Conformação e efeitos estereoeletrônicos sobre as constantes de acoplamento 1 J C-F em compostos heterocíclicos fluorados

Abstract

Halogenated methanol derivatives (halo = F, Cl and Br) were theoretically studied in the gas phase and implicit/explicit aqueous solution, taking into account the anti and gauche orientation obtained by rotating the X-C-O-H dihedral angle. It was found that the gauche coformation is more stable because of the hyperconjugative interaction nO*C-X (generalized anomeric effect). In addition, an assessment of the dependence of the 1JC,F coupling constant with the molecular dipole moment and hyperconjugation effects was performed, making an allusion to the effects that govern the known Perlin effect, . The Perlin effect refers to the fact that 1JC,Hax<1JC,Heq in six-membered rings - cyclohexanes or pyranoses - and may be useful to define the stereochemistry of, for example, carbohydrates. Thus, evaluation of the effects operating on the 1JC,F coupling constant in tetra-acetylated - e -D-fluoro-D-glucose in polar and nonpolar solution, using experimental (NMR) and theoretical techniques, revealed that the so-called "reverse fluorine Perlin-like effect" is correlated with dipole effects, and not hyperconjugation, as well as in 2-fluoro-1,3-dioxane and 2-fluoro-1,3-dithiane. However, according to a detailed analysis based on CMO's (Canonical molecular orbitals), it is suggested that the reverse fluorine Perlin-like effect in 2-fluoropiperidine and compounds containing the C-X-CHF-X fragment (where X = O and S) should be interpreted in terms of a spatial compression related to the heteroatom electron lone pairs on the fluorine atom, resulting in a shortening of the C-F bond. This interaction is proportional to dipolar effects, making them difficult to distinguish. In 2, 3 and 4-monofluorinated piperidines and respective ionic species, inductive, steric, electrostatic and hyperconjugative effects are responsible for the behavior of 1JC,F.