Carbon-sulphur bond, cleavag

Some benzenesulphonic acids with an electron withdrawing carboxyl group in the ring will also undergo carbon-sulphur bond cleavage [83]. 

The sulphonyl chloride 23 is proposed as a protecting group for the Gabriel synthesis of secondary amines from primary amines. At the deprotecting stage, carbon-sulphur bond cleavage is achieved using zinc and acid as the electron source [109]. 

Dialkyl sulphones are not reducible at a mercury cathode. Aryl alkyl and diaryl sulphones are however reduced with cleavage of a carbon-sulphur bond. Polaro-graphic half-wave potentials for this process are given in Table 5.6. One-electron addition in aprotic media to phenyl methyl sulphone [66] and to diphenyl sulphone [67] leads in both cases to a delocalised radical-anion in which the sulphone grorqj can be described as contributing a vacant symmetrical dx-orbital to the conjugated system. Phenyl methyl sulphone radical-anion is prepared and characterised in liq-... 

Electrochemical reduction of aryl sulphones in methanol leads to cleavage of one carbon-sulphur bond in a two-electron process. Alkyl aryl sulphones with an electron donating substituent in the aryl ring give the arenesulphinic acid and an alkane [68, 69]. Methyl phenyl sulphone and alkyl aryl sulphones with an electron withdrawing substituent in the aryl ring are cleaved to the substituted arene and the alkylsulphinic acid [70], see Scheme 5.2. In addition, a bulky orfAo-substituent such as tert.-butyl favours cleavage to the arene and alkanesulphinic acid [71]. 

Benzene mono- di- and tri-sulphonic acids are stable towards electrochemical reduction. More highly sulphonated compounds undergo a two-electron cleavage of a carbon-sulphur bond with elimination of sulphurous acid [82]. Thus benzene-hexa-sulphonic acid 14, in aqueous solution at pH 8, shows three two-electron po-larographic waves with half-wave potentials -1.00, -1.45 and 1.65 V vs. see, due to the sequential loss of substituents. The half-wave potentials are independent of pH. 

One synthetic route to methyl ketones involves the cleavage of carbon-sulphur bonds in either sulphoxides or sulphones using aluminium amalgam as the electron... 

Polymetalated systems of this type without phenyl substitution at the lithiated carbon centre are only accessible when solutions of LiCioHg (144a) or LiDBB (145) in THF instead of a suspension of metallic lithium in THF are reacted with bis(phenylthiomethyl)silanes of type 155. In our group, variously substituted bis(lithiomethyl)silanes 117a, 156b-e and 101 were synthesized by reductive cleavage of the carbon-sulphur bond with LiCioHg... 

A much more general synthesis of these silyl enol ethers, however, is based on the reductive cleavage of the carbon-sulphur bond of the silyl enol ether of a thiolester using sodium metal and chlorotrimethylsilane, once again in a silyl acyloin reaction (Scheme 22)97,98. 

The formation of disulphides (143) on irradiation of S-alkyl-3-oxobutanethioates (144) in benzene or ethanol appears to involve a Type I cleavage, as shown in Scheme 6, rather than direct carbon-sulphur bond homolysis. °° In contrast, the phenacyl sulphides (145) are converted in high yield into the thiones (146) via a Type II cleavage. " Examples of the... 

Ring cleavage initiated by carbon-sulphur bond homolysis... 

The following simplified scheme represents the most important reactions. The primary step is cleavage of carbon—sulphur bonds. Above 450°C, the evolution of hydrogen becomes very important and results in extensive crosslinking in the residue [302]. 

The thio-Claisen rearrangement is a [3,3]sigmatropic process, which consists of synchronous cleavage of a carbon—sulphur bond and formation of a new carbon—carbon bond (equation 6). 

Harder nucleophiles often react with thiirans to give a predominance of attack at carbon, with consequent cleavage of the weak carbon-sulphur bond. This strategy has been used to prepare 7-oxa-4-thia-alk-l-enes from alkoxides, thiiran, and allyl chlorides, yS-mercaptoethyl sulphides from thiiran and thiolate ions, thiazines by the CujO-catalysed cyclization of the adducts of a-lithio-isocyanides with thiirans, tautomeric mixtures of (18) and (19) or (20) from thiiran and hydrazones, 7V-methylpyrrolidines from bis(epithio)linoleic acid and methylamine, and 2,4-dimethylhex-5-ene-2-thiol by addition of the Grignard reagent derived from 3-chlorobut-l-ene with 2,2-dimethylthiiran. Methylthiiran has been found to open... 

While there are normally two reactions of thiiranium ions with nucleophiles, namely ring-opening (with cleavage of a carbon-sulphur bond) or desulphurization to an alkene, a third reaction, i.e. dealkylation to (56), was reported for (57) when chloride or bromide ion was used as the nucleophile. When fluoride, iodide, azide, or... 

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