Conjugation in a,p-unsaturated aldehydes and ketones

Reduction of conjugated double bonds in a,p-unsaturated aldehydes and ketones proceeds with high selectivity by the use of sodium dithionite under PTC conditions ... 

FIGURE 18 7 Nucleophilic addition to a p unsaturated aldehydes and ketones may take place either in a 1 2 or 1 4 manner Direct addition (1 2) occurs faster than conjugate addition (1 4) but gives a less stable product The product of 1 4 addition retains the carbon-oxygen double bond which is in general stronger than a carbon-carbon double bond... 

Aldol condensation offers an effective route to a,p-unsaturated aldehydes and ketones. These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group. As shown in Figure 18.6, the tt systems of the carbon-carbon and carbon-oxygen double bonds overlap to fonn an extended tt system that permits increased electron delocalization. 

The product is a P-hydroxy aldehyde (called an aldol) or ketone, which in some cases is dehydrated during the course of the reaction. Even if the dehydration is not spontaneous, it can usually be done easily, since the new double bond is in conjugation with the C=0 bond so that this is a method of preparing a,P-unsaturated aldehydes and ketones as well as P-hydroxy aldehydes and ketones. The entire reaction is an equilibrium (including the dehydration step), and a,P-unsaturated and P-hydroxy aldehydes and ketones can be cleaved by treatment with OH (the retrograde aldol reaction). There is evidence that an SET mechanism can intervene when the substrate is an aromatic ketone. ... 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

The reduction of a,p-unsaturated aldehydes and ketones by NaBH4 leads, in general, to substantial amounts of fully saturated alcohols. In alcoholic solvents, saturated -alkoxy alcohols can be formed via conjugate addition of the solvent. This latter process becomes the main reaction path when reduction is performed in 2-propanol in the presence of sodium isopropoxide. In base, a homoallylic alcohol can become the major product of borohydride reduction of an enone. Analysis of the influence of substrate structure on NaBH4 reduction has shown that increasing steric hindrance on the enone increases 1,2-attack. ... 

Sodium cyanoborohydride (NaBHsCN) or tetrabutylammonium cyanoborohydride in acidic methanol or acidic HMPA reduces a,p-unsaturated aldehydes and ketones to the corresponding allylic alcohols. This system is limited to enones in which the double bond is not further conjugated, in which case the allylic hydrocarbon is formed in substantial amounts. Thus, reduction of chalcone gives mainly 1,3-di-phenylpropene (48%) as well as 26% of the allylic ether. Cyclic enones are also not good substrates, as competing 1,4-addition gives large fractions of saturated alcohols. ... 

Micelle A spherical cluster in aqueous solution, such as the cluster of long-chain carboxylic acids in soap. Michoel addition The conjugate addition of enolates to a.p-unsaturated aldehydes and ketones. 

Even in the presence of other silicon reagents like PhjSiHj, Michael acceptors are totally unaffected. The addition of catalytic amounts of zinc chloride to the Pd(0)/silane system, however, creates a three-component mixture that allows rapid conjugate reduction of a,p-unsaturated aldehydes and ketones. The conjugate reduction was shown to be both regio- and stereoselective. The use of dideuterodiphenylsilane in the reduction of unsaturated ketones yielded satmated ketones containing one deuterium atom at the P-position. On the other hand, when traces of D2O were added to the nondeuterated mixture, incorporation of deuterium occurred in the a-position (Scheme 16). ... 

Interestingly, a,P-unsaturated aldehydes and ketones generally react with Grignard reagents at the carbonyl. Reactions in which the major product is derived from a process involving the double bond and the carbonyl (conjugate addition) are rare. 

First, in common with other conjugated rr-electron systems that we have seen, a,p-unsaturated aldehydes and ketones are more stable than their nonconjugated isomers. Under conditions chosen to bring about their interconversion, for example, the equilibrium between a p,y-unsaturated carbonyl and its a,p-unsaturated analog favors the conjugated isomer. 

In the presence of a catalytic amount of Sc(OTf)3, conjugated linoleic acid ethyl ester could be used as a diene in Diels-Alder reactions to react with quinines, a,P-unsaturated aldehydes, and ketones in good yields [26]. Sc(OTf)3 catalyst could be recovered by extraction of organic layers with water. After removal of water, the recovered Sc(OTf)3 could be reused in the Diels-Alder reaction without appreciable loss of catalytic activity. 

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