Bergman cyclization

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.

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The reaction proceeds by a thermal reaction or pyrolysis (above 200°C) forming a short-lived and very reactive para-benzyne biradical species. It will react with any hydrogen donor such as 1,4-cyclohexadiene which converts to benzene. When quenched by tetrachloromethane the reaction product is a 1,4-dichlorobenzene and with methanol the reaction product is benzyl alcohol.

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.

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