Group IVb Metalloidal and Mercury Substituent Effects in the Naphthalene System by Fluorine-19 Nuclear Magnetic Resonance

A number of substituted α (1)- and β (2)-fluoronaphthalenes with metallo substituents of the type MR (when M is a group IVb metalloid; R = methyl) and HgX (X = halogen or aryl) in the 4, 6, and 7 positions have been synthesized and their fluorine nmr spectra have been measured. The 19F substituent chemical shift data (the extent to which a substituent electronically perturbs the C–F neighborhood is indicated by the substituent chemical shift (SCS), which is defined as the difference between the chemical shift of the unsubstituted fluoroaromatic and the substituted fluoroaromatic) provide experimental evidence for d–p and 6p–p conjugative electron withdrawal (–M) in the ground state by the metalloidal and mercuri substituents, respectively. Comparisons with SCS data from metallo-substituted fluorobenzenes and fluorobiphenyls suggest that this conjugating ability depends on the energy and size of the outer lying orbital with π symmetry. This supports previous conclusions concerning the electronic behavior of metalloidal substituents as well as providing additional insight into the nature of aryl-carbon-mercury bonds. Further, the data from the 7β disposition suggest that certain approaches to the analysis of 19F SCS may be invalid. SCS information from two dicyclopentadienyltitanium derivatives indicates that hyperconjugation is probably an unnecessary concept to invoke for ground-state alkyl substituent effects