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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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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<jats:title>Abstract</jats:title><jats:p>The title compound<jats:bold>1b</jats:bold>ideally meets the theoretical requirements for the occurrence of an adiabatic photoisomerization in the lowest excited state (<jats:sup>1</jats:sup><jats:bold>1b</jats:bold>*) and, indeed, the predominant primary photoreaction observed is the conversion to its fluorescent valence isomer 10b, 10c‐methano‐<jats:italic>cis</jats:italic>‐10b 10c‐dihydropyrene (<jats:sup>1</jats:sup><jats:bold>1a</jats:bold>*). The mechanism for the formation often previously observed photoproduct 8b, 9a‐dihydro‐9<jats:italic>H</jats:italic>‐ cyclopropa[<jats:italic>e</jats:italic>]pyrene (<jats:bold>4a</jats:bold>) has been analyzed in some detail (<jats:italic>Scheme</jats:italic>). 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 (<jats:bold>2b</jats:bold>) and 9<jats:italic>H</jats:italic>‐cyclohepta[def]‐phenanthrene (<jats:bold>3b</jats:bold>). At room temperature the rearrangement<jats:bold>2b→4a</jats:bold>proceeds with a single excitation step bypassing the ground state intermediate<jats:bold>3b</jats:bold>. Finally, upon prolonged irradiation of (<jats:bold>4a</jats:bold>), the methylene group is lost to yield pyrene. Compound<jats:bold>2b</jats:bold>completes the series of all possible adducts of methylene to a CC bond of pyrene.</jats:p>