36.Photoisomerization Pathways of 8,16-Methano<2.2)metacyclophane-1,9-diene. A Model Case for Adiabatic Electrocyclic Ring Closure in the Excited Singlet State

Article abstract of DOI:10.1002/hlca.19840670137

The title compound 1b ideally meets the theoretical requirements for the occurrence of an adiabatic photoisomerization in the lowest excited singlet (¹1b*) and, indeed the predominant primary photoreaction observed is the conversion to its fluorescent valence isomer 10b,10c-methano-cis-10b,10c-dihydropyrene (¹1a*).The mechanism for the formation of the previousley observed photoproduct 8b,9a-dihydro-9H-cyclopropapyrene (4e) has been analyzed in some detail (Scheme).Below -30 deg C the reaction path consist of a tree quantum process (two di-?-methane rearrangements and a photochemical 1,7-H shift) involving two thermally stable, but light-sensitive isomers, 8,11b-methanocyclodecanaphthalene (2b) and 9H-cyclohepta-phenantrene (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.