Effect of Substituents and Conformations on the Optical Rotations of Cyclic Oxides and Related Compounds. Relationship between the Anomeric Effect and Optical Rotation¹

Abstract

The effect of substituents on the specific rotation of substituted cyclic oxides (X ) F, Cl, CN, and HCC) and related compounds was studied via geometry optimization at the B3LYP/6-311+G** level followed by calculations of the specific rotation with B3LYP/aug-cc-pVDZ and, when practical, also with B3LYP/ aug-cc-pVTZ. In some cases chiral samples were prepared so that the calculated specific rotations could be compared with experimental data. With most compounds there was only a minor effect of the basis set on the specific rotations. With the oxiranes and oxetanes, the chloro derivative gave a different behavior than the other substituents, but all substituents behaved in the same fashion with trans-2-methyl-1-Xcyclopropanes. Therefore the unusual behavior of chlorooxirane probably results from an interaction between oxygen and chlorine rather than being due to the presence of a three-membered ring. Chlorine is also an unusual substituent for the tetrahydrofurans. The effect of conformation on the calculated specific rotations was examined with the axial and equatorial 2-substituted tetrahydropyrans, where the anomeric effect is operative with the axial substituent, and also the 3-substituted tetrahydropyrans that would not be subject to the anomeric effect. The unusual effect of chlorine was seen only when it is antiperiplanar with respect to the oxygen.

The Role of Substituents in Optical Rotation of Oxiranes, Oxetanes, and Oxathietanes

“…Vibrational frequency calculations carried out upon optimization confirm that the optimized structures correspond to real minima on the potential energy surface. As we will compare our results to the findings of Wiberg et al, we have carried out numerical tests for four of the systems they investigated, employing the above given approximation for geometrical parameter calculation. Namely, using the DFT­(B3LYP)/6-311++G** level of theory, we have reoptimized the structures reported in ref for fluoro-, chloro-, cyano-, and ethynyl-oxirane, and next we have computed their OR values at the DFT­(B3LYP)/aVTZ level for three wavelengths, that is, λ = 633.0, 589.0, and 355 nm.…”

“…As we will compare our results to the findings of Wiberg et al, we have carried out numerical tests for four of the systems they investigated, employing the above given approximation for geometrical parameter calculation. Namely, using the DFT­(B3LYP)/6-311++G** level of theory, we have reoptimized the structures reported in ref for fluoro-, chloro-, cyano-, and ethynyl-oxirane, and next we have computed their OR values at the DFT­(B3LYP)/aVTZ level for three wavelengths, that is, λ = 633.0, 589.0, and 355 nm. Results of these tests reveal that changes in geometrical parameters due to the different bases used in the geometry optimizations do influence the property; however differences with respect to the results of Wiberg et al are small, and most pronounced in the case of ethynyloxirane, for which they amount to around 3%.…”

“…These results differ from those found by Wiberg et al on substituted oxiranes, where the substituent that provided a different behavior was the chlorine. Since the molecules in ref lack the methylene group we have in 2 , we can conclude that this group is the one responsible of the change in OR behavior.…”

Concerning the conformational preferences of the 2-cyano derivatives of oxane, thiane, and selenane

“…16 The (±)-2-cyanothiane (3) and (±)-2-cyanoselenane (4) were prepared, as outlined in the Scheme 1, by the cyclization of (±)-2,5-dibromohexanoic acid (5) 17,18 with sodium sulfide 17,18 or selenide, followed by purification of the heterocyclic carboxylic acids (6, 7) as their methyl esters (8,9), 17,18 and ammonolysis of these afforded the corresponding amides (10,11), which were then dehydrated to yield the target nitriles (3,4) .…”

Inducing chirality ex nihilo by an electric field

“…The geometries were constrained to enforce the appropriate symmetry, when applicable, for each molecule. The optical rotations were then calculated for these optimized geometries at the B3LYP/aug-cc-pVTZ level recommended by Wiberg et al 6 at various external electric field strengths, using electricfield-dependent gauge including atomic orbital (GIAO) functions and electric-field coupled perturbed Hartree-Fock (CPHF). The same integration grid used for the geometry optimizations was also used for the optical rotations.…”