¹³C and ¹⁷O chemical shifts and conformational analysis of mono‐ and di‐methyl‐substituted thiane 1‐oxide and thiane 1,1‐dioxide

Abstract

The -C and 1 7 0 (~tural abundance) chemical shifts of several mono-and di-methyl ring-substituted thiane 1-oxides and thiane 1,l-dioxides are reported. The cis and trans isomers of methyl-substituted thiane l-oxide are readily identified by -C and 170 NMR. In particular, the "0 NMR signals of axial SO groups are found several ppm upfield of those of the equatorial counterparts. The proportion of axial and equatorial conformers of S a n e 1-oxide in Merent solvents has been measured by low-temperature -C NMR. In THF the proportion of the axial conformer is nigher than in CDzClz, whereas in CDcl, or CHF&l the conformational preference is reversed and the equatorial conformer is slightly favoured. INTRODUCI'IONWe report a 13C and 170NMR study of thiane 1-oxide, thiane 1,l-dioxide and several of their methyl ring-substituted derivatives.The conformational energy, AG,a%AG,,a for an SO group of a six-membered ring was first estimated to be approximately 5.4 kJ mol-' (1.3 kcal mol-I), on the basis of thermal and hydrogen chloride catalysed equilibration of cis-and trans-4-tea-butylthiane 1-oxide.' Later studies showed that the conformational energy for the parent compound, thiane 1-oxide (l), was much lower, i.e. 0.8 kJ mol-' (0.2 kcal mol-l) as measured by 'H NMR.' Force-field calculations have been found to be in agreement with the NMR At present, the reasons governing the conformational preference of thiane 1-oxide are not well understood. According to an early interpretation, 1,3 the axial preference of the sulphinyl oxygen in 1 is accounted for in terms of an unusually attractive van der Waals interaction between the oxygen atom and the syn-axial protons in the axial conformer. In agreement with this interpretation, when the axial hydrogen at C-3 is replaced with a methyl group, as in 3,3-dimethylthiane 1-oxide, only the equatorial conformer is observed at low ternperat~re.~ However, according to a more recent interpretation the axial preference of thiane 1-oxide is best ascribed to the destabilizing interactions existing between the S-0 bond and the adjacent C-€3 bonds: whereas there are only two such interactions in the axial conformer, four are present in the equatorial conformer.4a Comparison of the experimental results with the theoretical predictions cannot be made correctly, however, until the influence of the * Author to whom correspondence should be addressed. solvent in the conformational equilibrium of 1 is known. The position of the conformational equilibrium is dependent on the solvent in several saturated heterocycles. This is the case, for instance, for some derivatives of 1,3-dioxane7 for which a strong dependence on the dielectric constant of the solvent was found for the conformational properties.6A better knowledge of the conformational preference of a sulphinyl group is also of relevance in the highly debated question of the stereochemistry of aHtmethylation of sulphoxides. It has been shown, for example, that in six-membered ring sulphoxides the stereochemistry of this reaction depends on the ...

Crystal structures and theoretical calculations of trans-2,4,4-trimethyl-4-silathiane 1-oxide and 4,4-dimethyl-4-silathiane 1,1-dioxide

“…In solution, more polar equatorial conformers can be stabilized and exist in equilibrium with axial conformers [24][25][26][27][28][29][30], the position of the equilibrium being dependent on the substituents in the ring. In our case, the existence of sulfoxide 1 in the form of the equatorial conformer is due to the method of its synthesis.…”

Oxygen-17 NMR spectroscopy: Basic principles and applications (Part I)

“…Calculated for the most stable of the ''open" forms and the intramolecularly hydrogen-bonded form.A comparison of the diastereomeric sulfoxides of trans-1-thiadecalin of Scheme 31 demonstrates the importance of c-gauche methyl or methylene shielding effects on sulfinyl oxygens in a six-membered ring. The17 O chemical shift of the axial sulfinyl oxygen in I, is more shielded by À17.0 ppm than the equatorial sulfinyl oxygen in II[527,536,537].The17 O chemical shifts of the sulfonyl (-SO 2 -) group in acyclic compounds appear between…”

Conformational behavior of 3,3-dimethyl-3-silathiane 1-oxide and its diastereomeric 2-methyl derivatives

“…The introduction of two methyl groups at C-3 atom strongly disfavors the axial conformer due to van der Waals repulsion between SO-ax and the syn-axial methyl group, so that 3,3-dimethylthiane 1-oxide 2 exists in CD 2 Cl 2 solution as the equatorial conformer 2-eq exclusively (Scheme 2) [17,19].…”

“…Conformational equilibria of these compounds have been shown to be strongly affected by the presence of a second heteroatom in the ring or substituents at the C-ring atoms and by the nature of the solvent [11]. Thus, for thiane oxide 1 a slight axial preference of the sulfinyl oxygen ( Gº −90 • C = +0.175 kcal mol −1 , CH 2 Cl 2 ) has been determined by low-temperature 1 H and 13 C NMR spectroscopy (Scheme 1) [16][17][18][19][20], but only axial chair form 1-ax has been found in the gas phase by an electron diffraction study [21]. The higher stability of the axial conformer as compared with the equatorial one was explained by an attractive 1,3-syn-axial interaction between the oxygen and hydrogen atoms in the 1-ax conformer and/or by a repulsion of the equatorial oxygen atom by its vicinal hydrogen atoms in 1-eq [11].…”