Photoreactions of trans ‐10b,10c‐Dimethyl‐10b,10c‐dihydropyrenes

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 .