decarbonylation
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Decarbonylation is a type of organic reaction that involves loss of CO. It is often an undesirable reaction since it represents a degradation. In the chemistry of
metal carbonyl Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe c ...
s, decarbonylation describes a substitution process, whereby a CO ligand is replaced by another ligand.


Organic chemistry

In the absence of metal catalysts, decarbonylation (vs decarboxylation) is rarely observed in organic chemistry. One exception is the decarbonylation of formic acid: :HCO2H → CO + H2O The reaction is induced by sulfuric acid, which functions as both a catalyst and a dehydrating agent. Via this reaction, formic acid is occasionally employed as a source of CO in the laboratory in lieu of cylinders of this toxic gas. With strong heating, formic acid and some of its derivatives may undergo decarbonylation, even without adding a catalyst. For instance,
dimethylformamide Dimethylformamide is an organic compound with the formula ( CH3)2NC(O)H. Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with water and the maj ...
slowly decomposes to give dimethylamine and carbon monoxide when heated to its boiling point (154 °C). Some derivatives of formic acid, like formyl chloride, undergo spontaneous decarbonylation at room temperature (or below). Reactions involving
oxalyl chloride Oxalyl chloride is an organic chemical compound with the formula (COCl)2. This colorless, sharp-smelling liquid, the diacyl chloride of oxalic acid, is a useful reagent in organic synthesis. Preparation Oxalyl chloride was first prepared in 1892 ...
(COCl)2 (e.g., hydrolysis, reaction with carboxylic acids,
Swern oxidation The Swern oxidation, named after Daniel Swern, is a chemical reaction whereby a primary or secondary alcohol is oxidized to an aldehyde or ketone using oxalyl chloride, dimethyl sulfoxide (DMSO) and an organic base, such as triethylamine. It is one ...
, etc.) often liberate both carbon dioxide and carbon monoxide via a fragmentation process. α-Hydroxy acids, e.g. (
lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...
and
glycolic acid Glycolic acid (or hydroxyacetic acid; chemical formula HOCH2CO2H) is a colorless, odorless and hygroscopic crystalline solid, highly soluble in water. It is used in various skin-care products. Glycolic acid is widespread in nature. A glycolate (s ...
) undergo decarbonylation when treated with catalytic concentrated sulfuric acid, by the following mechanism: Silacarboxylic acids (R3SiCOOH) undergo decarbonylation upon heating or treatment with base and have been investigated as carbon monoxide generating molecules.


Aldehyde decarbonylation

A common transformation involves the conversion of
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s to alkanes.Kreis, M.; Palmelund, A.; Bunch, L.; Madsen, R., "A General and Convenient Method for the Rhodium-Catalyzed Decarbonylation of Aldehydes", Advanced Synthesis & Catalysis 2006, 348, 2148-2154. :RCHO → RH + CO Decarbonylation can be catalyzed by soluble metal complexes. These reactions proceed via the intermediacy of metal acyl hydrides. An example of this is the Tsuji–Wilkinson decarbonylation reaction using
Wilkinson's catalyst Wilkinson's catalyst is the common name for chloridotris(triphenylphosphine)rhodium(I), a coordination complex of rhodium with the formula hCl(PPh3)3(Ph = phenyl). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as ...
. (Strictly speaking, the noncatalytic version of this reaction results in the formation of a rhodium carbonyl complex rather than free carbon monoxide.) This reaction is generally carried out on small scale in the course of a complex natural product total synthesis, because although this reaction is very efficient at slightly elevated temperatures (e.g., 80 °C) when stoichiometric rhodium is used, catalyst turnover via extrusion of CO requires dissociation of a very stable rhodium carbonyl complex and temperatures exceeding 200 °C are required. This conversion is of value in organic synthesis, where decarbonylation is an otherwise rare reaction. Decarbonylations are of interest in the conversions of sugars. Ketones and other carbonyl-containing functional groups are more resistant to decarbonylation than are aldehydes. :


Pericyclic reactions

Some cyclic molecules containing a ketone undergo a cheletropic extrusion reaction, leaving new carbon–carbon π bonds on the remaining structure. This reaction can be spontaneous, as in the synthesis of hexaphenylbenzene. Cyclopropenones and cyclobutenediones can be converted to alkynes by elimination of one or two molecules of CO, respectively.


Biochemistry

Carbon monoxide is released in the degradation (catabolism) of heme by the action of O2, NADPH and the enzyme heme oxygenase:Ryter, S. W.; Tyrrell, R. M., "The Heme Synthesis and Degradation Pathways: Role in Oxidant Sensitivity: Heme Oxygenase Has Both Pro- and Antioxidant Properties", Free Radical Biology and Medicine 2000, volume 28, pages 289-309. :


Inorganic and organometallic synthesis

Many metal carbonyls are prepared via decarbonylation reactions. The CO ligand in
Vaska's complex Vaska's complex is the trivial name for the chemical compound ''trans''-carbonylchlorobis(triphenylphosphine)iridium(I), which has the formula IrCl(CO) (C6H5)3sub>2. This square planar diamagnetic organometallic complex consists of a central iridi ...
arises by the decarbonylation of DMF: :IrCl3(H2O)3 + 3 P(C6H5)3 + HCON(CH3)2 + C6H5NH2 → IrCl(CO) (C6H5)3sub>2 + CH3)2NH2l + OP(C6H5)3 + 6H5NH3l + 2 H2O The conversion of Fe(CO)5 and Mo(CO)6 to their many derivatives often involves decarbonylation. Here decarbonylation accompanies the preparation of Cyclopentadienyliron dicarbonyl dimer: :2 Fe(CO)5 + C10H12 → (''η''5-C5H5)2Fe2(CO)4 + 6 CO + H2 Decarbonylation can be induced photochemically as well as using reagents such as trimethylamine ''N''-oxide: :Me3NO + L + Fe(CO)5 → Me3N + CO2 + LFe(CO)4


References

{{Reflist Chemical reactions Carbon monoxide