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Photochemically excited triphenylpyrylium tetrafluoroborate catalyzes Diels‐Alder reactions that proceed via an electron‐transfer chain mechanism. By means of a “redox umploung”, an electron‐rich alkene (En) is transformed into the radical cation, which functions as an appropriate electron‐poor dienophile (Sens ⊕ = triphenylpyrylium). equation image
Mlcoch et al. (Wed,) studied this question.