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 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.     

Conformational analysis of 2-isopropyl-5-methyl-5-methoxymethyl-1,3,2-dioxaborinane

Study of conformational transformations of 2-isopropyl-5-methyl-5-methoxymethyl-1,3,2-dioxaborinane using DFT approximation PBE/3ζ and the second order perturbation theory method RI-MP2/λ2 revealed beside the interconversion route sofa–sofa through a transition state corresponding to 2,5-twist form a number of local minima due to internal rotation of isopropyl and methoxymethyl substituents in sofa conformers. Over 88% of the molecules of the studied compound are present in a sofa form with the equatorially oriented CH₂OCH₃ group.     

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-methyl-1,3,2-oxaza- and 2-methyl-1,3,2-oxathiaborinane associates with water molecule

The structure and conformational behavior of 1: 1 molecular assotsiates of 2-methyl-1,3,2-oxazaand 2-methyl-1,3,2-oxathiaborinanes with water were studied ab initio in terms of HF/6-31G(d) and PBE/3z quantum-chemical approximations. The most stable complexes are formed via hydrogen bonding with the heteroatoms (oxygen and nitrogen or sulfur). Their conformational behavior implies equilibrium between sofa and half-chair conformers, and intermolecular hydrogen bond is retained in the course of ring interconversion. No associate with a dative O→B bond is formed.     

Conformational analysis of 2,4,5-trimethyl-1,3,2-dioxaborinane individual stereoisomers

The study of the conformational isomerization of cis- and trans-isomers of 2,4,5-trimethyl-1,3-dioxaborinane I by means of RHF//STO-3G, 3-21G and 6-31G(d) quantumchemical methods led to the conclusion that its route includes equilibrium between sofa conformers with a different steric orientation of substituents at the C-4 and C-5 ring atoms. These conformers are interconverted through the maxima, the conformations of equatorial and axial 2,5-twist-forms. A comparison between experimental ¹H NMR and theoretical vicinal spin-spin coupling constants was used to determine the quantitative conformational composition of stereoisomers and a value of ΔG ⁰ for conformational equilibrium.     

Conformational analysis of 2-methyl-1,3,2-dithiaborinane sulfonium ions

Russian Journal of Organic Chemistry -     

Conformational analysis of 4-methyl-2-trimethylsiloxy-1,3,2-dioxaphosphinane

Conformational analysis of 4-methyl-2-trimethylsiloxy-1,3,2-dioxaphosphinane was performed by the dipole moment method and quantum-chemical calculations. The 1,3,2-dioxaphosphinane heteroring was found to adopt a chair conformation with equatorial orientation of the 4-methyl group and axial orientation of the irregular trimethylsiloxy substituent. The conformational equilibrium involves non-eclipsed gauche and trans conformers (the latter prevailing) interconvertible through rotation about the exocyclic P-O bond.     

Conformational composition of the 2,4,6-trimethyl-1,3,2-dioxaborinane stereoisomers

A study of conformational transformations of 2,4,6-trimethyl-1,3,2-dioxaborinane cis- and trans-isomers in non-empirical quantum-chemical HF/6-31G (d) and PBE/3z approximations showed that the transformation routes include the sofa conformers with different orientations of the substituents at the ring atoms C⁴ and C⁶ through the transition states corresponding to diequatorial, axial-equatorial, and dixial conformations of the 2,5-twist form. The calculation and ¹H NMR data show that the conformational equilibrium of cis-isomer is almost entirely shifted to diequatorial sofa, and the trans-form is characterized by the interconversion between the two axial-equatorial sofa conformers.     

Conformational analysis of stereoisomers of 2,4-dimethyl-5-isopropyl-1,3,2-dioxaborinane

The investigation of conformation conversions of cis- and trans-isomers of 2,4-dimethyl-5-isopropyl-1,3,2-dioxaborinane using ab initio quantum-chemical approximation HF/6-31G(d) and the hybrid DFT-method PBE/3z showed that the routes of the transformations presuppose an equilibrium between sofa conformers with the different orientation of substituents at the atoms C⁴ and C⁵ of the ring that convert into each other through transition states corresponding to the equatorial and axial conformations of the 2,5-twist form. Based on experimental data of ¹H NMR spectra and calculated vicinal spin-spin coupling constants the quantitative conformational composition of cis- and trans-isomers was established and also the ΔG^o value of the conformation equilibrium was determined.     

Relative stability of 2-methyl-1,3,2-dioxaborinane complexes with alkoxy anions

Russian Journal of Organic Chemistry -     

Reaction of 2-isobutyl-5-methyl-4-phenyl-1,3,2-dioxaborinane with acetonitrile

Reaction of 2-isobutyl-5-methyl-4-phenyl-1,3,2-dioxaborinane (a mixture ofcis- andtrans-isomers in a 81 : 19 ratio) with acetonitrile yielded 2,5-dimethyl-4-phenyl-5,6-dihydro1, 3-oxazine as a mixture ofcis- andtrans-forms in a 50 : 50 ratio. The possible mechanism of this transformation is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1297–1298, May, 1996.     

Conformational analysis of 2-methyl-5-alkyl- and 5-aryl-1,3,2-dioxaborinanes

Quantum chemical study of conformational isomerization of 2-methyl-5-alkyl- and 5-aryl-1,3,2-dioxaborinanes using RHF/6-31G(d) method led to the conclusion that the equilibrium between equatorial and axial sofa conformers is shifted to the latter form. Based on the experimental and theoretically calculated vicinal coupling constants J _HH the quantitative conformational composition and the values of ΔG ⁰ for substituents at the C⁵ ring atom were established.