7,8-Unsaturated carbonyl compounds

y-unsaturated carbonyl compounds are non-conjugated systems and show remarkable properties in ultra-violet region. Bicylo [2.2.1]-hept-5-en- 2-one is an example and shows absorption at 308 nm. 

The n - ir excitation is most favoured transition but some times, the absorption shifts to longer wavelength, because of mixing of n Ji and n - k  

y-unsaturated ketones exhibit two reactions that depends upon the bichromophoric interaction. These two characteristic reactions are [1, 2] and [1, 3] acyl shift. 

Beside [1, 2] shift and [1, 3] shift p, y-unsaturated carbonyl compounds undergo characteristic reactions of chromophores, i.e., dimerisation, oxetane formation, reduction, Norrish t3T)e I and Norrish type II reactions, are also reported. 

Reduction of unsaturated carbonyl compounds to the saturated carbonyl is achieved readily and in high yield. Over palladium the reduction will come to a near halt except under vigorous conditions (73). If an aryl carbonyl compound, or a vinylogous aryl carbonyl, such as in cinnamaldehyde is employed, some reduction of the carbonyl may occur as well. Carbonyl reduction can be diminished or stopped completely by addition of small amounts of potassium acetate (i5) to palladium catalysts. Other effective inhibitors are ferrous salts, such asferroussulfate, at a level of about one atom of iron per atom of palladium. The ferrous salt can be simply added to the hydrogenation solution (94). Homogeneous catalysts are not very effective in hydrogenation of unsaturated aldehydes because of the tendencies of these catalysts to promote decarbonylation. 

Of greater challenge is the selective hydrogenation of those carbonyl compounds having two or more double bonds. Sometimes conditions can be adjusted so that either or both double bonds can be reduced at will. Quantitative yields of either 19 or 21 could be obtained by hydrogenation of 20 with appropriate use of modifiers (114). 

Selective conversion of l,4-androstadiene-3,l7-dione to 4-androsten-3,17-dione was achieved with (Ph Pi RuClj (SI), prepared by refluxing ruthenium 

Reduction of vinylic and allylic compounds without hydrogenolysis may present a problem. The ratio of olefin saturation to hydrogenolysis depends importantly on catalyst, temperature, solvent, and pH. In both classes of compounds, hydrogenolysis is favored by polar solvents, acid, and elevated temperatures hydrogenation, by nonpolar solvents and low temperatures. 

In general, hydrogenolysis of vinylic compounds is favored by platinum and hydrogenation by ruthenium and rhodium 31,55,59,72,106). In the reduction of 4-methyl-1-cyclohexenyl ether, the order of decreasing hydrogenolysis to give methylcyclohexane was established as Pt Ir Rh Os Ru = Pd (52). 

The selective hydrogenation of the double bond of an a,p-unsaturated carbonyl compound is rather easily accomplished over most metal catalysts under moderate conditions. Because double bond isomerization does not take place in these systems, palladium catalysts are often used in the liquid phase at ambient temperature and atmospheric pressure. An added advantage here is that palladium is essentially inert for aliphatic aldehyde and ketone hydrogenations under these conditions. Vapor phase hydrogenations should be run at temperatures as low as possible to minimize carbonyl group hydrogenations. Catalysts such as Ni(B) are 

In liquid phase hydrogenations of the double bond of a,P-unsaturated carbonyl compounds, mechanistic considerations are complicated because the 

Effect of solvent and hydrogen pressure on the stereochemistry of the hydrogenation of A -octaione-2 (31).53-5S 

The stereochemistry at the a-carbon on hydrogenation of a,P-unsaturated carbonyl compounds in neutral solvents depends primarily on the nature of the 

The combination of a carbonyl function and a double bond in the same molecule leads to exceptional properties only when the groups are close to one another. The cumulated and conjugated arrangements are of particular interest. We shall consider first the conjugated, or a,/3-unsaturated, carbonyl compounds, because their chemistry is related closely to that of the substances already discussed in this chapter and in Chapter 16. 

The most generally useful preparation of a,/3-unsaturated carbonyl compounds is by dehydration of aldol addition products, as described in Section 17-3D. Conjugation of the carbonyl group and double bond has a marked influence on spectroscopic properties, particularly on ultraviolet spectra, as the result of 

Such resonance is much less important in the ground state but is still sufficiently 

17 Carbonyl Compounds II. Enols Enolate Anions. Unsaturated and Polycarbonyl Compounds 

The effect of conjugation also is reflected in infrared carbonyl frequencies (Section 16-3A) and nmr spectra. With respect to the latter, it is found that the protons on the (3 carbon of a,/3-unsaturated carbonyl compounds usually come at 0.7 to 1.7 ppm lower fields than ordinary alkenic protons. The effect is smaller for the a protons. 

