Comparative conformational analysis of 1,3-dioxane and 1,3-dithiane

Abstract

It was shown by ab initio quantum-chemical approximations HF/6-31G(d) and MP2/6-31G(d)// HF/6-31G(d) that the conformational isomerization of 1,3-dioxane and 1,3-dithiane proceeded along common routes. The potential energy surface of both compounds contains six minima including the chair invertomers and enantiomeric fl exible forms. They are separated by several potential barriers. It was established by molecular dynamics method that the fl exible conformers at heating and keeping at 295-300 K transformed into each other and in the chair conformer.The interest to the structural investigation of 1,3-dioxanes and 1,3-dithianes originates from the features of their structure, their application in the industrial and fi ne organic synthesis, and also their promising physiologically active properties [1][2][3][4][5][6][7][8][9]. In particular the great possibilities should be mentioned of the industrial application of cyclic acetals in the production of isoprene, 1,3-diols, and a number of drugs [1], and also in the synthesis of other six-membered 1,3,2-heterocyclic compounds [8], aminoalcohols and their derivatives [9]. The microwave spectroscopic studies established the geometrical characteristics of the most stable conformers of 2-methyl-[10], 4-methyl- [11], and 5-methyl-1,3-dioxanes [12]. The recent stereochemical studies actively utilized the quantum-chemical methods. Thus in the framework of nonempirical approximations the minima on the potential energy suface (PES) were thoroughly studied for 1,3-dioxane [13] and 1,3-dithiane [14]. In the latter study also some transition states were investigated on the route of the conformational isomerization chairtwist and twist-twist. However up till now the general picture of the conformational transformations of the studied heterocyclic systems is absent in the literature. We formerly by the computer simulation established the main routes of the conformational isomerization both of the proper 1,3-dioxane [15] and its 2-methyl [16], 4-methyl [17], 4-phenyl [18], 4-trifl uoro methyl [19], 5-methyl [20], 5-isopropyl [21], and 2,5-dimethyl analogs [22]. This study concerns the comparative characteristic of the conformational isomerization of the molecules 1,3-dioxane (I) and 1,3-dithiane (II) by the use of ab initio approximations HF/6-31G(d) and МР2/6-31G(d)// HF/6-31G(d) calculated with HyperChem software [23]. O O I S S II According to 1 Н NMR data the molecules of the studied compounds at the room temperature undergo fast interconversion of the ring [2, 3, 24]. X X X X C inv CWe established that the PES of formals (I) and (II) contained six minima: invertomers of the chair and of fl exible forms. The routes of the conformational isomerization chair-chair (C-C inv) are identical in both compounds and suggest the interconversions by four independent routes through the conformers of flexible forms 2,5-twist-(2,5-Т) and 1,4-twist (1,4-Т). The

Computer simulation of conformational transformations of 1,3-dioxanes and their 2-sila and 2-bora analogs

“…1,4-T 2,5-T 1,4-T 2,5-T 1,4-T C Under these temperature conditions, the chair conformer taken as initial structure undergoes only slight distortion and is not transformed into a flexible conformer [13]. The calculated geometric parameters of the chair conformer of I are very consistent with the data of microwave spectroscopy [15].…”

“…The ring inversion chair-chair (C C inv ) implies four independent paths through flexible conformers, 2,5-twist (2,5-T) and 1,4-twist (1,4-T) (Scheme 1). Each twist conformer occurs in equilibrium with a similar one, and their number is doubled due to formation of energy-degenerate enantiomeric conformers [13] (Scheme 2). The relative energies of local minima indicate some preference of the path involving 2,5-T.…”

“…According to the 1 H NMR data, molecules I at room temperature undergo rapid ring inversion [6,14]. As shown in [11][12][13], the PES of I includes six minima occupied by chair invertomers and flexible structures. The ring inversion chair-chair (C C inv ) implies four independent paths through flexible conformers, 2,5-twist (2,5-T) and 1,4-twist (1,4-T) (Scheme 1).…”

“…However, up to the beginning of new millennium detailed pattern of conformational transformations of 1,3-dioxane (I) remained unclear. It was presented for the first time in [11] and was supplemented by the results of computer simulation [12,13]. According to the 1 H NMR data, molecules I at room temperature undergo rapid ring inversion [6,14].…”