Formation of C-S bonds

Carbon-sulfur bonds can be formed by the reaction of elemental sulfur with a lithio derivative, as illustrated by the preparation of thiophene-2-thiol (399) (700S(50)104). If dialkyl or diaryl disulfides are used as reagents to introduce sulfur, then alkyl or aryl sulfides are formed sulfinic acids are available by reaction of lithium derivatives with sulfur dioxide. 

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The reductive formation of C-S bonds via the reaction of carboxylic acids with phosphorus pentasulphide and red phosphorus has been utilized in the synthesis of 3-arylthiophenes starting from 2-arylsuccinic acids [66, 67]. The reaction is catalysed by benzyltriethylammonium chloride, but it has been suggested that co-catalysis with 18-crown-6 is advantageous [67]. 

Despite the prevalence of the C-S bond in nature and the importance of the sulfur group in many biological processes the organocatalysed formation of C-S bonds has received significantly less attention than C-N, C-0 and C-C construction. 

Figure 5-74. The nucleophilic displacement of a leaving group by a thiolate is one of the commonest methods for the formation of C-S bonds.

This is an example of a macrocyclisation reaction involving formation of C-S bonds that gives a [1 + 1] major product and a [2 + 2] minor product. 

Cyclizations by formation of carbon-sulfur-bonds belong to the best studied reactions which use the dilution principle. Usually bromides react with thiols [7 b, 17, 18], thiuronium salts [19], or with sodium sulfide [20] and thioacetamide, respectively. The formation of C—S bonds represents one of the most valuable methods to synthesire medium-membered cyclophanes such as 4-8 [7 b, 18 b, 20d, 21]. 

Even severely strained ring systems such as the bridged biphenylene 9 [20c] and the helical chiral [2.2]metacyclophane 10 [22,23] could be synthesized by formation of C —S bonds. 

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