1.3- Dithianes conformations

Dithiane, 2-phenyl-bond angles, 3, 949 conformation, 3, 949 structure, 3, 949 sulfoxide... 

Table 3.8. Comparison of Conformational Free-Energy Values for Substituents on Tetrahydropyran, 1,3-Dioxane, and 1,3-Dithiane Rings with Those for Cyclohexane...

The conformational preference of the monosulfoxides of 1,2-, 1,3- and 1,4-dithianes (179-181) were determined by NMR experiments which included variable-temperature studies, double irradiation, solvent effects and the influence of lanthanide shift reagents167. For 179 and 181, the axial conformers were the dominant species in CD3OD, but for 180, the equatorial conformer was in excess. 

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

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. 

Fig. 2 Calculated low energy conformation of the protonated dithiane oxide cation (R=H) in zeohte Y (Si/Al = 1). The bottom view shows a view through the twelve ring containing e deprotonated framework oxygen, the top view is perpendicular to this. For clarity the zeolite framework is shown using a stick model and the adsorbed molecule is drawn in space filled form represented by tlie Van der Waals radii for the atoms being in the order S>0>C>H.

Fig. 3 Calculated low energy conformation of the protonated dithiane oxide cation (R=phenyl) in zeolite Y (Si/Al = 1). The views and presentational details are as for figure 2.

Figure 4 Calcxilated low energy conformations for enantiomers of < - — s butan-2-ol in dithiane oxide (R=H)...

The decrease of the anomeric effect in polar solvents was also supported by quantum mechanics calculations.13 Nevertheless further studies on the anomeric effect demonstrated the limitations of the electrostatic model. In particular, Juaristi et al.14 demonstrated that, at low temperature, the dependence of conformational equilibria of 2-carbomethoxy-l,3-dithiane upon solvent shows an opposite trend to the stronger anomeric effect in less polar media observed at 25 °C (Table 4). 

Similar observations have been reported by Pinto15 for the conformational equilibrium in 2-[(4-methoxyphenyl)seleno]-l,3-dithiane (Fig. 9). Although, at 273 and 300 K, the proportion of the more polar equatorial isomer 17 increases as the dielectric constant of the medium increases, no apparent correlation exists at low temperature. 

Table 4 Conformation dependence on solvent polarity according to Juaristi for 2-carbomethoxy-l,3-dithiane... 

Fig. 9 Equilibrium data for conformational equilibrium in 2-[(4-methoxyphenyI)seIeno]-l,3-dithiane according to Pinto.

Finally, mention should be made of the striking conformational preference of trans 2,3- and tram 2,5-dioxanes, dithianes, and thioxanes. Each of these compounds exists in a chair conformation with two diaxial halogens334- 341 ... 

Both the measured dipole moments and H NMR spectra of 1,2-dithiane, its 4,4,5,5-tetradeutero analog, 3,3,6,6-tetramethyl-l,2-dithiane, and the m//ra r-isomers of 3,6-dimethyl-l,2-dithiane provided unequivocal evidence for the chair conformation adopted by the saturated six-membered ring, supported further by X-ray solid-state structures. Other conformers, for example the twist conformer, were not detected . 

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