Stereoelectronic interactions: A booster for 4 J HF transmission

Long-range proton-fluorine coupling constants ( J ) 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 J . In this compound, the J was measured to be +5.1 Hz, which is five times larger than the remaining J in the same molecule ( J = +1.0 Hz and J = +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 J . Interactions containing the π system as an electron acceptor (e.g., σ → π and σ → π ) increase the value of the J , while the interaction of the π system as an electron donor (e.g., π → σ ) decreases it. Additionally, the carbonyl group was shown not to be part of the transmission pathway of the diequatorial J 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 J in these systems.