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SummaryIrradiation of pale yellow 5-methyl-l,4-naphthoquinone (1, Scheme I ) yields the blue photoenol 4-hydroxy-5-methylidene-1(5 H)-naphthalenone (2) which is stable at 77 K, At room temperature the enol retautomerizes to starting material, the reaction rate being strongly dependent on the hydrogen-bond-acceptor basicity of the solvent. The enol is trapped in the presence of acid by protonation at the remaining carbonyl oxygen atom and subsequent electrophilic reaction of the exocyclic methylene group.

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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.Introduction. -The homologization of aromatic hydrocarbons by the replacement of one or more C=C subunits by cyclopropane rings has been a rich source of exciting new compounds. The addition of methylene to pyrene is formally possible at six nonequivalent C=C bonds giving rise to six constitutionally isomeric cyclopropa-dihydropyrenes ( l a to 6a). Each of these is associated with a ring-opened valence tautomer (lb to 6b) in analogy to the well-known norcaradiene (7a) -cycloheptatriene (7b) pair. In practice, the above reaction yields the single isomer 8b,9a-dihydro-9H-cyclopropra[e]pyrene (4a) [ 13. Four other isomers, 9H-~yclohepta[deJ]phenanthrene (3b) [2], 8,16-methano[2.2]metacyclophane-1,9-diene (lb) [3] and a mixture of 6H-cyclohepta[cd]phenalene (5h) and 7H-~yclohepta[cd]phenalene (6b) [4] have been synthesized by different methods. The preparative photolysis of l h has yielded the isomer 4a in near quantitative yield; the valence isomer lob, 10c-methano-&-lob, I0c-dihydropyrene (la) was proposed as a plausible intermediate [3]. The radical anion of 1 has been observed by ESR spectroscopy, but an unambiguous identification of its structure (a or b) has not been possible; it converts readily to the radical anion of pyrene [5a, b]. *) We are most grateful to Prof. Huns Fritz, Ciba-Geigy SA, Basel, for providing this spectrum and for valuable assistance in the analysis.