Thioethers from alkenes

See also Section 134 (Ethers, Epoxides, and Thioethers from Alkenes)... 

SECTION 134 ETHERS, EPOXIDES AND THIOETHERS FROM ALKENES... 

Sulfides, or thioethers, are sulfur analogues of ethers, and like ethers they can be either symmetrical (R2S) or unsymmetrical (RSR1, where R and R are different). Sulfides can be prepared from alkyl halides by a Williamson-type synthesis with sodium hydrogen sulfide, sodium thiolate or sodium sulfide from alkyl or aryl halides via the Grignard reagent (11) from alkenes by radical-catalysed addition of thiols or by reduction of sulfoxides (Scheme 9).2b... 

Oxidative transformations of substituents may be found in Section 7.2.2.1.1.5.6. (oxidations of thioethers, aldehydes, alkenes, and methyl groups). Included for formal reasons are nitrosations (formation of azides and diazonium cations from hydrazines or amines) (Section 7.2.2.I.1.5.6.). 

Recent mechanistic studies from our laboratory point to peroxopolyoxo complexes behave as electrophilic oxidants toward nucleophilic substrates such as thioethers and alkenes, whereas with sulfoxides incursion of SET processes might be a likely event. ... 

Electrophilic addition of bromodimethylsulphonium bromide to alkenes gives, after dehydrobromination, variable yields of vinylsulphonium salts. Hydrolysis of the vinyl thioether derivatives gives a formal conversion of an alkene into a carbonyl compound with some degree of control over regioselect-ivity. Halohydrins (derived from alkenes) can be converted into ketones by dehydrohalogenation with potassium carbonate, catalysed by palladous acetate. ... 

The oxidative cleavage of ethers to give carbonyl compounds has been demonstrated using UFe (readily available deficient in U) as oxidant. The reaction is regiospecific with methyl ethers (89 90). Phenyl thioethers have also been shown to undergo oxidative cleavage via a-chlorination (Scheme 41) giving, as shown, a route from alkenes or alkyl halides to aldehydes. 

Representative data illustrating the influence of Lewis base functional groups in the ADMET reaction are shown in Table 1. When molybdenum catalysts are used to polymerize ether or thioether dienes, little change in reaction rate is observed as compared with the standard, 1,9-decadiene, which possesses no heteroatoms in its structure. When a sulfur atom is three carbons atoms away from the alkene site, the reaction rate is reduced approximately one order of magnitude otherwise, the kinetics are all essentially unaffected [20a]. 

Aliphatic onium ions such as immonium, oxonium, and sulfonium ions have been introduced as even-electron ionic products of the a-cleavage occurring from molecular ions of amines, alcohols, and ethers or thiols and thioethers, respectively (Chap. 6.2.5). All these and analogous onium ions are capable of further fragmentation reactions, the majority of which are alkene losses [141] yielding fragments of high relevance for structure elucidation. 

Interestingly, sulfonium ylides generated from electrophilic carbene complexes and sulfides can react with carbonyl compounds, imines, or acceptor-substituted alkenes to yield oxiranes [1320-1325], aziridines [1321,1326,1327] or cyclopropanes [1328,1329], respectively. In all these transformations the thioether used to form the sulfonium ylide is regenerated and so, catalytic amounts of thioether can be sufficient for complete conversion of a given carbene precursor into the... 

If alkenes are used instead of silanes, the intermediate oxocarbenium ion undergoes an acetal -ene reaction. An example is the reaction of acetal 11 with methylenecy-clohexane to give the cyclohexenylmethyl-substituted product 12 (Scheme 8.4) [20]. Silenol ethers are electron-rich alkenes particularly suited for addition to cationic species. Pinacolone-derived enol ether 13, for example, adds to an thioxocarbenium ion generated in situ from S,S-acetal 14 to give thioether 15 (Scheme 8.4) [21]. 

Selenides are more readily cleaved by tin radicals than thioethers. Products bound to cross-linked polystyrene by a C-Se bond can be released from the support either by treatment with an oxidant to yield alkenes [1, 55-58], or by treatment with tin radicals to yield alkanes... 

Acetals result from oxidative coupling of alcohols with electron-poor terminal olefins followed by a second, redox-neutral addition of alcohol [11-13]. Acrylonitrile (41) is converted to 3,3-dimethoxypropionitrile (42), an intermediate in the industrial synthesis of thiamin (vitamin Bl), by use of an alkyl nitrite oxidant [57]. A stereoselective acetalization was performed with methacrylates 43 to yield 44 with variable de [58]. Rare examples of intermolecular acetalization with nonactivated olefins are observed with chelating allyl and homoallyl amines and thioethers (45, give acetals 46) [46]. As opposed to intermolecular acetalizations, the intramolecular variety do not require activated olefins, but a suitable spatial relationship of hydroxy groups and the alkene[13]. Thus, Wacker oxidation of enediol 47 gave bicyclic acetal 48 as a precursor of a fluorinated analogue of the pheromone fron-talin[59]. 

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