Double Bonds Conjugated with a Carbonyl Group

2 Double Bonds Conjugated with a Carbonyl Group 

Hydrogenation of cinnamaldehyde to 3-phenylpropionaldehyde over palladium catalyst may be accompanied by the formation of 3-phenyl-1-propanol and propyl-benzene,218 although the formation of 3-phenylpropionaldehyde usually predominates.219,220 The composition of the products are widely affected by the nature of palladium catalysts, solvents, supports, and additives.216,221 The hydrogenation over Pd-Al203 in ethanol or over Pd-kieselguhr in acetic acid gave 3-phenylpropionalde-hyde quantitatively at room temperature and atmospheric pressure. The addition of a 1 1 ratio of ferrous chloride to palladium also resulted in quantitative formation of 3-phenylpropionaldehyde in the hydrogenation over 5% Pd-C in methanol.221 This result was contrasted with those obtained with platinum oxide where iron additives led 

The hydrogenation of a-ionone over prereduced Cu-A1203 in toluene is accompanied by partial isomerization of the unconjugated C-C double bond and gives a mixture of products. The isomerization could be suppressed by using dioxane as the solvent (eq. 3.61), thus suggesting participation of weakly electrophilic Cu(I) surface site or of Al3+ acidic sites.233 

No difficulty exists in the hydrogenations where aliphatic and alicyclic ketones are formed. Thus, acetylcyclohepetene was hydrogenated to acetylcycloheptane in 97% yield over Pd-C (eq. 3.63).235 

Freidlin et al. found that 6-methyl-3,5-heptadien-2-one (103) was hydrogenated to the 3-en-2-one over unmodified Ni-Al203 and Ni-ZnO by 1,2 addition of hydrogen to the 5,6 double bond, while the 5-en-2-one was formed preferentially in the presence of Pb, Pb(OAc)2 or Cd(OAc)2 (eq. 3.65), by 1,4 addition of hydrogen to the C=C-C=0 system, as was presumed from the results on the hydrogenation of 103 in MeOD solution.237 238 

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This is quite different to the reactivity of a C=C double bond conjugated with a carbonyl group, which, as you have just seen, reacts with nucleophiles such as cyanide, amines, and alcohols. The conjugated system is different from the sum of the isolated parts, with the C=0 group profoundly affecting the reactivi- true electJ on distribution lies somewhere ty of the C=C double bond. To show why, we can use... 

The procedure permits the synthesis of compounds with three double bonds conjugated with a carbonyl group. In all cases the y-proton of the phosphorane is replaced by the 3-carbon of the unsaturated ester or ketone. 

Structures with a carbon-carbon double bond conjugated with a carbonyl group exhibit characteristic reactions that can be rationalized with Lewis... 

Addition of an enolate anion to a carbon—carbon double bond conjugated with a carbonyl group presents an entirely new synthetic strategy. In this section, we study a type of conjugate addition involving nucleophrhc addition to an electrophrhc double bond. 

The addition of a carbanion to a double bond conjugated with a carbonyl, nitrile, or sulfonyl group can give the usual simple addition product or it can give an intermediate carbanion which continues the reaction as a chain polymerization. 

In enediones in which two carbonyl groups of a diketone are linked by an ethylenic bond tin [174] and chromous chloride [196] reduce only the double bond, and none of the conjugated carbonyl groups. A double bond conjugated with one carbonyl group only is not reduced. Refluxing cholest-4-ene-3,6-dione with chromous chloride in tetrahydrofuran yielded 49% of 5/S-cholestane-3,6-dione, and a similar reduction of cholesta-l,4-diene-3,6-one gave 5)S-cholest-l-ene-3,6-dione [196]. 

Knoevenagel Condensation. The Knoevenagel condensation is a very useful reaction for preparing molecules containing a double bond conjugated with a carbonyl function by a condensation of a molecule with a carbonyl group and another with activated methylene group. 

Third, dehydration to introduce a double bond conjugated with the carbonyl group by loss of the proton (H+) a to the carbonyl (C=0) and the hydroxyl (OH")... 

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