Conformational analysis of 2-substituted 5,5-bis(chloromethyl)-1,3-dioxanes

Abstract

Quantum-chemical study on stationary points on the potential energy surfaces of 2-alkyl-, 2-phenyl-, and 2-methyl-2-phenyl-5,5-bis(chloromethyl)-1,3-dioxanes in terms of HF/6-31G(d), B3PW91/6-31G(d,p), and B3PW91/LanL2Dz nonempirical approximations revealed only one pathway for conformational isomerization of the equatorial and axial chair conformers through intermediate energy minimum corresponding to the 2,5-twist conformer. I, R = H, R′ = Me; II, R = H, R′ = i-Bu; III, R = H, R′ = Ph; IV, R = Me, R′ = Ph.Interest in structural studies on 1,3-dioxanes is determined by specificity of their structure and by the use of these compounds as reagents in fine organic synthesis [1-3]. 5,5-Bis(chloromethyl)-1,3-dioxanes occupy a specific place among cyclic acetals. The chlorine atoms therein are relatively readily replaced by iodine with formation of mono-and diiodo derivatives; this process is stereoselective, and chlorine atom is replaced preferentially in the equatorial chloromethyl substituent [4]. Conformational behavior of 5,5-bis(halomethyl)-1,3-dioxanes having no substituent on C 2 at room temperature is characterized by fast (on the NMR time scale) ring inversion [5]. By contrast, molecules of 2-substituted analogs exist mainly as chair conformers with equatorial orientation of the substituent on C 2 (C eq ) [6, 7]. According to the X-ray diffraction data, 5,5-bis(chloromethyl)-2-phenyl-1,3-dioxane in crystal has chair structure with the phenyl group occupying equatorial position [6]. In the recent years, computer simulation of conformational transformations of various molecular systems, in particular of cyclic acetals, attracts increasing attention [8]. In the present work we examined conformational isomerization pathways of 2-methyl-, 2-isobutyl-, 2-phenyl-, and 2-methyl-2-phenyl-5,5-bis(chloromethyl)-1,3-dioxanes I-IV in terms of HF/6-31G(d), B3PW91/6-31G(d,p), and B3PW91/LanL2Dz quantum-chemical approximations using GAMESS software package [9].In the first step we estimated the relative energies of conformers differing by mutual orientation of chlorine atoms in the chloromethyl substituents. For this purpose, the energies of four possible chair conformers A-D of compounds I-III with equatorial orientation of the substituent on C 2 were calculated at the B3PW91/6-31G(d,p) level of theory. In keeping with the X-ray diffraction data [6], the most stable is conformer A with gauche orientation of the chloromethyl groups (Table 1). This structure occupies the global

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

“…In particular, the chair-chair inversion of 5,5-bis(chloromethyl)-1,3-dioxane (XIII) is mediated by only one local minimum (2,5-T) and involves only one transition state (halfchair conformation) [40]. The interconversion of 2,2-dimethyl- [41], 2-alkyl-5,5-bis(chloromethyl)-, and 2,2-disubstituted 5,5-bis(chloromethyl)-1,3-dioxanes [42] follows a similar pattern. The character of conformational transformations of 4,4-dimethyl-1,3-dioxane (XIV) is more complex due to ring asymmetry, and it is rather close to 4-substituted analogs: the chair-chair interconversion involves a family of flexible conformers (1,4-T, 2,5-T, and 3,6-T) [43].…”