Tetrahydrothiophenes, formation

The second observation comes from studies on the photolysis of ethylene episulfide itself. When this substrate is photolyzed in the presence of ethylene, small yields of tetrahydrothiophene are detected among the products. The most obvious reaction to account for tetrahydrothiophene formation would be the addition of thiodimethylene to the added ethylene. 

The initial step of the adsorption of cyclic sulfides on a Mo(100) surface is also the formation of adsorbed thiolate groups.395-397 Adsorbed alkyl thiolates decompose to adsorbed sulfur, carbon, and hydrogen on the clean Mo surface, but once the surface is deactivated by adsorbed sulfur, alkanes and alkenes evolve from the surface. Tetrahydrothiophene (34) and trimethylene sulfide decompose on Mo(110) to alkanes and alkenes by way of a common intermediate, which is proposed to be a surface thiolate (33). The thiolate undergoes hydrogenation or dehydrogenation, depending on the surface hydrogen concentration (Scheme 4.115).398 399... 

The formation of five-membered tetrahydrothiophene rings using nonsymme-trical reaction partners usually occurs with high regioselectively. In most cases, a stereospecific reaction was observed (9,27,94). Recently, a diastereoselective cycloaddition of the parent species la to various chiral a,p-unsaturated amides was also reported (32,95). 

An attempted synthesis of biotin using thiocarbonyl ylide cycloaddition was carried out (131,133,134). The crucial step involves the formation of the tetrahydrothiophene ring by [3 + 2] cycloaddition of a properly substituted thiocarbonyl ylide with a maleic or fumaric acid derivative (Scheme 5.27). As precursors of the thiocarbonyl ylides, compounds 25a, 72, and 73 were used. Further conversion of cycloadducts 74 into biotin (75) required several additional steps including a Curtius rearrangement to replace the carboxylic groups at C(3) and C(4) by amino moieties. 

The reaction of the sterically crowded thiocarbonyl ylide 69 with highly electron-deficient alkenes such as 2,3-dicyano fumarate and maleate, tetracya-noethene, a-cyano cinnamates, and l,2-bis(trifluoromethyl)ethene-l,2-dicarboni-trile occurred in a nonstereospecific manner (27,89,96,97,136-138). The formation of a mixture of cis/trans tetrahydrothiophenes of type 82 is the result of a stepwise reaction involving zwitterionic intermediates of type 81 (Scheme 5.29). Ylide 69 fulfills the fundamental requirements for a two-step reaction with electron-deficient alkenes. This species corresponds to an electron-rich 1,3-dipole that also contains a bulky substituent at one terminus (89). 

An intramolecular cycloaddition reaction was also used in the synthesis of the annelated tetrahydrothiophene (97), starting from l,3-oxathiolan-5-one (96) (131) (Scheme 5.36). Thiocarbonyl ylide formation occurred by thermal extrusion of CO2 at 250 °C, yielding 97 in 62% yield. 

This structural change is suppressed by the addition of tetrahydrothiophene (THT)19b. It prevents the formation of polymethylene zinc, i.e. (—CH2Zn—) . Without THT, a solution of 3 in THF yields polymethylene zinc at 60 °C. Monomeric bis(iodozincio)methane (3) is much more active for methylenation as compared to polymethylene zinc. As shown in Table 3 (entry 3), the addition of THT to the reaction mixture at 60 °C improved the yield of the alkene dramatically. Practically, however, its stinking property makes the experimental procedure in large scale uncomfortable. Fortunately, an ionic Uquid, l-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), plays the same role. Ionic liquid also stabilizes the monomeric structure of 3 even at 60 °C and maintains it during the reaction at the same temperature. The method can be applied to various ketones as shown in Scheme 14.4... 

Tetrahydrothiophenes.1 Reaction of K.SCOCH, (2.4 equivalents) with 1 in DMF in the presence of air results in formation of 2 in 90% yield. In the absence of oxygen the yield of 2 is negligible. The yield is also low if only 1 equivalent of the salt is present. 

The catalytic properties of unsupported transition metal sulphides have been examined for the reaction of dehydrogenation of tetrahydrothiophene. This study has shown that a selectivity higher than 90% for thiophene formation can be obtained for the most active catalysts, essentially the second row sulphide catalysts. The comparison between the catalytic activities in both dehydrogenation of tetrahydrothiophene and... 

Photolysis of 8-thia-bicyclo[3.2.1]octan-3-one 115 in /butyl alcohol resulted in the formation of 4-but-3-enylthietan-2-one 116 in 19% yield and a tetrahydrothiophene derivative 117 in 5% yield accompanied by traces (4% and 3%) of exo- and . 

Both the 2-thiahex-5-enyl and the 2-sulfonylhex-5-enyl radicals exhibit a significant preference for 5-exo cyclisa-tion to the thiophene over formation of tetrahydrothiopyran by 6-endo ring closure (Equation 144) <2000TL7987> and only at higher I2 concentration does the formation of tetrahydrothiopyrans compete successfully with thietane and tetrahydrothiophene production when methylene-intermpted dienoates 455 react with dimethyl disulfide (Equation 145) <1994TL5575>. 

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