Photochemical synthesis of stable peroxides

A procedure was developed by which 9-phenylxanthene could be readily photooxidised to produce 9-hydroperoxy-9-phenylxanthene in good yield. Initially the effect of different lamps was investigated to find a suitable light source. This was followed by optimizing the conditions required to maximise the hydroperoxide yield and to minimise its decomposition. The reaction had an induction period which was overcome by the addition of the sensitizer benzophenone. The mechanism by which the photooxidation was sensitised was investigated and it was found that no singlet oxygen was involved but that triplet benzophenone brought about hydrogen atom abstraction from the substrate followed by hydroperoxide formation. A series of 9-aryl and 3-substituted-xanthenes were synthesised and photooxidised. However it was found that the substituents did not influence the rate of photooxidation. Also the influence of the ether oxygen of xanthene was determined by comparing a number of analogues having either no atom, a carbon, nitrogen or sulphur atom in the 10-position. It was found that the rate of photooxidation was retarded for these analogues when compared to 9-phenylxanthene except for 9,10-diphenylacridan which was enhanced. However the formed hydroperoxides were all less stable than 9-hydroperoxy-9-phenylxanthene