Steady-Light Irradiation and Flash Photolysis Studies of [2.2]Metacyclophane-Iodine Systems. Mechanistic Implications of Transannular Dehydrogenation Reaction

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Irradiation of cyclohexane solutions of [2.2]metacyclophane in the presence of iodine is shown to yield 4,5,-9,10-tetrahydropyrene as the initial product. In order to clarify the mechanism of the cyclodehydrogenation reaction, steady-light irradiation, and flash photolysis experiments have been carried out. The rate of formation of 4,5,9,10-tetrahydropyrene is followed by UV spectra measurements using a characteristic band maximum at 292 nm. The rate of appearance of 4,5,9,10-tetrahydropyrene is found to be proportional to the absorption of light. The reaction is a one photon process. The reaction rate is almost constant when the concentration of iodine is less than 7×10<SUP>−4</SUP> mol/<I>l</I>, but decreases gradually with higher concentrations. A long-lived species, whether radical or excited triplet, can not be detected by flash techniques. It is most likely that the first step involves the excited singlet. Involvement of CT-complex is proposed.
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