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Benzophenone‐Photosensitized Reactions of Xanthinic Compounds. A Mechanistic Study
Author(s) -
Murgida Daniel H.,
Aramendía Pedro F.,
ErraBalsells Rosa
Publication year - 1998
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1998.tb09083.x
Subject(s) - benzophenone , chemistry , photochemistry , combinatorial chemistry
Photosensitized reaction of xanthinic compounds (XH) as caffeine (CF), theobromine (TB) and theophylline (TF) by benzophenone (BZ) in ethanol solution was investigated. In the three cases four main reaction products (benzopinacol; diphenylcarbinol; 1,1‐diphenyl‐l,2‐propanediol and 8‐[1‐(1‐hydroxyethyl)] xanthine) were identified and then characterized by melting point, 1 H NMR, 13 C NMR and mass spectrometry. The quenching of trip‐let BZ by the three XH was detected and a thorough kinetic 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 support to the proposed reaction scheme. We demonstrate that the mechanism leading to XP formation does not proceed through the X. radical directly obtained by H abstraction. An alternative reaction path through an intermediate radical originated on the addition of ethanol radical to XH is proposed. Redox potentials for the oxidation of XH were estimated by cyclic voltametry and by using the Rehm‐Weller equation the redox quenching of triplet BZ by XH was discarded.