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-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.     

Molecular structure and conformational preference of 2-methyl-5-nitro-5-bromo-1,3,2-dioxaborinane and its complex with pyridine

The molecular structure of 2-methyl-5-nitro-5-bromo-1,3,2-dioxaborinane and its complex with pyridine is determined by single crystal X-ray diffraction. The structure of the synthesized compounds in the solution is also confirmed by the NMR spectroscopy data, and the conformational preference of their molecules is confirmed by the calculations using hybrid density functional theory.     

Structure and reactions of benzo-4-diphenylphosphino-2-phenyl-1,3,2-dioxaborinane

The crystal structure of 5,6-benzo-4-diphenylphosphino-2-phenyl-1,3,2-dioxaborinane 1 indicates the occurrence of an interaction between the π-systems of the 1,3,2-dioxaborinane ring and the appropriate phenyl group of the diphenylphosphino group. Complexes of compound 1 with PtCl₂, PdCl₂, and Cul were synthesized. Compound 1 and its complexes are readily hydrolyzed in solution to give diphenylphosphine and its corresponding complexes (NMR).     

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.     

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.     

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.     

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 transformations and autooxidation of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane

Conformational study of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane at the DFT PBE/3ξ level of theory revealed the only sofa–sofa interconversion pathway through a transition state corresponding to 2,5-twist conformer. The barrier to internal rotation of the axial nitro group is several times higher than that for the equatorial nitro group. According to the ¹H, ¹³C, and ¹¹B NMR, IR, and X-ray diffraction data, the main autooxidation products of 5-bromo-2-(2-methylpropyl)-5-nitro-1,3,2-dioxaborinane are 2-bromo-2-nitropropane-1,3-diol and boric acid.     

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.     

Autooxidation of 2-isobutyl-5,6-benzo-1,3,2-dioxaborepane

Using PMR, IR, and mass spectrometric methods it was shown that the main autooxidation product upon prolonged storage of 2-isobutyl-5,6-benzo-1,3,2-dioxaborepane is 2,2′-oxybis(5,6-benzo-1,3,2-doxaborepane). A. V. Bogatskii Physicochemical Institute, National Academy of Sciences of Ukraine, Odessa 270080. Organic Chemistry Institute, Ufa Science Center, Russian Academy of Sciences, Ufa 450054. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1418–1421, October, 1998.