A packaging material with exceptional importance for food is PE. Depending on the source of this plastic, water brought in contact can produce various nuances of a characteristic odor as well as flavor changes. The description of this PE odor ranges from candle-like, stale, stuffy, musty, to soapy or rancid. For this reason PE used for food applications has particularly high quality standards set. 

It is well known for example in the saturated LDPE polymer chains that a certain number of double bounds exist which can be measured with IR spectroscopy. By extraction with non-polar solvents and GC separation, numerous alkanes and alkenes can be identified which are dissolved in small concentrations in the PE. The odor thresholds of these compounds are in general so high that these hydrocarbons play no sensory role. As a result no correlation can be made between the total amount of volatile compounds isolated from PE or the fingerprint chromatogram from a GC separation and the sensory properties of a sample. The relevant sensory compounds as a rule are the (order of magnitude) less concentrated oxygenated compounds in the 

The oxidation of the 1-alkene occurs most likely through the formation of a free radical from the splitting off of hydrogen on the third C-atom (allylic CH-compound). It is expected that the oxygen subsequently attacks in the 1 and 3 position due to the allylmesomerism  

With this, the formation of the 2-alkene-l-ol, 2-alkene-l-al, 2-alkene-1-carboxylic acid, l-alkene-3-ol and l-alkene-2-one is predestined. 

Tabic 13-4 Absolute odor thresholds (ng) and OTa concentrations (mg/m3) of alkenes and the corresponding alcohols and unsaturated carbonyl compounds. The number of C-atomes in the molecule is designated with n. 

The proton spectrum consists of two signals for the vinylic protons, each a doublet of quartets. The signal for the proton at C2 is centered at 6.53 ppm with a trans three-bond H-H coupling constant of 15.8 Hz, and four-bond F-H coupling of 2.0 Hz. The signal of the proton at C3 is centered at 6.91 ppm, with respective coupling constants of 15.8 and 6.5 Hz. 

FIGURE 5.18. HNMRspectrum of 4,4,4-trifluorocrotonic acid (vinylic area) 

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So far in this section we have combined enolate anions with other carbonyl compounds by direct attack at the carbonyl group. We can expand the scope of this reaction by using a,p-unsaturated carbonyl compounds as the electrophiles. This is the Michael reaction. Remind yourself of tliis by writing out the mechanism of a Michael reaction such as ... 

The Michael reaction plays a part in some more extended synthetic sequences of great importance. Analyse TM 116 as an a,p-unsaturated carbonyl compound and continue your analysis by the Michael reaction. 

Analysis Another lactone FGl reveals the true TM (A). Our normal discormection a of an a,p-unsaturated carbonyl compound gives us the 1,5-dicarbonyl compound (B) and the ketone (C) clearly derived from phenol. Alternatively we could disconnect bond b to the keto-ester (D) with the further discormection shown ... 

Aldol additions and ester condensations have always been and still are the most popular reactions for the formation of carbon-carbon bonds (A.T. Nielsen, 1968). The earbonyl group acts as an a -synthon, the enoi or enolate as a d -synthon. Both reactions will be treated together here, and arguments, which are given for aldol additions, are also valid for ester condensations. Many famous name reactions belong to this category ). The products of aldol additions may be either /J-hydroxy carbonyl compounds or, after dehydration, or, -unsaturated carbonyl compounds. 

The Michael reaction is of central importance here. This reaction is a vinylogous aldol addition, and most facts, which have been discussed in section 1.10, also apply here the reaction is catalyzed by acids and by bases, and it may be made regioselective by the choice of appropriate enol derivatives. Stereoselectivity is also observed in reactions with cyclic educts. An important difference to the aldol addition is, that the Michael addition is usually less prone to sterical hindrance. This is evidenced by the two examples given below, in which cyclic 1,3-diketones add to o, -unsaturated carbonyl compounds (K. Hiroi, 1975 H, Smith, 1964). 

Another widely used route to cyclopropanes involves the addition of sulfoniutn ylides to a,/3-unsaturated carbonyl compounds (S.R. Landor, 1967 R. Sowada, 1971 C.R. Johnson, I973B, 1979 B.M. Trost, 1975 A). Non-activated double bonds are not attacked. Sterical hindrance is of little importance in these reactions because the C—S bond is extraordinarily long... 

The allylic geminal diacetate 141 undergoes the monoallylation of malonates to give 142 and the two regioisomers 143 and 144[93,94]. The dimethylacetal 145 or ortho esters of aromatic and a,/3-unsaturated carbonyl compounds react with trimethylsilyl cyanide to give the methyl ether of cyanohydrin[95]. 

