A comparative investigation of 1,4‐pentamethylene and 1,4‐hexamethylene Dewar benzene: Evidence for the intermediate formation of [5]paracyclophane

Abstract

Abstract The synthesis of the title compounds 1a and 1b is described. Starting from the corresponding 1,2‐dimethylenecycloalkanes 6 , the compounds 1 were obtained in four steps, viz . addition of dichlorocarbene, reduction with triphenyltin hydride, treatment with potassium tert ‐butoxide and silver‐ion‐catalyzed rearrangement. In the last step, the 1,2‐isomers 11 of 1 were also formed, and their thermal rearrangement to the benzocycloalkenes 4 is briefly described. Compound 1b rearranged to its aromatic isomer [6]paracyclophane (2b) both thermally (60°C in solution, 100‐460°C flow pyrolysis) and under silver‐ion catalysis at room temperature; in this latter reaction, the initially formed 2b was gradually further isomerized to 4b . At higher temperatures, 2b rearranged to the spirotrienes 3b and finally fragmented to give p ‐ethylstyrene (17) . From 1a , the spirotriene 3a and benzocycloheptene (4a) were obtained by thermolysis and by silver‐ion catalysis, respectively. The mechanism of these reactions is discussed and it is concluded that [5]paracyclophane (2a) is a transient intermediate in the reaction of 1a .