Thianes, conformation

The preference for the axial position in unhindered thiane-1-oxides has been known for some time. The spectra of the cis and trans isomers of the 2-, 3- and 4-methyl thiane-1-oxides, 169-171, were also measured. It was concluded from the 13C chemical shifts that the methyl groups preferred the equatorial positions. A comparison of the 170 chemical shifts obtained for sulfoxides 169-174 with those obtained for the cis and tram sulfoxide isomers of trans- 1-thiadecalin, 175 and 176, was consistent with this proposal. Sulfoxide 175 with the S=0 axial gave a shift about 17 ppm upfield from that of its equatorial isomer 176. For sulfoxides 169-174, the conformers proposed to have the S=0 axial gave shifts that were upfield from those of the supposed equatorial conformers. For tram-3, (rans-5-dimethylthiane-1 -oxide (177) with the oxygen axial, the 170 signal was 21 ppm upfield from the signal observed for the equatorial oxygen in cis-3, cis-5-dimethylthiane-l-oxide (178). 

The molecular mechanics method314 has been applied to the calculation of conformational properties of the thiane, dithiane and trithiane oxide systems315, which are... 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

In the equilibrium mixtures of thiane oxide (2) and 1,3-, 1,4-dithiane dioxides (3) and (4), the axial conformers are present predominantly over the equatorial conformers at low temperatures (— 90 °C). For instance in the monoxide (2) a ratio of 62% axial and 48% equatorial conformers has been observed. The prevalence of the axial conformers in the sulphoxides (2) and (4) has been explained to be due to hydrogen bonds between the oxygen in the axial sulphinyl group and the hydrogen atom at the 3-position as shown in Scheme 7121 127. 

Thiadiazoline S-oxides, synthesis of 275 Thiane dioxides formula of 382 synthesis of 468 Thiane oxides 750-752 conformational analysis of 465-467 formula of 382... 

Other studies have also established the preference of the chair conformation with the oxygen in the axial position the rationale for this preference is different from the attractive interaction between the sulfoxide oxygen and the syn-axial hydrogens proposed previously . Rather, a repulsion effect is advocated the equatorial oxygen is squeezed between four vicinal hydrogens, while there are only two corresponding repulsions if it is in the axial position. The correlation between the predicted and observed conformational/orientational preferences in 3,3-dimethylthiane oxide (e.g., equatorial preference in the chair conformation) corroborates this interpretation. The axial preferences of the sulfur-oxygen bond in the thiane oxide is reversed in 3,3-dimethylthiane oxide because of the syn-axial interaction. 4,4-Dimethylthiane oxide, however, maintains a predominance of the axial isomers as deduced from the analysis of NMR data . ... 

The NMR spectrum of protonated thiane-3,3,5,5-c/ has also been studied in HS03F-SbF5 to determine the conformational position of the proton on sulfur in the six-membered ring and to study the ring inversion process.162 The proton on sulfur resides exclusively in the axial position. 

Thiosulfonium ions derived from methylation of substituted 1,2-dithianes (94a) were found to exist in undistorted chair conformation with the methyl group in axial position.254 The phenomenon was interpreted to result from minimizing electron repulsion in orthogonal position (94b) (ab initio calculation at STO-3G //STO-3G level). Unlike 1,2-dithianes, methylation of 1,2-dithiolanes and substituted thianes to form the corresponding. S -methylsulfonium hexafluorophosphate and perchlorate salts255 is non-stereoselective. 

In 2-benzoyl-oxane (also in 2-benzoyl-thiane, -1,3-dithiane, 1,3-oxa-thiane) the benzoyl substituent adopts the equatorial conformation in the solid state (99IJC617). 

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. 

Very recently, the HF, MP2, and DFT calculated conformational equilibria of 4-X substituted thiane-1,1-dioxides (X = Me, CH2OH, COCH3, COC2H5, CN, F, Cl, Br, and OCOCH3) were published (02STC115). 

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