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

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. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1860–1865, December, 2007.     

Conformational analysis of 2-methyl-1,3,2-dioxaborinane in the presence of a water molecule

Russian Journal of Organic Chemistry - By means of nonempirical quantum-chemical approximations RHF//STO-3G, 3-21G, and 6-31G(d) conformational characteristics were studied of molecular associate...     

Conformational analysis of the complex of 2-methyl-1,3,2-dioxaborinane with hydroxyl anion

Studies of the conformationsl transformations of the complex of 2-methyl-1,3,2-dioxaborinane with the hydroxyl anion performed by the quantum-chemical method HF/6-31+G(d) showed that they are more versatile than those in the initial boric ester, include the existence of six minima, five transition states, and are close to the conformational behavior of 1,3-dioxanes.     

Conformational analysis of 2,4-dialkyl-1,3,2-dioxaborinanes

Computer modeling using the quantum-chemical empirical MM+ method and nonempirical RHF//6-31G(d), MP2//6-31G(d), and, in individual cases, RHF//3-21G methods was employed to study the potential energy surface of 2,4-dialkyl-1,3,2-dioxaborinanes. The optimal geometry and ¹H NMR spectral data gave values for ∆G ⁰ of the methyl and hexyl substituents at C-4 of the heterocyclic ring equal to 0.6 and 1.6 kcal/mol, respectively.     

Nonempirical approach to the conformational analysis of 2,4-dimethyl-1,3,2-dioxaborinane and its oxonium ions

Using nonempirical quantum-chemical approximations RHF//STO-3G, 3-21G, 6-31G(d) and MP2//6-31G(d,p) a conformation isomerism of 2,4-dimethyl-1,3,2-dioxaborinane and its oxonium ions was studied. It was shown that the potential energy surface of the studied molecules has minima corresponding to equatorial (main minimum) and axial sofa forms and maxima corresponding to equatorial and axial conformations of 2,5-twist forms. Calculated values of the barriers of internal rotation of methyl group at the ring C⁴ atom were found. It was also established that the heat of protonation of the cyclic boric ester was smaller than of the non-boric analog, cis-2,4-dimethyl-1,3-dioxane, owing to decrease in basicity of the oxygen atoms in the cyclic boric ester due to partial double bond character of B-O bond.     

Conformational analysis of 2-methyl-5-nitro-1,3,2-dioxaborinane

Quantum-chemical methods HF/6-31G(d), HF/6-31+G(d), MP2/6-31G(d)//HF/6-31G(d), and MP2/6-31+G(d)//HF/6-31+G(d) were used to investigate the conformational isomerization of 2-methyl-5-nitro-1,3,2-dioxaborinane. It has been shown that a potential energy surface of this compound includes two minima: an axial form of semi-chair and equatorial sofa together with a transition state belonging to the conformation of 2,5-twist-form. A comparison between experimental NMR ¹H and theoretical vicinal coupling constants was used to determine the quantitative conformational composition of cyclic boric acid ester and a value of ΔG ⁰ for nitro group at the ring carbon atom C⁵ in CCl₄ and C₆D₅NO₂ solutions.     

Quantum-chemical investigation of conformational characteristics of the complex of 2-methyl-1,3,2-dioxaborinane with dimethyl ether

With the help of the quantum-chemical approximation MP2/6-31G(d)//HF/6-31G(d) conformational characteristics were investigated of donor-acceptor associate of 2-methyl-1,3,2-dioxaborinane with dimethyl ether, 1:1. The character of the conformational isomerization of the adduct differs from that established for the proper cyclic boric ester and involves an interconversion between two conformers nondegenerate by the energy. The stability of the associate under study is comparable with those of the molecular complexes of 2-methyl-1,3,2-dioxaborinane with water and methanol.