Oxidation vs. fragmentation in radiosensitization. Reactions of α-alkoxyalkyl radicals with 4-nitrobenzonitrile and oxygen. A pulse radiolysis and product analysis study
α-Monoalkoxyalkyl radicals produced from 1,4-dioxane (100%), 1,3-dioxane (56%), tetrahydrofuran (92%) and dimethyl ether (100%) by H-abstraction by hydroxyl radicals generated in the radiolysis of water were found to react with 4-nitrobenzonitrile (NBN) by addition to give N-alkoxyaminoxyl-type radicals, which have absorption maxima at about 310 nm and decay very slowly (k= 0.4 – 1.0 s–1). On the other hand, the reaction of the α-dialkoxyalkyl radical, 1,3-dioxan-2-yl 3[from the reaction of hydroxyl radicals with 1,3-dioxane (32%)] with NBN leads to the rapid formation of the radical anion NBN˙– .
The N-alkoxyaminoxyl-type radicals (A in the case of 1,4-dioxane and D in the case of dimethyl ether) react with ascorbate (k≈ 2 × 104 dm3 mol–1 s–1). They have a very low reactivity with oxygen (k < 103 dm3 mol–1 s–1 in the case of tetrahydrofuran). On the other hand, they are rapidly reduced by NBN˙–(k≈ 109 dm3 mol–1 s–1 as observed with A and with B derived from 1,3-dioxane).
The products [G values (in parenthesis) in units of 10–7 mol J–1] in the γ-radiolysis of N2O-saturated solution of 1,4-dioxane in the presence of NBN are 1,4-dioxan-2-one (0.3), 2-hydroxy-1,4-dioxane (2.5), ethane-1,2-diol monoformate (2.1), ethane-1,2-diol diformate (0.7), formaldehyde (2.1), 4-nitrosobenzonitrile and other reduction products of 4-nitrobenzonitrile. These products are accounted for as resulting from the fragmentation of the aminoxyl radical A by (a) heterolysis of the C–O bond (45%, leading to the one-electron oxidation of the 1,4-dioxan-2-yl radical) and (b) homolysis of the N–O bond (55%, leading to the formation of the 1,4-dioxan-2-oxyl radical which undergoes further fragmentation).
The products from the reaction of methoxymethyl radicals with NBN under γ-radiolysis conditions are formaldehyde (5.7), methanol (2.5) and methyl formate (1.3). It is concluded that also in this case the decay of the aminoxyl radical D occurs by two pathways: the heterolysis route (46%) and the homolysis route (54%).
In the presence of oxygen the 1,4-dioxan-2-yl radicals are converted into the corresponding peroxyl radicals. Their bimolecular decay (2k= 2.0 × 108 dm3 mol–1s –1) yields the same products as in the case of NBN (albeit with a different product distribution and the formation of some peroxides): 1,4-dioxan-2-one (0.4), 2-hydroxy-1,4-dioxane (0.4), ethane-1,2-diol monoformate (0.6), ethane-1,2-diol diformate (2.8) and formaldehyde (0.6).
These results indicate that fragmentation reactions involving the carbon-skeleton of organic radicals are important not only in the case of peroxyl radicals but they can also be induced by nitroaromatic sensitizers. In cells, reduction of the long-lived sensitizes adduct radicals by reducing agents such as ascorbate to give (toxic) hydroxylamine type products may compete with the homolytic or heterolytic fragmentation of the N-alkoxyaminoxyl radicals.