Addition and Cycloaddition Reactions of Arenediazonium Ions with 1,3‐Dienes: A Shift From a Concerted to a Stepwise Mechanism

Arenediazonium ions 1 undergo [2+ + 4] cycloadditions with (E)‐1,3‐pentadiene (2a), 2,3‐dimethylbutadiene (2b), and (E)‐2‐methyl‐1,3‐pentadiene (2c) to give dihydropyridazines or pyridazinium salts. While highly electrophilic diazonium ions and the unsymmetrical dienes 2a and 2c predominantly yield those regioisomers that are expected for a stepwise cycloaddition process, the opposite regioselectivity is found in the cycloadducts of less electrophilic diazonium ions. Kinetic investigations and product studies indicate that all cycloaddition reactions proceed concertedly. The dienes 2d and 2e, in contrast, undergo ordinary azo coupling reactions in acetonitrile/methanol with formation of the hydrazones 17 and the azo compounds 18, respectively. It is demonstrated that the linear free enthalpy relationship lgk = s (N + E) can also be used to roughly estimate the rates of ionic cycloaddition reactions.