Sulfur-carbon bonds formation

G. Sharma, R. Kumar, and A. K. Chakraborti, A novel environmentally friendly process for carbon-sulfur bond formation catalyzed by montmorillonite clays, J. Mol. Cat. A Chem., 263 (2007) 143-148. 

Scheme 6.63 Ullmann-type carbon-nitrogen and carbon-sulfur bond formations.

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. 

A palladium-catalyzed protocol for carbon-sulfur bond formation between an aryl triflate and para-methoxybenzylthiol was introduced by Macmillan and Anderson (Scheme 6.66) [138], Using palladium(II) acetate as a palladium source and 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) as a ligand, microwave heating of the two starting materials in N,N-dimethylformamide at 150 °C for 20 min in the presence of triethylamine base led to the formation of the desired sulfide in 85% yield. 

Although examples are sparse, a Pd-catalyzed carbon-sulfur bond formation leading to 379 was the penultimate reaction in a synthesis of ( )-chuangxinmycin [470, 194], Earlier, Widdowson described the thiolation of 3-acetyl-4-iodoindole (Me02CCH2SsnMe3/Pd(Ph3P)4/83%) [471]. 

Hydrothiolation of Allenes The first example of a gold-catalyzed carbon-sulfur bond formation was published by Kraus et al. who synthesized 2,5-dihydrothiophenes by allenyl thiocarbinols [38]. The best results were obtained with AuCI in CH2C12, providing an impressive diastereoselectivity. In the same study, other coin-metal precatalysts were tested but only gold afforded the cydization product. 

The first published example of the anodic cyclization with carbon-sulfur bond formation was the oxidation of thiobenzanilide to 2-phenyl-l,3-benzothiazole.120 The oxidation of the thioacetanilides (106) and /V-(3-coumaryl)thioacetamide (108) at a platinum electrode in CH3CN-Et4NC104 gave the expected 1,3-thiazole derivatives 107 and 109 in 60-75% yield169 [Eqs. (81) and (82)]. 

The synthesis of thiepanes by double nucleophilic reaction in dipolar aprotic solvents improves the yield even under at normal dilution (79SC857). Li2S generated in situ from hexamethyldisilathiane and methyllithium was convenient for carbon-sulfur bond formation (Equation (2)) (85JOC4969). 

Carbon-carbon bond formation by Friedel-Crafts reactions was a versatile procedure for benzo- and dibenzothie-pines. Some new methods to synthesize thiepine rings by Friedel-Crafts-type reactions have been reported. Carbon-sulfur bond formation has been used for dihydro-, tetrahydrothiepines, and thiepanes. New cyclization methods involving carbon-sulfur and carbon-carbon bond formation have been added. 

Carbon-sulfur bond formation by oxidative cyclization of a thiourea derivative using bromine will in the case of (521) furnish a thiazolo[/,/]quinoline (522 Z = NH). The same imine is obtained from 8-mercapto-l,2,3,4-tetrahydroquinoline on treatment with cyanogen bromide with phosgene the 2-oxo derivative (522 Z = 0) is formed (63JOC2581). 

An interesting asymmetric carbon-sulfur bond formation occurs during the interaction of thiofenchone-S-methylide 21 with 1.1 equivalents of thiophenol in tetrahydrofuran. Two thioac-etals 22 are formed in 94% yield. The ratio of diastereomers, originally 7 3, is reversed to 1 9 on contact with silica gel. It was suggested that the latter ratio refers to the equilibrium139. 

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