Photoisomerization pathways of 8,16-methno[2.2]metacyclopane-1,9-diene. A model case for adiabatic electrocyclic ring closure in the excited singlet state |
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Authors: | Jakob Wirz Gabriele Persy Erika Rommel Ichiro Murata Kazuhiro Naksuji |
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Abstract: | The title compound 1b ideally meets the theoretical requirements for the occurrence of an adiabatic photoisomerization in the lowest excited state (1 1b *) and, indeed, the predominant primary photoreaction observed is the conversion to its fluorescent valence isomer 10b, 10c-methano-cis-10b 10c-dihydropyrene (1 1a *). The mechanism for the formation often previously observed photoproduct 8b, 9a-dihydro-9H - cyclopropa[e]pyrene ( 4a ) has been analyzed in some detail (Scheme). Below ? 30°C the reaction path consists of a three quantum process (two di-π-methane rearrangements and photochemical 1,7-H shift) involving two thermally stable, but light-sensitive isomers 8,11b-methanocyclodeca[cde]naphthalene ( 2b ) and 9H -cyclohepta[def]-phenanthrene ( 3b ). At room temperature the rearrangement 2b→4a proceeds with a single excitation step bypassing the ground state intermediate 3b . Finally, upon prolonged irradiation of ( 4a ), the methylene group is lost to yield pyrene. Compound 2b completes the series of all possible adducts of methylene to a C?C bond of pyrene. |
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