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-p entadiene (2a), 2,3-dimethylbutadiene (2b), and (E)-2-methyl-1,3-penta diene (2c) to give dihydropyridazines or pyridazinium salts. While hig hly electrophilic diazonium ions and the unsymmetrical dienes 2a and 2 c predominantly yield those regioisomers that are expected for a stepw ise cycloaddition process, the opposite regioselectivity is found in t he cycloadducts of less electrophilic diazonium ions. Kinetic investig ations and product studies indicate that all cycloaddition reactions p roceed concertedly. The dienes 2d and 2e, in contrast, undergo ordinar y azo coupling reactions in acetonitrile/methanol with formation of th e hydrazones 17 and the azo compounds 18, respectively. It is demonstr ated that the linear free enthalpy relationship lgk = s (N + E) can al so be used to roughly estimate the rates of ionic cycloaddition reacti ons.