Cyclopentadienone

Structural formula
General
Surname	Cyclopentadienone
other names	Cyclopentadien-1-one
Molecular formula	C 5 H 4 O
External identifiers / databases
CAS number 13177-38-3 PubChem 139405 ChemSpider 122931 Wikidata Q21099674
properties
Molar mass	80.09 g mol −1
safety instructions
GHS hazard labeling no classification available
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Cyclopentadienone is a chemical compound from the group of ketones .

Extraction and presentation

Cyclopentadienone can be produced photolytically or pyrolytically from several precursors (e.g. from 1,2-benzoquinone ) at low temperatures and isolated in an argon matrix at 10 K. It already dimerizes when the matrix is ​​thawed (38 K).

properties

Substituted cyclopentadienones are important synthetic building blocks that are used in natural product synthesis or in the representation of strained compounds , such as. B. Tetrahedrans are used. The first and probably best known example of a monomeric cyclopentadienone, tetraphenylcyclopentadienone , was published in 1925 by A. Löwenbein and G. Ulich. More recently, organometallic chemistry has discovered the cyclopentadienones for itself.

Therefore, the search for its basic structure began at an early stage, with the first work going back to Klaus Hafner and Charles Herbert DePuy . They synthesized by Cyclopentadienone bromide - elimination from the corresponding bromide or ester pyrolysis of the acetate . Cyclopentadienone was not directly detectable because it immediately dimerizes in a Diels-Alder reaction . In addition to the preparative, there was also a theoretical interest in the compound due to the unusual reaction behavior of the substance class. Cyclopentadienones, for example, cannot generally be converted to cyclopentadienes or hydroxycyclopentadienes by reducing the carbonyl group , but give cyclopentenones .

Two mesomeric boundary structures were discussed for cyclopentienone as the basic structure , which should provide an explanation. Here, boundary structure I leads to an anti-aromatic destabilization of the ring, since it is a 4π system. In contrast to this, an aromatic 6π system is formed in boundary structure II with umpolung of the carbonyl function. As a result of the delocalization, both boundary structures should lead to an easier reducibility of the cyclopentadienone than in a system with localized bonds . In addition, the involvement of polarized boundary structures such as II would also explain the lower reactivity of the carbonyl function. As the investigations showed, these boundary structures only play a very subordinate role in reality.

Another explanation was provided by the molecular orbital theory , which describes the properties of the cyclopentadienones well, whereby these are determined by the small HOMO - LUMO distance and the energetically very low and strongly delocalized LUMO. The already mentioned intense colors of this class of compounds can be traced back to the π-π * transitions.

See also

• Tetraphenylcyclopentadienone
• Fulven

Individual evidence

1. ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.
2. ^ Roger Brown: Pyrolytic Methods in Organic Chemistry Application of Flow and Flash Vacuum Pyrolytic Techniques . Elsevier, 2012, ISBN 978-0-323-15417-8 , pp. 173 ( limited preview in Google Book search).
3. ^ Günther Maier, Lothar Hermann Franz, Hans-Georg Hartan, Klaus Lanz, Hans Peter Reisenauer: Small rings, 54. Cyclopentadienone. In: Chemical Reports. 118, 1985, p. 3196, doi : 10.1002 / cber.19851180819 .
4. ↑ William M. Horspool, Francesco Lenci: CRC Handbook of Organic Photochemistry and Photobiology, Volumes 1 & 2, Second Edition . CRC Press, 2003, ISBN 978-0-203-49590-2 , pp. 12-8 ( limited preview in Google Book search).
5. ↑ Michael A. Ogliaruso, Michael G. Romanelli, Ernest I. Becker: Chemistry of Cyclopentadienones , Chem. Rev. , 1965, Volume 65, pp. 261-367. doi : 10.1021 / cr60235a001 .
6. ↑ Dissertation: Scheppelmann, Imke. “ Pyridyl-Functionalized Cyclopentadienones: Synthesis and Coordination Chemistry. ” Bielefeld University , 2002.