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 lambda = 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 D2O 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-dimethyl[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 approximate to 2-5,13-S-2 > 2-1,5,10-S-3.