Stereoelectronic interactions: A booster for 4 J HF transmission.

Long-range proton-fluorine coupling constants ( n J HF ) 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 4 J HF . In this compound, the 4 J H6axF was measured to be +5.1 Hz, which is five times larger than the remaining 4 J HF in the same molecule ( 4 J H4F  = +1.0 Hz and 4 J H6eqF  = +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 4 J H6axF . Interactions containing the π system as an electron acceptor (e.g., σ C6H6ax  → π * C═O and σ CF  → π * C═O ) increase the value of the 4 J H6axF , while the interaction of the π system as an electron donor (e.g., π C═O  → σ * CF ) decreases it. Additionally, the carbonyl group was shown not to be part of the transmission pathway of the diequatorial 4 J H6eqF 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 4 J HF in these systems.