Substituent Effects in the Bicyclooctane Ring System. A Carbon-13 and Fluorine-19 Nuclear Magnetic Resonance Study of 4-Substituted Bicyclooct-1-yl Fluorides

Article abstract of DOI:10.1021/jo00136a029

19F and 13C NMR spectra have been recorded for a large number of 4-substituted bicyclo<2.2.2>oct-1-yl fluorides (1) in which the substituents cover a wide range of electronic effects.Correlations of the 19F substituent chemical shifts (SCS) for several solvents with substituent constants (?_I, ι, and ?_R⁰) indicate that these NMR substituent-probe parameters are predominantly manifestations of electric field and electronegativity effects.Moreover, it is also revealed that whereas the former effects are solvent dependent, the latter are essentially independentof the nature of the medium.In addition, the statistical analysis indicates that resonance effects involving orbitals of ? symmetry appear not to be transmitted through the bicyclo<2.2.2>octyl skeletal framework.Compelling support for the validity of the overall statistical dissection is provided by an independent measure of the polar susceptibility parameters (ρ_I values) for each solvent.The coefficient (A) of the Buckingham equation for linear electric field effects on C_sp3-F bonds is calculated for c-C6H12 as the solvent.A good correlation of solvent induced changes (CDCl3 to CF3CO2H) in the 19F SCS vs. similar differential changes in the corresponding 13C SCS for C1-F confirms the solvent independence of the electronegativity effect.The origin of this latter effect is considered, and some of the possibilities are probed by correlative analysis of the substituent-induced changes in the one-bond carbon-fluorine spin-spin coupling constants (Δ¹J_CF).The polar effects of alkyl groups are alluded to in the light of the new results for system 1.