Conformational analysis of 5-methyl- and 2,2,5-trimethyl-1,3-dithianes

Abstract

Conformational isomerization of 5-methyl-and 2,2,5-trimethyl-1,3-dithianes was studied using quantum-chemical HF/6-31G(d), HF/pVDZ, and PBE/3z approximations. The potential energy surface of molecules of both compounds is shown to contain the main minimum corresponding to the equatorial chair conformer C-5e. The calculated potential barriers of the conformational isomerization were found. Based on the experimental ( 1 H NMR) and theoretical vicinal coupling constants the ΔG 0 values were determined of the methyl group at the ring C 5 atom. The reasons for the distinctions between the values of this parameter from the ΔG 0 , obtained by energy minimizations of the equatorial and axial chair conformers are discussed.The interest in substituted 1,3-dithianes are due both to the peculiarities of their structure [1][2][3][4][5][6][7][8][9][10][11] and to their use as reagents in fine organic synthesis [12][13][14][15][16][17][18][19]. Earlier using 1 H and 13 C NMR spectroscopy it has been shown that the main minimum on the potential energy surface (PES) of 5-alkyl-1,3-dithianes is the equatorial chair conformer (C e ) [2, 3, 5-9]. Computer simulation applied to the conformational analysis of unsubstituted 1,3-dithiane has revealed the main conformer, the chair (C), and the local minima, 1,4-twist (1,4-T) and 2,5-twist (2,5-T), as well as transition states (TS): the semi-chair, symmetric boat and asymmetric boat conformations [20,21]. In this paper conformational analysis is performed of the molecules of 5-methyl-(I) and 2,2,5-trimethyl-1,3-dithianes (II) using ab initio quantum chemical approximations HF/6-31G(d), HF/pVDZ, and PBE/3z within the HyperChem [22] and PRIRODA [23] software packages.compounds I and II contains five minima: the equatorial chair and axial chair conformers (C e , C a ), as well as flexible conformers, 1,4-T and two enantiomeric forms of 2,5-T. The overall picture of the conformational transitions in this case corresponds to a trigonal bipyramid.Note that all the used approximations show only one of the two possible 1,4-T forms, with pseudoequatorial methyl group at the C 5 atom of the ring. In the course of the geometry optimization and energy minimization the alternative conformer is irreversibly converted into 2,5-T form.Analysis of the calculated parameters of the conformational equilibria of studied 1,3-dithianes (Table 1) shows that the difference in the energy of C e and C a forms (ΔH) is 1.2-1.5 kcal mol -1 , except for the case of HF/pVDZ approximation. Accordingly, the value of ΔG 0 for the C e ↔ C a equilibrium is equal to 1.3-1.4 kcal mol -1 (PBE/3z). However, the calculated value of ΔH for similar conformers of 5-methyl-1,3-dioxane in the framework of the HF/6-31G(d) does not exceed 0.6 kcal mol -1 [25] . The large value of this parameter for compounds I and II is difficult to explain by steric interactions of the axial methyl group at the C 5 atom with the heteroatomic ring fragment, because an increase in the C-S bond length compared with C-O, should, conversely, reduce the ΔH a...