Bergman cyclization

Bergman cyclization
Named after	Robert George Bergman
Reaction type	Ring forming reaction
Identifiers
Organic Chemistry Portal	bergman-cyclization
RSC ontology ID	RXNO:0000240

The Bergman cyclization or Bergman reaction or Bergman cycloaromatization is an organic reaction and more specifically a rearrangement reaction taking place when an enediyne is heated in presence of a suitable hydrogen donor (Scheme 1). It is the most famous and well-studied member of the general class of cycloaromatization reactions. It is named for the American chemist Robert G. Bergman (b. 1942). The reaction product is a derivative of benzene.

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 such as calicheamicin contain the same 10-membered ring and are found to be cytotoxic. These compounds generate the diradical intermediate described above which can cause single and double stranded DNA cuts. There are novel drugs which attempt to make use of this property, including monoclonal antibodies such as mylotarg.

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