PMR study of the conformational behavior of 2,5,5-trisubstituted 1,3,2-dioxaborinanes

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

“…The singlet nature of the signal of the ring methylene protons in the 1 H NMR spectra at room temperature with half-width (∆ν) not exceeding 0.01-0.03 ppm in most cases indicates ring inversion rapid on the NMR time scale. Decreasing the temperature leads to broadening of this signal but an experimental activation free energy could be determined only for esters 4, 5 [15] and 10 [1, 18] due to a low coalescence temperature.…”

“…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].…”

“…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 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]. All the 2,5,5-substituted borates studied 4-28 may be divided into three groups relative to their conformational behavior clearly reflected by the 1 H NMR signal of the ring methylene protons [4,18]. Group I compounds have identical substituents at C (5) (esters 4-17).…”

“…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].…”