Bergman Cyclization

Abstract

The Bergman cyclization is an endothermic reaction and the formed 1,4‐biradical is believed to be a reactive σ,σ‐biradical that abstracts hydrogen from the proton donor (usually 1,4‐cyclohexadiene) to give an aromatic compound; this intermediate sometimes also abstracts a chlorine atom from CCl 4 to form substituted 1,4‐dichlorobenzenes or reacts with dissolved oxygen to form quinone. The electronic configurational mixing from a closed‐shell structure to the open‐shell character of 1,4‐biradical in this cyclization has led extensive mechanistic studies. Various factors have been found to affect the Bergman cyclization. The Bergman cyclization can be carried out either thermally or photochemically. When a carbon‐carbon double bond (ethylene moiety) is replaced by a heteroatom (nitrogen), the corresponding cyclization is called aza‐Bergman cyclization.