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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 12, Number 4
BKCSDE 12(4)
April 20, 1991 

Mechanistic Studies on the Anormalous Photocycloaddition Reaction of 5-Styryl-1,3-dimethyluracil and 2,3-Dimethyl-2-butene: Formation of the “Apparent Forbidden” [π4s + π2s] Cycloadduct
Eun Ju Shin*, Ho Kwon Kang, Sang Chul Shim
Irradiation of 5-styryl-1,3-dimethyluracil (5-SDU) with 2,3-dimethyl-2-butene (DMB) gives a [4+2] cycloadduct which is converted into a [2+2] cycloadduct on the prolonged irradiation. Triplet sensitization, quenching, and external heavy atom effect on the [4+2] photocycloaddition reaction demonstrate the singlet pathway and salt effect excludes a radical ion pair precursor possibility. Polar solvents increase the reaction efficiency implying a polar exciplex involvement in the [4+2] photocycloaddition reaction. Inverse temperature dependence both on the reaction and DMB fluorescence quenching of 5-SDU indicates the presence of a singlet exciplex intermediate.
434 - 437
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