Conformational analysis of 1,3-oxathiane

Abstract

Computer simulation of pathways of conformational isomerization of 1,3-oxathiane molecule carried out with the help of HF/6-31G(d), MP2.6-31G(d)/HF/6-31G(d), and PNE/3z quantum-chemical approximations showed that interconversion between the degenerate in energy chair conformers proceeds through seven independent pathways: directly and via six flexible forms. Potential energy surface contains eight minimum points including chair conformers and enantiomeric pairs of twist forms, and also five transition states, among them different modification of semi-chair, symmetric and unmmetrical boat. Molecular dynamics methods show that flexible forms at room temperature convert into one another and into the chair conformers.1,3-Oxathianes attract attention as valuable reagents used in fine organic synthesis [1][2][3][4][5]. Besides, these substances from the outset of stereochemical studies of saturated six-membered heterocycles were regaeded as convenient models for invest-tigation of the effect of heteroatom on the changes in the conformational characteristics of cyclohexane heteroanalogs [6][7][8]. According to NMR data the chair conformation is preferred for the majority of 1,3-oxathianes [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. With the help of non-empiric quantum-chemical approximations the evaluation of stereoelectronic interactions was obtained and the presence of homoanomeric effects in the 1,3-oxathiane ring was established [20,21]. But unlike cyclohexane [22], 1,3-dioxanes [22][23][24][25][26][27][28][29][30][31][32], and 1,3-dithiane [33, 34], pathways of conformational isomerization of 1,3-oxathiane molecules were not considered. In the majority of cases "chair-chair" inversion was postulated [6,[9][10][11][12][13], and in some cases the participation of flexible forms was suggested [14][15][16]18]. In [35,36] with the help of empiric and semiempiric methods the relative stability of chair conformers and twist-forms of unsubstituted as well as 5-and 2,2,5-substituted 1,3-oxathianes have been studied. At the same time it is known that the potential energy surfaces of substituted 1,3-heterocycles with different heteroatoms in the ring, for example, tetrahydro-1,3-oxazines, have a fairly large number of stationary points [37]. Recently it was shown by computer simulation that for 1,3-oxathianes the unusual direct chair-chair inversion takes place avoiding the intermediate flexible forms [38,39]. Such direction of conformational isomerization is absent in the series of 1,3-dioxanes and 1,3-dithianes [24-34], but it is possible for 1,3-dioxa-2-sila-[40] and 2-germacyclohexanes [41]. In this study carried out by means of nonempiric HF/6-31G(d), MP2/6-31G(d)/HF/6-31G(d) approximations and density functional PBE/3z methods within the frame of HyperChem [42] and PRIRODA [43] complexes of programs all possible conforma-tional transformations of unsubstituted 1,3-oxathiane I were revealed.