α,β-Unsaturated carbonyl compounds areorganic compounds with the general structure (O=CR)−Cα=Cβ−R.[1][2] Such compounds includeenones andenals, but also carboxylic acids and the corresponding esters and amides. In these compounds, thecarbonyl group isconjugated with analkene (hence the adjectiveunsaturated). Unlike the case for carbonyls without a flanking alkene group, α,β-unsaturated carbonyl compounds are susceptible to attack by nucleophiles at the β-carbon. This pattern of reactivity is calledvinylogous. Examples of unsaturated carbonyls areacrolein (propenal),mesityl oxide,acrylic acid, andmaleic acid. Unsaturated carbonyls can be prepared in the laboratory in analdol reaction and in thePerkin reaction.
α,β-Unsaturated carbonyl compounds can be subclassified according to the nature of the carbonyl and alkene groups.
α,β-Unsaturated carbonyl compounds featuring a carbonyl conjugated to an alkene that is terminal, orvinylic, contain theacryloyl group (H2C=CH−C(=O)−); it is theacyl group derived fromacrylic acid. Thepreferred IUPAC name for the group isprop-2-enoyl, and it is also known asacrylyl or simply (and incorrectly) asacryl. Compounds containing an acryloyl group can be referred to as "acrylic compounds".
Anα,β-unsaturated acid is a type of α,β-unsaturated carbonyl compound that consists of analkeneconjugated to acarboxylic acid.[3] The simplest example is acrylic acid (CH2=CHCO2H). These compounds are prone to polymerization, giving rise to the large area ofpolyacrylate plastics.Acrylate polymers are derived from but do not contain the acrylate group.[4] The carboxyl group of acrylic acid can react withammonia to formacrylamide, or with analcohol to form an acrylateester. Acrylamide andmethyl acrylate are commercially important examples of α,β-unsaturatedamides and α,β-unsaturated esters, respectively. They also polymerize readily. Acrylic acid, its esters, and its amide derivatives feature the acryloyl group.
α,β-Unsaturateddicarbonyls are also common. The parent compounds aremaleic acid and the isomericfumaric acid. Maleic acid forms esters, an imide, and an anhydride, i.e.diethyl maleate,maleimide, andmaleic anhydride. Fumaric acid, as fumarate, is an intermediate in theKrebs citric acid cycle, which is of great importance in bioenergy.
Anenone (oralkenone) is an organic compound containing bothalkene andketone functional groups. In an α,β-unsaturated enone, the alkene isconjugated to the carbonyl group of the ketone.[3] The simplest enone ismethyl vinyl ketone (butenone,CH2=CHCOCH3).
Enones are typically produced through analdol orKnoevenagel condensation, both in industry and in the laboratory.
Some commercially significant enones produced by condensations ofacetone aremesityl oxide (dimer of acetone) andphorone andisophorone (trimers of the same).[5]
Certain specializedenolic oxidations also form enones. Theselenoxide elimination andSaegusa-Ito oxidation both effect aleaving group near the ketone that spontaneously eliminates. WhenN-tert-butyl benzenesulfinimidoyl chloride oxidizes an enolate, the result is 1,4-elimination.
Finally, theMeyer–Schuster rearrangement converts apropargylic alcohol to an enone with acid.
The cyclic enones includecyclopropenone, cyclobutenone,[6]cyclopentenone,cyclohexenone, and cycloheptenone.[7]
Cyclopentenones can be prepared via thePauson–Khand reaction orNazarov cyclization.
Anenal (oralkenal) is an organic compound containing bothalkene andaldehyde functional groups. In an α,β-unsaturated enal, the alkene isconjugated to the carbonyl group of the aldehyde (formyl group).[3] The simplest enal isacrolein (CH2=CHCHO). Other examples includecis-3-hexenal (essence of mowed lawns) andcinnamaldehyde (essence of cinnamon).
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α,β-Unsaturated carbonyls areelectrophilic at both thecarbonyl carbon as well as the β-carbon. Depending on conditions, either site is attacked bynucleophiles. Additions to the alkene are calledconjugate additions. One type of conjugate addition is theMichael addition, which is used commercially in the conversion ofmesityl oxide intoisophorone. Owing to their extended conjugation, α,β-unsaturated carbonyls are prone to polymerization. In terms of industrial scale, polymerization dominates the use of α,β-unsaturated carbonyls. Again because of their electrophilic character, the alkene portion of α,β-unsaturated carbonyls is good dienophiles inDiels–Alder reactions. They can be further activated by Lewis acids, which bind to the carbonyl oxygen. α,β-Unsaturated carbonyls are good ligands for low-valent metal complexes, examples being(bda)Fe(CO)3 andtris(dibenzylideneacetone)dipalladium(0).
α,β-Unsaturated carbonyls are readily hydrogenated. Hydrogenation can target the carbonyl or the alkene (conjugate reduction) selectively, or both functional groups.
Enones undergo theNazarov cyclization reaction and in theRauhut–Currier reaction (dimerization).
When appropriately irradiated, they undergoenone–alkene cycloadditions.
α,β-Unsaturated thioesters are intermediates in several enzymatic processes. Two prominent examples arecoumaroyl-coenzyme A andcrotonyl-coenzyme A. They arise by the action ofacyl-CoA dehydrogenases.[8]Flavin adenine dinucleotide (FAD) is a required co-factor.
Since α,β-unsaturated compounds are electrophiles and alkylating agents, many α,β-unsaturated carbonyl compounds are toxic. The endogenous scavenger compoundglutathione naturally protects from toxic electrophiles in the body. Some drugs (amifostine,N-acetylcysteine) containing thiol groups may protect from such harmful alkylation.