PHOTOCHEMISTRY OF 1-ALKOXY-9,10-ANTHRAQUINONES AND 1-(BENZYLOXY)-9,10-ANTHRAQUINONES IN METHANOL - A FACILE PROCESS FOR THE PREPARATION OF ALDEHYDES AND KETONES

The facile production of acid sensitive aldehydes and ketones via photochemical intramolecular delta-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-antharahydroquinones (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, Bu, 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 a-position. These rate constants are ascribed to equilibrium populations of confonmers in the geometry required for reaction in the n,pi* triplet state.