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

Computer simulation of conformational transformations of 1,3-dioxanes and their 2-sila and 2-bora analogs

“…The ∆G° value of the latter, determined according to [23][24][25][26] (Table 2), indicates strong prevalence (86-98%) of the HC ax structure in the conformational equilibrium of cyclic ester XLVI. As might be expected, increase in the solvent polarity favors increased fraction of the axial conformer due to its higher dipole moment [97].…”

“…This is confirmed by the fact that the major conformer of nitrocyclohexane is equatorial [99] and that the axial chair conformer predominates for the boron-free analog, 5-nitro-1,3-dioxane [6,98]. Fairly high calculated barrier to the rotation about the C 5 -N bond (4.5 kcal/mol [97]) suggests an appreciable stabilizing effect of the COBOC fragment on the axial nitro group. The ∆G° value of the latter, determined according to [23][24][25][26] (Table 2), indicates strong prevalence (86-98%) of the HC ax structure in the conformational equilibrium of cyclic ester XLVI.…”

“…The ∆G° values for most alkyl substituents in 1,3,2-dioxaborinanes XXXVII-XLV (determined according to [23][24][25][26]) are close to those observed for similarly substituted boronfree analogs, 1,3-dioxanes [6]. The PES of 5-nitro derivative XLVI also contains three stationary points: half-chair (HC ax ) with the axial nitro group (global minimum according to most approximations used), sofa (S eq ), and transition state 2,5-T [97] (Scheme 20).…”

“…Parameters of conformational equilibria of 2,5-disubstituted 1,3,2-dioxaborinanes XXXVII-XLVI[96,97] …”