Computer modelling of the conformational equilibrium of 2-and 2,5,5-substituted 1,3,2-dioxaborinanes

Abstract

Empirical, semiempirical, and nonempirical quantum-chemical methods were used to study the conformational equilibrium of 2,5,5-substituted 1,3,2-dioxaborinanes. The sofa invertomers were found to correspond to the local and global minima on the potential energy surface. The position of the equilibrium between these forms is a function of the substituents at C (5) of the heterocycle.Interest in cyclic borates, namely, 1,3-dioxa-2-boracycloalkanes, is linked to the increasing importance of these compounds in the fine organic synthesis -production of enantiomeric alcohols and polyenes. These borates have a series of useful practical properties -they are biologically active compounds, corrosion inhibitors, components of polymers fuel and lubrications products. These compounds have interesting structural features involving electronic and steric intramolecular interactions [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. These interactions are largely attributed to the presence of an electron-deficient boron atom and electron-donor oxygen atoms in a single molecule [4,10,11,13].It is know that molecules of 2-alkyl-or 2-alkoxy-5,5-dimethyl-1,3,2-dioxaborinanes exist at room temperature with a relatively low barrier to ring inversion, which is rapid on the NMR time scale [1,4,[15][16][17][18]. Early the determinant effect of the substituents at C (5) atom in the ring on the conformational equilibrium of the molecules of 2,5,5-substituted 1,3,2-dioxaborinanes has been shown by 1 H NMR spectroscopy [1,4,18].The potential energy surfaces (PES) of molecules unsubstituted 1,3,2-dioxaborinane (1) as well as 2-and 2,5,5-substituted analogs 2-28 were calculated with the aim of further understanding the effect of structural factors on this equilibrium using empirical (MM+), semiempirical (AM1), and nonempirical methods [RHF/STO-3G, 3-21G, 6-31G, 6-31G(d), 6-31G(d,p)] using the Hyperchem computer program package [19].The PES of ester 1 and 2-substituted 1,3,2-dioxaborinanes 2 and 3 molecules have a single minimum corresponding to the sofa conformer (C and energy-degenerate form C * ) and also a single maximum corresponding to the 2,5-twist form (2,5-T). The values of ∆E ≠ of this process found in an MM+ calculation are independent of the nature and conformational bulk of the substituent at the boron atom (H, OMe, i-Pr, Table 1). Similar results were obtained in our previous work for 2-phenyl-1,3,2-dioxaborinane [20]. The experimental determination using low-temperature 1 H NMR spectroscopy of the value of ∆G ≠ for ester 3 gave only the upper limit (<9.0 kcal/mole) due to the relatively low coalescence temperature [4].

Routes of conformational isomerization of 4-substituted 1,3,2-dioxaborinanes

“…For the maxima the form 2,5-Ta is also more stable than 2,5-Te. However in all cases the character of the substituent R influences the calculated height of the potential barrier (∆Е ≠ ) relatively weakly which is in good agreement with the experimental data [15][16][17]. Thus a detailed analysis of the conformational conversions of 4-substituted 1,3,2-dioxaborinanes indicates two different routes for the transition Ce ↔ Ca; their relative contributions are determined by the steric and electronic characteristics of the substituents at the atom C-4 of the heterocyclic ring.…”

Theoretical barriers to the internal rotation of a nitro group in 2-methyl-5-nitro-1,3,2-dioxaborinane

“…It was shown previously that molecules of 5-nitro-1,3,2-dioxaborinanes continue in the preferred conformation with an axial nitro group [2][3][4][5]. In the present work the barriers to internal rotation of the nitro group in axial and equatorial conformers of the molecule of 2-methyl-5-nitro-1,3,2-dioxaborinane have been investigated for the first time.…”

“…Interest in 1,3,2-dioxaborinane, linked with the special features of the structure, chemical behavior, and a complex of practically useful properties [1], makes the study of the potential energy surface (PES) of molecules of these compounds by computer modeling methods urgent [2,3]. It was shown previously that molecules of 5-nitro-1,3,2-dioxaborinanes continue in the preferred conformation with an axial nitro group [2][3][4][5].…”

Unusual conformational isomerization of 2,2-dimethyl-1,3-dioxa-2-germacyclohexane

“…Besides the case of 1,3-dioxa-2-silacyclohexanes [4], such a single-barrier transition to an alternative form has been found previously only for cyclohexane heteroanalogs with an sp 2 atom at the second ring position, in particular, for 2-oxo-1,3-dioxanes [8] and substituted 1,3,2-dioxaborinanes [9,10]. _______ * To whom correspondence should be addressed, e-mail: kuzmaggy@mail.ru.…”