Photochemistry of 1-Alkoxy- and 1-(Benzyloxy)-9,10-anthraquinones in Methanol: A Facile Process for the Preparation of Aldehydes and Ketones

Article abstract of DOI:10.1021/jo00126a040

The facile production of acid sensitive aldehydes and ketones via photochemical intramolecular δ-hydrogen atom transfer in 1-alkoxy- and 1-(benzyloxy)-9,10-anthraquinones (1) was investigated.Irradiation of 1 in argon purged methanol generates the primary photoproducts, 1-(RCH(OMe)O)- and 1-(ArCH(OMe)O)-9,10-anthrahydroquinones (2), respectively.Upon exposure to air, the intermediate anthrahydroquinone is rapidly converted to the corresponding aldehyde and 1-hydroxy-9,10-anthraquinone (3), which can be recycled.Aldehydes containing an acetal or ketal were prepared in high yields using this photoprocess.Apparent rate constants for the photodemethylation of 1-methoxy-2-X-9,10-anthraquinones (X = H, Me, Et, Pr, i-Bu, and benzyl) were measured and found to vary by a factor of 10 separating the slowest anthraquinone (X = H) and the fastest (X = benzyl), indicating a strong dependency upon the size of the substituent at the 2-position.These rate constants are ascribed to equilibrium populations of conformers in the geometry required for the reaction in the n,?^* triplet state.