BENZOPHENONE-PHOTOSENSITIZED REACTIONS OF XANTHINIC COMPOUNDS - A MECHANISTIC STUDY

Photosensitized reaction of xanthinic compounds (XH) as caffeine (CF), theobromine (TB) and theophylline (TF) by benzophenone (BZ) in ethano l solution was investigated. In the three cases four main reaction pro ducts (benzopinacol; diphenylcarbinol; 1,1-diphenyl-1,2-propanediol an d 8-[1-(1-hydroxyethyl)] xanthine) were identified and then characteri zed by melting point, H-1 NMR, C-13 NMR and mass spectrometry. The que nching of triplet BZ by the three XH was detected and a thorough kinet ic analysis was performed. Caffeine produces mainly physical quenching , while TF reacts by N-H hydrogen abstraction. For TB both mechanisms are operative. Heats of reactions were calculated for chosen reactive steps of the mechanism by the PM3 method. They provide additional supp ort to the proposed reaction scheme. We demonstrate that the mechanism leading to XP formation does not proceed through the X . radical dire ctly obtained by H abstraction. An alternative reaction path through a n intermediate radical originated on the addition of ethanol radical t o XH is proposed. Redox potentials for the oxidation of XH were estima ted by cyclic voltametry and by using the Rehm-Weller equation the red ox quenching of triplet BZ by XH was discarded.