A dicarbonyl is a molecule containing two carbonyl (C=O) groups. Their properties often differ from those of monocarbonyls. The simplest are symmetrical dialdehydes, diketones, and diesters. Related to the latter are dicarboxylic acids and diamides. Unsymmetrical dicarbonyls are also prevalent, e.g. keto-aldehydes, keto acids, etc.
A single 1,2-dialdehyde exists, glyoxal, (CHO)2. Like most alkyldialdehydes, it is rarely encountered as such but instead is encountered almost exclusively as its hydrate and oligomers thereof. These derivatives often behave like the aldehyde. Glyoxal condenses readily with amines. Via such reactions, it is a precursor to many heterocycles, e.g. imidazoles.
- RCH(OH)CH(OH)R → RC(O)C(O)R + 2 H2
2,3-Butanedione, 2,3-pentanedione, and 2,3-hexanedione are found in small amounts in various foods. They are used as aroma components in alcohol-free beverages and in baked goods. Benzil, (PhCO)2, is the corresponding diphenyl derivative.
A distinctive feature of 1,2-diketones is the long C-C bond linking the carbonyl groups. This bond distance is about 1.54 Å, compared to 1.45 Å for the corresponding bond in 1,3-butadiene. The effect is attributed to repulsion between the partial positive charges of the carbonyl carbon atoms.
1,2-Diesters and diacids
Oxalic acid and its esters define this family of compounds. The diacid is produced industrially by oxidation waste sugars. It occurs naturally (as the conjugate base), notably in members of the plant species Oxalis. Condensation of the diesters with diamines gives cyclic diamides.
The parent 1,3-dialdehyde is malondialdehyde (CH2(CHO)2). Like most dialdehydes, it is rarely encountered as such. Instead it is handled almost exclusively as its hydrate, methyl acetal, and oligomers thereof. These derivatives often behave like the parent. Many 2-substituted derivatives are known. They are often prepared by alkylation of the enolate of malondialdehyde.
An important member is acetylacetone, CH3C(O)CH2C(O)CH3. Dimedone is a cyclic 1,3-diketone. Acetylacetone is prepared industrially by the thermal rearrangement of isopropenylacetate. Another cyclic 1,3-diketone is 2,2,4,4-tetramethylcyclobutanedione, which is a precursor to a useful diol.
- CH2(CH3)COC(O)Me → MeC(O)CH2C(O)Me
1,3-Diketones that can tautomerize to an enol that is conjugated to the other carbonyl usually exist predominantly in the enol form, and especially when the product can be further stabilized by a six-membered ring containing a hydrogen bond. For example, the percent enol in acetylacetone, trifluoroacetyacetone, and hexafluoroacetylacetone are 85, 97, and 100%, respectively (neat, 33 °C).
The conjugate base derived from 1,3-ketones can serve as ligand s to form metal acetylacetonate coordination complexes. In the DeMayo reaction 1,3-diketones react with alkenes in a photochemical pericyclic reaction to form (substituted) 1,5-diketones.
Classically, 1,3-diketones are prepared by the Claisen condensation of a ketone with an ester.
1,3-Diesters and diacids
Malonic acid and its esters are the parent members of this class of dicarbonyls. Also common are the 2-substituted derivatives with the formula RCH(CO2R)2, which arise by C-alkylation of the conjugate base (the enolate) NaCH(CO2R)2.
Succinaldehyde (CH2CHO)2 is the simplest and parent 1,4-dialdehyde. It hydrates readily to give 2,5-dihydroxytetrahydrofuran.
Diketones with two methylene groups separating the carbonyl groups, also called γ-diketones, typically coexist with their enol tautomers. The preeminent member is acetonylacetone. 1,4-Diketones are useful precursors to heterocycles via the Paal–Knorr synthesis, which gives pyrroles:
This reactivity is the basis of the neurotoxicity of γ-diketones. 1,4-Diketones are also precursor to furans and thiophenes. The condensation of 1,4-diketones (and related substrates) with hydrazines afford dihydropyridazines, which can be converted to pyridazines.
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