¹³C Nuclear Magnetic Resonance Studies. 45. The ¹³C Spectra of Some β,γ Unsaturated Polycyclic Ketones

Abstract

The 13 C spectra of several β,γ unsaturated polycyclic ketones have been recorded to examine the shielding differences for all carbons relative to those for their saturated analogs and the parent olefins. The variations of the sp 2 carbon shieldings provide qualitative evidence for the existence of homoconjugative interactions between the olefinic and carbonyl groups in the ground state. Similar evidence of interaction between γ,δ double bonds and carbonyl groups was found from spectra for a few examples. A simple interpretation of the shielding trends entirely in terms of charge density variations arising from through space polarization is shown to be inadequate, thereby emphasizing the existence of other major shielding contributions.