Abstract
The photoisomerization of trans‐10b,10c‐dimethyl‐10b,10c‐dihydropyrene (1), its 2,7‐di‐tert‐butyl derivative 5 and a series of potassium sulfonate derivatives of 1, in both [D~8~]1,4‐dioxane and [D~2~]dichloromethane as solvent, has been studied using λ = 420–570 nm irradiation. Irradiation of a deoxygenated solution of 1 in dioxane leads to the exclusive formation of anti‐8,16‐dimethyl[2.2]metacyclophane‐1,9‐diene (2), whereas the photoconversion of 1 in a non‐degassed solution leads to the successive formation of 2 and 5,8‐epidioxy‐anti‐8,16‐dimethyl[2.2]metacyclophane‐1,9‐diene (7). Similarly, irradiation of the 2,7‐di‐tert‐butyl derivative 5 in oxygen containing [D~8~]dioxane as solvent leads to the consecutive formation of the corresponding 5,13‐di‐tert‐butyl derivatives 6 and 8. Irradiation of mixtures of potassium sulfonate derivatives of 1 (1‐S~n~) in D~2~O as solvent leads to photoisomerization with formation of the correspondingly substituted sulfonate derivatives of 2. Only with the dipotassium 2,4‐ and 2,5‐disulfonates of 1 eventually small amounts of the corresponding epidioxy‐bridged products 7 were formed. The photoisomerization 1‐S~n~ → 2‐S~n~ appeared to be most effective for 1‐2‐S and 1‐2,5‐S~2~ and least effective for 1‐2,4,7‐S~3~ and 1‐2,4,7,10‐S~4~. In the dark, the epidioxy products 7 and 8 reverted to give initially the corresponding metacyclophanes 2 and 6, and subsequently the respective dihydropyrenes 1 and 5, and the dark reactions of the potassium sulfonate derivatives 2‐S~n~ yielded the correspondingly substituted 1‐S~n~. A quantitative study showed that the dark reactions steps 7 → 2, 8 → 6, and 2‐S~n~ → 1‐S~n~ (n = 1–4) exhibit first‐order kinetics. For the deoxygenation of 5,8‐epidioxy‐anti‐8,16‐di‐methyl[2.2]metacyclophane‐1,9‐diene (7) the rate coefficient is ca. 2.7 times that of its 5,13‐di‐tert‐butyl derivative (8), and the rate coefficient for the isomerization 2‐S~n~ → 1‐S~n~ follows the order 2‐5‐S → 2‐5,13‐S~2~ > 2‐1,5,10‐S~3~.