Photodeoxygenation of Dibenzothiophene S-Oxide Derivatives in Aqueous Media

The use of atomic oxygen (O((3)P)) as potent oxidant in water has suffered from the lack of a facile, efficient source. The photodeoxygenation of aromatic sulfoxides to the corresponding sulfides in organic solvents has been suggested to produce O((3)P) in low quantum yields. The photolysis of 4,6-dihydroxymethyldibenzothiophene S-oxide and 2,8-dihydroxymethyldibenzothiophene S-oxide in water results in deoxygenation at significantly higher quantum yields than in organic solvents. Depending upon conditions, a variable amount of oxidation of the hydroxymethyl substituent into an aldehyde was observed to accompany deoxygenation. Analysis of the photoproducts indicated the deoxygenation occurred by at least two different pH-sensitive mechanisms. Under basic conditions, photoinduced electron transfer yielding a hydroxysulfuranyl radical that decomposed by heterolytic S-O cleavage was thermodynamically feasible. The thermodynamics of photoinduced electron transfer were expected to become increasingly unfavorable as the pH of the solution decreased. Thus, at neutral and acidic pH, an S-O bond scission mechanism was suspected. The observed increase in the photodeoxygenation quantum yields was consistent with charge separation accompanying S-O bond scission. Oxidative cleavage of alkenes in aerobic conditions suggested O((3)P) was produced during photolysis in these conditions; however, the formation of discrete O(*-)/HO(*) may occur, particularly at low pH.