The Bergman cyclization or Bergman reaction or Bergman cycloaromatization is an organic reaction and more specifically a rearrangement reaction taking place when an enyne is heated in presence of a suitable hydrogen donor (Scheme 1) [1]. It is named for the American chemist Robert George Bergman (b. 1942). The reaction product is a derivative of benzene.
When the enyne moiety is incorporated into a 10-membered hydrocarbon ring (e.g. cyclodeca-3-ene-1,5-diyne in scheme 2) the reaction, taking advantage of increased ring strain in the reactant, is possible at the much lower temperature of 37°C.
Naturally occurring compounds exist containing the same 10-membered enyne ring and are found to be cytotoxic. These compounds generate the diradical intermediate described above which cause single- and double stranded DNA cuts. Novel drug research attempts to make use of this property, mylotarg one of them [2].
A biradical mechanism is also proposed for the formation of certain biomolecules found in marine sporolides that have a chlorobenzene unit as part of their structure. In this mechanism a halide salt provides the halogen. A model reaction with the enediyene cyclodeca-1,5-diyn-3-ene, lithium bromide as halogen source and acetic acid as hydrogen source in DMSO at 37°C supports the theory [3][4]:
The reaction is found to be first order in enediyne with the formation of p-benzyne A as the rate-limiting step. The halide ion then donates its two electrons in the formation of a new Br-C bond and radical electron involved is believed to shuttle over a transient C1-C4 bond forming the anion intermediate B. The anion is a powerful base, stripping protons even from DMSO to final product. The dichloride or dihydrogen product (tetralin) never form.
External links
Bergman CycloaromatizationPowerpoint Whitney M. Erwin 2002
References
^p-Benzyne. Generation as an intermediate in a thermal isomerization reaction and trapping evidence for the 1,4-benzenediyl structure Richard R. Jones, Robert G. Bergman J. Am. Chem. Soc.; 1972; 94(2); 660-661. Abstract
^Design and synthesis of heterocycle fused enediyne prodrugs activable at willLuca Banfi, Andrea Basso, Giuseppe Guanti, and Renata Riva Arkivoc2006 HL-1786GR 261-275 Abstract
^ Nucleophilic Addition to a p-Benzyne Derived from an Enediyne: A New Mechanism for Halide Incorporation into Biomolecules Charles L. Perrin, Betsy L. Rodgers, and Joseph M. O'Connor J. Am. Chem. Soc.; 2007; ASAP Web Release Date: 23-Mar-2007; (Article) doi:10.1021/ja070023e
