| Autor: |
Francisco CB; Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil., Fernandes CS; Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil., de Melo UZ; Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil., Rittner R; Chemistry Institute, University of Campinas, 6154, Campinas 13083-970, Brazil., Gauze GF; Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil., Basso EA; Chemistry Department, State University of Maringá, 5790, Maringá 87020-900, Brazil. |
| Abstrakt: |
The presence of strong stereoelectronic interactions involving the substituents in cis -2-substituted cyclohexanes may lead to results different from those expected. In this work, we studied the conformational behavior of cis -2-fluoro- ( F ), cis -2-chloro- ( Cl ), cis -2-bromo- ( Br ) and cis -2-iodocyclohexylamine ( I ) by dynamic NMR and theoretical calculations. The experimental data pointed to an equilibrium strongly shifted toward the ea conformer (equatorial amine group and axial halogen), with populations greater than 90% for F , Cl and Br in both dichloromethane- d 2 and methanol- d 4 . Theoretical calculations (M06-2X/6-311++G(2df,2p)) were in agreement with the experimental, with no influence of the solvent or the halogen on the equilibrium. A principal component analysis of natural bond orbital energies pointed to the σ* C-X and σ C-H orbitals and the halogen lone pairs (LP X ) as the most significant for the hyperconjugative interactions that influenced the equilibrium. The σ C-H → σ* C-X hyperconjugation and the interactions involving the LP X counterbalance each other, explaining the non-influence of the halogen on the conformational equilibrium. These interactions are responsible for the strong preference for the ea conformer in cis -2-halocyclohexylamines, being strong enough to restrain the shift in the equilibrium due to other factors such as steric repulsion or solvent effects. |