Some organosilicon compounds undergo transmetallation. The allylic cyanide 461 was prepared by the reaction of an allylic carbonate with trimethylsi-lyl cyanide[298]. The oriho esters and acetals of the o. d-unsaturated carbonyl compounds 462 undergo cyanation with trimefhylsilyl cyanide[95]. 

Preparation of o,/3-Unsaturated Carbonyl Compounds by the Reactions of Silyl Enol Ethers and Enol Acetates with Ally Carbonates... 

Addition of dihydrosilane to a, /J-unsaturated carbonyl compounds such as citral (49), followed by hydrolysis, affords saturated citroneJlal (50) directly. The reaction is used for the selective reduction of conjugated double bonds[45,46]. In addition to Pd catalyst, the use of a catalytic amount of... 

Triethylammonium formate is another reducing agent for q, /3-unsaturated carbonyl compounds. Pd on carbon is better catalyst than Pd-phosphine complex, and citral (49) is reduced to citronellal (50) smoothly[55]. However, the trisubstituted butenolide 60 is reduced to the saturated lactone with potassium formate using Pd(OAc)2. Triethylammonium formate is not effective. Enones are also reduced with potassium formate[56]. Sodium hypophosphite (61) is used for the reduction of double bonds catalyzed by Pd on charcoal[57]. 

Cycloaddition of COj with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiocheraistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /3-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

In resonance terms electron delocalization map unsaturated carbonyl compounds IS represented by contributions from three principal resonance structures... 

The carbonyl group withdraws rr electron density from the double bond and both the carbonyl carbon and the p carbon are positively polarized Their greater degree of charge separation makes the dipole moments of a p unsaturated carbonyl compounds signifi cantly larger than those of comparable aldehydes and ketones... 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

On the other hand the polarization of electron density map unsaturated carbonyl compounds makes their p carbon atoms rather electrophilic Some chemical conse quences of this enhanced electrophilicity are described m the following section... 

Ordinarily nucleophilic addition to the carbon-carbon double bond of an alkene is very rare It occurs with a p unsaturated carbonyl compounds because the carbanion that results IS an enolate which is more stable than a simple alkyl anion... 

CONJUGATE ADDITION OF ORGANOCOPPER REAGENTS TO a,p-UNSATURATED CARBONYL COMPOUNDS... 

Stabilized anions exhibit a pronounced tendency to undergo conjugate addition to a p unsaturated carbonyl compounds This reaction called the Michael reaction has been described for anions derived from p diketones m Section 18 13 The enolates of ethyl acetoacetate and diethyl malonate also undergo Michael addition to the p carbon atom of a p unsaturated aldehydes ketones and esters For example... 

Ethyl acetoacetate behaves similarly to diethyl malonate in its reactivity toward a p unsaturated carbonyl compounds Give the structure of the product of the following reaction sequence... 

Ammonia and amines undergo conjugate addition to a 3 unsaturated carbonyl compounds (Section 18 12) On the basis of this information predict the pnncipal organic product of each of the following reactions... 

Acrolein (H2C=CHCH=0) undergoes conjugate addition with sodium azide in aqueous solution to give N3CH2CH2CH=0 Propanal is not an a 3 unsaturated carbonyl compound and cannot undergo conjugate addition... 

Vinyl ethers and a,P unsaturated carbonyl compounds cyclize in a hetero-Diels-Alder reaction when heated together in an autoclave with small amounts of hydroquinone added to inhibit polymerisation. Acrolein gives 3,4-dihydro-2-methoxy-2JT-pyran (234,235), which can easily be hydrolysed to glutaraldehyde (236) or hydrogenated to 1,5-pentanediol (237). With 2-meth5lene-l,3-dicarbonyl compounds the reaction is nearly quantitative (238). 

A particularly useful reaction has been the selective 1,2-reduction of a, P-unsaturated carbonyl compounds to aHyUc alcohols, accompHshed by NaBH ia the presence of lanthanide haUdes, especially cerium chloride. Initially appHed to ketones (33), it has been broadened to aldehydes (34) and acid chlorides (35). NaBH by itself gives mixtures of the saturated and unsaturated alcohols. 

AldolRea.ctlons, In the same way, hydroxybenzaldehydes react readily with aldehydes and ketones to form a,P-unsaturated carbonyl compounds in the Claisen-Schmidt or crossed-aldol condensation (60). 

Mesityl Oxide. Mesityl oxide (MSO) (4-metliyl-3-penten-2-one) is an oily colorless liquid with an unpleasant odor. It exhibits the versatiUty and unusual reactivity associated with conjugated a,P unsaturated carbonyl compounds (172). On standing ia air, mesityl oxide slowly forms bis(3,5,5-trimethyl-l,2-dioxolanyl)-3-peroxide (173). 

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