¹³C Nuclear magnetic resonance studies. XIII. ¹³C Carbonyl carbon shieldings of some β,γ-unsaturated ketones

Abstract

The 15.1 MHz I3C nuclear magnetic resonance spectra of 13 aliphatic and alicyclic ketones having an olefinic bond in the D, y position with respect to the carbonyl group have been recorded and the carbonyl shieldings measured. The major aim of this study was to investigate the possibility of detecting homoconjugative interactions between the n-systems by comparing the carbonyl shieldings with those for thc corresponding saturated and a,D-unsaturated systems. Some evidence of homoconjugation is presented on the basis of our previous interpretation of the factors governing carbonyl shieldings. A qualitative assessment of the observed geometrical dependence of the effect is offered.

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

“…relative to 12. Although the marked effect of the P,y olefinic bond in 5 was well known (1,17) it seemed somewhat surprising that 21 exhibited a shift of -9.0 p.p.m. since ketones 9, 16, and 23 show shifts near -4 p.p.m.…”

“…Infrared and proton spectra were determined for each to ensure their identities. Compounds 1-10, 23, and 24 were available from earlier studies (1,3,4) (6). Reduction of the latter with Na-NH, gave the unsaturated alcohol which was oxidized using Cr0, in CH,CI,-pyridine (7) to 1 3 (8) and subsequently reduced catalytically to 12; compound 12 was also obtained by catalytic hydrogenation of the unsaturated alcohol over PtO, in ether followed by CrO, oxidation (7).…”

“…Some years ago, the carbonyl carbon shieldings for a number of P,y unsaturated bicyclic ketones were measured to investigate the possibility of detecting homoconjugative interactions between olefinic and carbonyl n systems (1). At that time instrumental limitations precluded definitive detection and assignments for the remaining carbon signals but the carbonyl data indicated an interaction between the P,y double bond and carbonyl group since the carbonyl carbons were shielded relative to their absorption positions for the saturated analogs.…”