The Diels-Alder cycloaddition, an intriguing problem in organic sonochemistry

Sonochemical Diels-Alder cycloadditions are discussed with the help of published examples and recent data from the authorsʹ laboratories. Most of the significant results, in terms of rate and/or yield increases, are obtained when quinones are the dienophiles. A possible mechanism implies a redox process between the diene and dienophile, giving radical-ion intermediates. Convergent data lead to the conclusion that in other cases, where a concerted (or closely related) mechanism is probable, the sonochemical effect should remain modest, making necessary a re-evaluation of the physical effects of cavitation.