Carbon-sulfur bonds, formation, copper

Recently, interest in copper-catalyzed carbon-heteroatom bond-forming reactions has shifted to the use of boronic acids as reactive coupling partners [133], One example of carbon-sulfur bond formation is displayed in Scheme 6.65. Lengar and Kappe have reported that, in contrast to the palladium(0)/copper(l)-mediated process described in Scheme 6.55, which leads to carbon-carbon bond formation, reaction of the same starting materials in the presence of 1 equivalent of copper(II) acetate and 2 equivalents of phenanthroline ligand furnishes the corresponding carbon-sulfur cross-coupled product [113]. Whereas the reaction at room temperature needed 4 days to reach completion, microwave irradiation at 85 °C for 45 min in 1,2-dichloroethane provided a 72% isolated yield of the product. 

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

An alternative to the synthesis of epoxides is the reaction of sulfur ylide with aldehydes and ketones.107 This is a carbon-carbon bond formation reaction and may offer a method complementary to the oxidative processes described thus far. The formation of sulfur ylide involves a chiral sulfide and a carbene or carbenoid, and the general reaction procedure for epoxidation of aldehydes may involve the application of a sulfide, an aldehyde, or a carbene precursor as well as a copper salt. This reaction may also be considered as a thiol acetal-mediated carbene addition to carbonyl groups in the aldehyde. 

Utility of Enol and Aryl Triflates. Enol and aryl triflates are extensively used for cross-coupling reactions, the formation of carbon-carbon, carbon-tin, carbon-nitrogen, carbon-sulfur, carbon-phosphorus, and carbon-halogen bonds, and reduction/ deoxygenation. In recent examples, they were used to form enamines or enamides or were eliminated to cyclooctynes for copper-free cycloadditions in biological systems. 

The formation of aryl chlorides from arenediazonium ions on reaction with copperfi) chloride is known as the Sandmeyer reaction and also involves the intermediacy of aryl radicals [192]. Copper salts are similarly involved in the reactions of diazonium ions with cyanide, nitrite, and sulfur dioxide. The uses of arenediazonium salts in synthetic reactions forming carbon-carbon, carbon-sulfur, and carbon-boron bonds have been summarized recently [193]. 

Due to sluggish reactivity of aryl and vinyl halides in nucleophiUc substitution reactions, the formation of sulfur-carbon(sp ) bonds is typically carried out using transition metal catalysis [22-27]. While the field is dominated by the use of palladium, copper, and nickel catalysts, considerable advances have been made using more abundant metal catalysts such as iron. Additionally, a number of transition metal-fiee approaches have been developed for the formation of sulfur-carbon(sp ) bonds. The following sections will highlight representative examples of C—S bond forming reactions. 

It has been shown that the amination of the carbon-hydrogen bond in benzox-azoles may be achieved using sulfamoyl chlorides as the nitrogen source with palladium-copper catalysis. It is likely that after the formation of intermediates such as (118), expulsion of sulfur dioxide forms another intermediate which, after reductive... 

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