13C N.M.R. STUDIES: PART VI. CARBON-13 SPECTRA OF α,β-UNSATURATED CARBONYL COMPOUNDS

The 15.1 Mc/s 13C n.m.r. spectra of 34 α,β.-unsaturated carbonyl compounds have been obtained and the chemical shifts of the olefinic and carbonyl carbon nuclei are reported. Examples were drawn from the acyclic and five- and six-membered ring alicyclic series. A few aldehydes are included although most of the compounds are ketones. In the case of compounds assumed to be planar, variations of the olefinic shieldings with alkyl substitution parallel those observed previously for olefinic hydrocarbons. The key factor governing the chemical shifts of the carbonyl-carbon nuclei appears to be the extent of conjugation in a given system. Consequently, the 13C results provide evidence for steric interference to coplanarity in the hindered systems. This approach is compared with other methods of estimating the extent of steric inhibition of conjugation and its merits are assessed.