2 -Alkyl substituted thianes

Thiane (00JMS(T)225), 2-, 3-, 4-alkyl substituted thianes (98JPOC831, 99JPOC176), thiane-1-oxide and a number of 3-substituted thiane-1-oxides (01JMS(T)287, 01JMS(T)203) were calculated with HF and DFT theories employing extended basis sets, and the results were compared with experimentally determined conformational equilibria. The conformational free energies (AG°) of conformers and rotamers are discussed in terms of repulsive non-bonded interactions in the axial)equatorial conformers. 

Thiane oxides have been shown to be reduced cleanly back to the thiane with phosphorus pentasulfide under conditions to which sulfones, sulfinates, ketones, esters and amides are inert (78CJC1423) the potential of this reaction, though not yet applied, is obviously considerable, especially when coupled with the old-established Raney nickel desulfurization technique. Thiane itself is desulfurized to pentane (with traces of cyclopentane) but the opportunity to construct alkyl chains of great complexity regio- and stereo-specifically is there. At the very least, the reduction to tetrahydrothiopyrans presents a very useful entree into a wide range of 2-substituted thianes (Scheme 2). 

A series of n-alkyl substituted tetrahydrothiophenes (thiolanes) and thiacyclohexanes (thianes) shown in Figure 3 have been isolated and identified from non-biodegraded petroleums (27). These /1-alkyl monocyclic sulfides were not found in petroleums which had been subjected to biodegradation in their reservoirs. These observations suggested that this group of compounds is biodegradable. 

The sulfide fraction of non-biodegraded petroleums usually contains a complement of n-alkyl substituted sulfides, thiolanes and thianes, in addition to the terpenoid class. In the biodegraded Alberta oil sand bitumens, however, n-alkyl substituted sulfides are absent. The two classes of sulfides can be separated by thiourea adduction, the n-alkyl-containing molecules being adducted. Raney nickel reduction of the Bellshill Lake sulfides (1 11 yielded a series of n-alkanes showing a distribution quite similar to that of the n-alkanes in the saturate fraction of this oil. In some oils the thiolane and thiane concentrations are commensurate, e.g. Houmann, Figure 13 in others, the concentration of thianes may be small compared to thiolanes. 

Under thermal stress the terpenoid sulfides tend to undergo side chain loss and consequently thermally more mature oils are leaner in the longer chain members of each series than are thermally less mature oils, Figure 14. Biodegradation does not appear to affect the terpenoid sulfides but has been shown to remove n-alkyl-substituted thiolanes and thianes (241. and thus the terpenoid fraction of the sulfides becomes relatively enhanced on biodegradation. 

The rest of the sulfur in oil sand asphaltenes, which is not in the form of thiane and thiolane, is present as thiophenes. Indeed, the asphaltene pyrolysis oil contains homologous series of a and a,a n-alkyl substituted thiophenes, 2-, 4- and 2,4-... 

Despite the greater thermal stability of the four-membered alkyl substituted cyclodisil-thianes, the cage compounds (RSi)4S6 are thought to possess the adamantane-like structure. The methyl derivative has such a structure (Si-S bond lengths 212.9 pm), Jike the germanium and tin derivatives36. 

In contrast to a thoroughly performed conformational analysis of 1,4-dioxanes and 1,4-oxa-thianes, the data on the conformational behavior of 1,4-dithianes are quite limited. According to NMR ( H, C) spectroscopy and molecular mechanics calculations, 2-alkyl-substituted 1,4-dithianes exist preferentially in the conformation with equatorially oriented substituents. For 1,4-dithianes with substituents of tbe CHjX type (X = aeetoxy, halogen), the predominance of axial conformers is observed originating from an intramolecular electrostatic attraction <91T9455). 

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