A sterically controlled recyclable system: reversible photoredox reactions between anthraquinone and hindered tertiary amines

Photochemical reactions of 9,10-anthraquinone (AQ) with sterically hindered tertiary amines have been studied. The reactivity and products are strongly dependent on the structure of the tertiary amine. Irradiation of AQ in the presence of the sterically hindered amine 1,2,2,6,6-pentamethyl-4-piperidinol (3) (or 1,2,2,6,6-pentamethylpiperidine (5)) in degassed dry benzene leads chiefly to the formation of 9-hydroxy-9-[N-(2,2,6,6-tetramethyl-4-piperidinol)-methyl]anthr one (8), which is found to be metastable at room temperature under vacuum. The reaction may be thermally reversed to regenerate the starting materials. The photolysis products and thermal reversion are solvent dependent. While in dry benzene adduct 8 is the major product, in dry acetonitrile the ionic redox products AQH-and iminium cation are detected and no thermal reversal occurs. The results are explained in terms of equilibrium between a product ion pair (AQH[sup [minus]], iminium[sup +]) and a metastable adduct 8. In the presence of water, the reaction leads to the formation of a stable product, 9-hydroxy-9-(hydroxymethyl)anthrone (6). Different reactivity is observed upon irradiation of AQ in the presence of the sterically less hindered tertiary amine trans-tropine (1). 47 refs., 4 figs., 3 tabs.