Stereoelectronic interactions: A booster for 4JHF transmission

• Published in: Magnetic resonance in chemistry 2022-05, Vol.60 (5), p.481-488
• Main Authors: Soares, João Vitor, Dal Poggetto, Guilherme, Viesser, Renan V., Couto, Uenifer R., Tormena, Cláudio F.
• Format: Article
• Language: English
• Subjects: Carbonyl groups; Carbonyls; Coupling (molecular); coupling constant; Fluorine; Molecular structure; stereoelectronic interaction; transmission pathway
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.5248

Long‐range proton‐fluorine coupling constants (nJHF) are helpful for the structure elucidation of fluorinated molecules. However, their magnitude and sign can change with the relative position of coupled nuclei and the presence of substituents. Here, trans‐4‐tert‐butyl‐2‐fluorocyclohexanone was used as a model compound for the study of the transmission of 4JHF. In this compound, the 4JH6axF was measured to be +5.1 Hz, which is five times larger than the remaining 4JHF in the same molecule (4JH4F = +1.0 Hz and 4JH6eqF = +1.0 Hz). Through a combination of experimental data, natural bond orbital (NBO) and natural J‐coupling (NJC) analyses, we observed that stereoelectronic interactions involving the π system of the carbonyl group are involved in the transmission pathway for the 4JH6axF. Interactions containing the π system as an electron acceptor (e.g., σC6H6ax → π*C═O and σCF → π*C═O) increase the value of the 4JH6axF, while the interaction of the π system as an electron donor (e.g., πC═O → σ*CF) decreases it. Additionally, the carbonyl group was shown not to be part of the transmission pathway of the diequatorial 4JH6eqF coupling in cis‐4‐tert‐butyl‐2‐fluorocyclohexanone, revealing that there is a crucial symmetry requirement that must be fulfilled for the π system to influence the value of the 4JHF in these systems. The highlighted interactions σC─H → π*C═O and σC─F → π*C═O are shown to cause an increase in the value of the 4JHF, thus serving as a booster for the transmission of coupling constants.