Cleavage of thioethers

The El mass spectra of thiols and thioethers also show a series of onium ions generated by a-cleavage of the molecular ion (Table 6.9). Sulfonium ions can easily be recognized from the isotopic pattern of sulfur (Fig. 6.9). The fragmentation patterns of thioethers will be discussed in greater detail later (Chaps. 6.5.2 and 6.12.4). 

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The above reactions in this section have been examples of addition alone or addition followed by elimination. Ligand reactions involving nucleophilic substitution are also known and these are of the dealkylation type. Lewis acids such as aluminum chloride or tin(IV) chloride have been used for many years in the selective demethylation of aromatic methyl ethers, where chelation is involved (Scheme 27). Similar cleavage of thioethers, specially using mercury(II) salts, is commonly used to remove thioacetal functions masking ketones (equation 27).104 In some cases, reactions of metal ions with thioether ligands result in isolation of complexes of the dealkylated organic moiety (equations 28 and 29).105-107... 

Sucholeiki I, Solid-phase photochemical C-S bond cleavage of thioethers—a new approach to the solid-phase production of nonpeptide molecules, Tetrahedron Lett., 35 7307-7310, 1994. 

Cleavage of thioethers. von Braun found that a dialkyl thioether is cleaved by cyanogen bromide at a somewhat elevated temperature (60-70°, sealed tube) to an alkyl thiocyanate and an alkyl bromide. Results for the first three mixed ethers disclose a clear relationship of Haftfestigkeit to size of group ... 

Itoh N, Yoshida M, Miyamoto T et al. (1997). Fungal cleavage of thioether bond found in Yperite. FEBS Lett, 412, 281-284. 

An ice-cooled soln. of diphenylphosphine in dry tetrahydrofuran treated with a 1.2 M soln. of butyllithium in petroleum ether (b.p. 40-60°), allyl phenyl ether added, and refluxed 4hrs. phenol (Y 78%) and allyldiphenylphosphine (Y 76%). — Anisole is cleaved much faster than phenetole and other simple alkyl phenyl ethers. F. e., also cleavage of thioethers and cleavage with diarylarsenide ions, s. F. G. Mann and M. J. Pragnell, Soc. 1965, 4120. 

Uchida, K. and Stadtman, E. R. Selective cleavage of thioether linkage in protein modified with 4-hydroxynonenal. Proc. Natl. Acad. Sci. USA 89 1992a 5611-5615. 

Scheme 6 Indirect anodic cleavage of thioethers with trisarylamines.

Stable cation is produced. The silver- or mercury (II) ion cleavage of thioethers appears to be similar to the proton-catalyzed cleavage with the important exception that the last step is irreversible. 

Reductive cleavage of thioethers and sulfones Mercaptans and hydrocarbons from thioethers Sulfinic acids and hydrocarbons from sulfones Extractor technique... 

Sue1994 Sucholeiki, I., Solid-Phase Photochemical C-S Bond Cleavage of Thioethers - A New Approach To The Solid-Phase Production of Non-Peptide Molecules, Tetrahedron Lett., 35 (1994) 7307-7310. 

Direct cleavage of thioether derivatives 402 can be performed resulting in alkanes 403 or in the iodo-substituted derivatives 406. lodo-transformations have been known since a publication by Crosby et al. in 1977 [272], Starting from a methyl sulfide resin and following hthiation with -butyllithium, alkylarylthioethers have been synthesized that could be cleaved in 82% yield via the addition of a mixture containing Mel and Nal (Scheme 57). 

Silyl-derived protective groups are also used to mask the thiol function. A complete compilation is not given here since silyl derivatives are described in the section on alcohol protection. The formation and cleavage of silyl thioethers proceed analogously to simple alcohols. The Si—S bond is weaker than the Si—O bond, and therefore sulfur derivatives are more susceptible to hydrolysis. For the most part silyl ethers are rarely used to protect the thiol function because of their instability. Silyl ethers have been used for in situ protection of the — SH group during amide formation. ... 

Na/NH3, >54% yield. Methyl thioether cleavage of BOC-protected methionine. ... 

The deprotection of the polyamine-thioethers 40a,b can also be accomplished in similar good yields with Li/naphthalene in THF (214). Cleavage of the thioether functions also commenced with [PdCl2(NCMe)2] in the case of 40b (216), and it is likely that the other macrobicycles will also undergo this metal-ion mediated cleavage reaction as well (Scheme 2). Note that this reaction cleaves only one of the two thioether bonds forming a macrocyclic ligand with one thiophenolate and one vinyl-thioether moiety, as... 

Example The oxidative addition of dimethyl disulfide (DMDS) transforms the double bond to its 1,2-bis-thiomethyl derivative (a). Induced by charge localization at either sulfur atom, the molecular ions of DMDS adducts are prone to a-cleavage at the former double bond position (b). This gives rise to sulfonium ions that are readily identified from the mass spectrum (Chap. 6.2.5). The method can be extended to dienes, trienes, and alkynes. [70,71] (For the mass spectral fragmentation of thioethers cf. Chap. 6.12.4). 

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