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Decarbonylation

Removing a carbonyl group by irradiation with UV light is called photochemical decarbonylation or photodecarbonylation. The reaction takes place by photoelimination of a small but stable CO molecule. The steps of reaction can be described as C–C-bond-breaking and C–C bond-formation, so it is considered a concerted reaction that follows trajectories determined by orbital symmetry. Photoinduced eliminations of carbonyl groups are endothermic reactions. The energy for the reaction results from the absorption of a photon. Due to this, the products have a higher energy content than the starting material.[1] For this kind of reaction high pressure xenonmercury lamps with emission in the UV region are usually used. The reaction takes place under irradiation with wavelengths of 200 nm to 400 nm. Several saturated cyclic and aromatic ketones as well transition metal carbonyl complexes undergo photochemical decarbonylations.[2]

dec1

Figure 1: Photodecarbonylation of thujone.

 

 dec2

Figure 2: Photodecarbonylation of tetramethyl-1,3-cyclobutanedione.

The photodecarbonylation of thujone is shown in Figure 1[2]. Under given conditions the CO formation rate is 0.79 mL min−1, yielding a diene as product. The irradiation of tetramethyl1,3-cyclobutanedione in benzene gives mainly and 2,3-dimethyl-2-butene (tetra-methylethylen). As shown in figure 2, it is assumed that the reaction proceeds via an intermediate and the formations of tetramethylcyclopropanone. The product is obtained in small yield when methanol is used as solvent. Major products are methyl isobutyrate and methyl 2,2,3-trimethylbutyrate. The irradiation of dispiro[5.1.5.1]tetradecane-7,14-dione in benzene and methylene chloride under avoidance of oxygen gives bicyclohexylidene associated with the release of carbon monoxide.[3] The photodecarbonylation reactions of aromatic ketones proceed rapidly and almost quantitatively. Irradiation of diphenylcyclopropenone gives diphenylacetylene as product and furnishes a yield of 68%.[4]

dec3

Figure.3: Photodecarbonylation of dispiro[5.1.5.1]tetradecane-7,14-dione.

 dec4

Figure 4: Photodecarbonylation of diphenylcyclopropenone.

 

  1. ALBINI, A.; FAGNONI, M. (Eds.): Handbook of Synthetic Photochemistry, 1. edn., 2009, Wiley-VCH Verlag GmbH& Co. KGaA, ISBN 978–3–5273–2391–3.
  2. STARR, J. E.; EASTMAN, R. H.: Structural Features Facilitating the Photodecarbonylation of Cyclic Ketˇones1,2, The Journal of Organic Chemistry, 1966-May, 31 (5), 1393–1402, DOI: 1021/jo01343a019.
  3. TURRO, N. J.; LEERMAKERS, P. A.; VESLEY, G. F.: CYCLOHEXYLIDENECYCLOHEXANE, Organic Syntheses, 1967, 47, 34, DOI: 15227/orgsyn.047.0034.
  4. QUINKERT, G.: Photochemistry of non-conjugated ketones in solution, Pure and Applied Chemistry, 1964-January, 9 (4), DOI: 1351/pac196409040607.

 



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