductive C-S cleavage is probably the best way of making benzylic organolithiums. 

Intermolecular PET from photoexcited JV-ethyl-2-ethylphenothiazine [87] and perylene [88], for example, to triphenylsulfonium salts produces C—S cleavage to provide diphenylsulfi.de and phenyl radical as well as the cation-radical of the sensitizer. 

Diphenyl sulfoxide and dimethyl sulfoxide are oxidized electrochemically to the coiresponding sulfones in acetonitrile and 1 M H2SO4, respectively. The product yields are enhanced the presence of transition metal salts or oxides of W, V, Mo or Se. In some cases sulfonic acid salts are formed with C—S cleavage. Sulfoxides react with oxygen under photochemical conditions to give sulfones in good yields. ... 

The photolytic cleavage of alkyl aryl sulfoxides has been shown to occur via initial C—S bond homolysis, in accordance with the common mechanistic assumption. Secondary and tertiary alkyl groups show high chemoselectivity for alkyl C—S cleavage. Uniquely, alkene products have been isolated, formed by disproportionation of the initial alkyl radical, with the formation of benzaldehyde and racemization of primary alkyl compounds. An investigation into the photochemical conversion of N-propylsulfobenzoic imides into amides in various solvents revealed a solvent dependence of the observed mechanism. In ethanol, sulfur dioxide extension forms a biradical which abstracts a hydrogen atom from the solvent, whereas in aromatic solvents biradical formation by a single electron transfer is implicated. The photolysis and thermolysis of l,9-bis(alkylthio)dibenzothiophenes and /7-aminophenyl disulfide have been studied. 

Presumably this reaction proceeds via an allylic carbocation intermediate 301 formed by Lewis acid-initiated C-S cleavage, then ring closure to a new carbocation 302 stabilized by the silyl group. As illustrated in Scheme 55, the subsequent steps, leading to final product 300, involve disulfide bond formation, double-bond shift, and hydride migration. 

A quantitative study of the rate of product formation as a function of exposure time and pressure showed that HjS and CH4 arise from secondary reactions in the photolysis, viz., sensitization of the substrate by an electronically or vlbrationally excited molecule of dimethyl disulfide the unit efficiency of the primary step was verified. Thiyl radicals were also observed by EPR and uv measurements on photol-yzed CHaSH in the solid state , in water clathrate matrices and in flash photolysis experiments . Recent evidence points to the formation of hot H-atoms in the primary process. The Hg( Pi)-sensitized decomposition of CH3SH also leads to the formation of CH3S + H but there is a minor (ca 10%) contribution from C-S cleavage, viz. 

The photolysis of ethyl mercaptan in an organic matrix at 77 °K was shown to produce CH3CH2S radicals by uv and esr spectrometry . The gas-phase photolysis at 2288 and 2537 A is also initiated by S-H bond rupture, and hot H-atoms are probably produced < (H2) = 1.0. In the Hg( Pi)-sensitized decomposition , C-S cleavage appears to be a major process, although its relative importance could not be determined. The pyrolysis, examined by toluene carrier in the range 512-665 C proceeds by two mechanisms depending on the temperature". At low temperature, H2S and C2H4 are the major products, the former found in quantitative yields a molecular mechanism, similar to the dehydration of alcohol, was proposed, viz. 

The thermolysis of benzyl mercaptan in the range 487-630° proceeds by a first order process to yield H2S and bibenzyl, again via C-S cleavage... 

Qualitative photochemical studies of a variety of simple alkyl monosulfides are consistent with primary C-S cleavage... 

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