13C-NMR spectra have been examined to obtain the structural informations on a series of alkyl-substituted β-diketones and we found correlation between chemical shift and substituent.
Structual formulas of β-diketones employed in our experiment are listed in Table 1 and 13C chemical shifts are given in Table 2.
Since most of those β-diketones are enolised at equilibrium state, satisfactory spectrum of the keto form could not be obtained except three compounds. Therefore, chemical shifts of enol carbons are discussed mainly here.
(1)The existence. of two forms of enol([1-A]and[1-B])is supposed to B-diketone, but we cannot observe their spectra. This fact suggests the rapid evchange between these two forms.
(2) the shifts of the enol carbons (C-2, C-3) decrease monotonously with the increased inductive (-I) effect of alkyl substituents. That is, the enol carbons of a molecule with two methyl groups resonate at the highest field, and those with two butyl groups, resonate at low field. The similar effect which appears in the mono ketone has been reported by several workers.
(3) The signal of the enolic methine carbon (C-1) appears in 90-lO2 ppM region. lt lies in -extraordinary high field compared with those of mono olefinic or aromatic carbons. ln chelate ring of enol form, lone-pair electrons on the oxygen atom could take part in rr-conjugation. This effect causes the increase of π-electron density of methine carbon to make them more shielded.
(4) Furthermore, an additivity is found in the observed 13C shifts of m.ethine carbon for each alkyl substituent. Chemical shifts of asymmetric β-diketones calculated from the value of symmetric β-diketone are in good agreement with the experiinental values as shown in Table 3.