Molecular orbital study of the conformational energies (−ΔG° orA values) of 2-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes)

Abstract

Ab initio 6-31G* and MP2/6-31G*// 6-31G* methods and density functional (pBPDN**) theory were used to calculate the geometries and relative energies of the chair, boat, and twist-boat conformations of tetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes). The chair conformation of thiacyclohexane is 5.3 and 8.0 kcal mol À1, respectively, (1 kcal = 4.184 kJ) more stable than the twist-boat and boat structures at the MP2/6-31G*// 6-31G* level. Ab initio methods were used to calculate the relative energies of the rotamers in the chair conformations of 2-alkylthiacyclohexanes (CH 3 , C 2 H 5 , i-C 3 H 7 , t-C 4 H 9 , neo-C 5 H 11 , SiMe 3 ). The MP2/6-31G*// 6-31G* conformational energies (ÀDG°or A values, kcal mol À1 ) of the 2-alkylthiacyclohexanes (Me = 1.00; Et = 1.16; i-Pr = 1.02; t-Bu = 3.56; neo-Pent = 1.04; SiMe 3 = 2.05) are smaller than those calculated for the corresponding alkylcyclohexanes and 4-alkylthiacyclohexanes. Plots of the calculated conformational energies (ÀDG°) for the 2-alkylthiacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope = 0.680 and r = 0.983 for 6-31G* and slope = 0.667 and r = 0.989 for MP2/6-31G*// 6-31G*). The C(2)-S(1) bond lengths are in the range 1.827-1.840 Å and the C(6)-S(1) bond lengths are in the range 1.815-1.819 Å . With the exception of the axial 2-t-Bu substituent (103.0°), the C-S-C angles vary from 98.6°to 100.4°. The S(1)-C(2)-C(7) angle in the most stable axial conformer is larger that the corresponding angle in its most stable equatorial conformer.

Silacyclohexanes, Sila(hetero)cyclohexanes and Related Compounds: Structure and Conformational Analysis

“…For example, while the value of A Me is 1.76 kcal/mol (Table 1), the measured ratio of 2-Me eq :2-Me ax conformers in the molecule of 2,3,3-trimethyl-3-silathiane is 60:40, that corresponds to ∆G o = 0.35 kcal/mol, coinciding with the calculated value of 0.36 kcal/mol [41]. The analogues of the latter compound with 2-Me 3 Si [41] and 2-Ph substituents [42] exist as single 2-eq conformers due to the large conformational energy of the Ph (2.87 kcal/mol, Table 1) and SiMe 3 group (experimental 2.5 kcal/mol, [43], theoretical 2.05 kcal/mol [44].…”

Conformational Energies in Organic Six-Membered Cyclic Sulfoxides

“…Yavari et al synthons in organic synthesis. 2,3 The preference by most substituents to occupy the equatorial positions on chair conformations of monosubstituted cyclohexanes, 3,4 oxanes, 3,5 and thianes 3,6 is occationally reversed in certain substituted heterocyclic systems. Examples include stereoelectronic interactions, such as the anomeric effect and hyperconjugative orbital interactions, where an electron-withdrawing (electronegative) substituent at C2 in a heterocycle preferes the axial position.…”

A Comparison of the ab Initio Calculated and Experimental Conformational Energies of Alkylcyclohexanes

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The conformations of cyclohexane (1-11) and heterocycles (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) are important in the teaching of stereochemistry and are of active mechanistic, pharmacological, synthetic, and theoretical interest. Computational chemistry can play a central role in delineating the structures of conformers and rotamers of molecules.

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