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
✍ Scribed by Jakob Wirz; Gabriele Persy; Erika Rommel; Ichiro Murata; Kazuhiro Naksuji
- Publisher
- John Wiley and Sons
- Year
- 1984
- Tongue
- German
- Weight
- 811 KB
- Volume
- 67
- Category
- Article
- ISSN
- 0018-019X
No coin nor oath required. For personal study only.
✦ Synopsis
The title compound l b ideally meets the theoretical requirements for the occurrence of an adiabatic photoisomerization in the lowest excited singlet state ('lb*) and, indeed, the predominant primary photoreaction observed is the conversion to its fluorescent valence isomer 10b,10c-methano-cis-lOb,lOc-dihydropyrene ('la*). The mechanism for the formation of the previously observed photoproduct 8b,9a-dihydro-9Hcyclopropa[e]pyrene (4a) has been analyzed in some detail (Scheme). Below -30°C the reaction path consists of a three quantum process (two di-n-methane rearrangements and a photochemical 1,7-H shift) involving two thermally stable, but light-sensitive isomers, 8,11b-methanocyclodeca[cde]naphthalene (2b) and 9H-cyclohepta[deflphenanthrene (3b). At room temperature the rearrangement 2h-+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.
') In the patent literature [15b, c] these derivatives are described as blue, sublimable solids. It is likely that the blue colour arose from the presence of impurities, e.g. derivatives of 15a or 16a.
*) We are most grateful to Prof. Huns Fritz, Ciba-Geigy SA, Basel, for providing this spectrum and for valuable assistance in the analysis.