Chemistry Reference
In-Depth Information
When
•
OH is generated by the Fenton reaction, i.e. in the presence of H
2
O
2
and
Fe
2+
/Fe
3+
, the transition metal ions (and possibly H
2
O
2
; in fact, photolysis of H
2
O
2
in the presence of Ura and Thy is a very convenient way of preparing their gly-
cols in good preparative yields; Hahn and Wang 1977) undergo redox reactions
with the radicals that are formed by
•
OH-attack. In the case of 1,3Me
2
Ura, the
otherwise dominating product, the dimers (Table 10.12), are no longer formed.
Instead, the 1,3Me
2
Ura glycol is now practically the only product (Theruvathu et
al. 2001). It is formed in a short chain reaction; 2.7 mol 1,3Me
2
Ura are consumed
and glycol formed for 1 mol of Fe
2+
consumed. The major radical, the reducing
C
(5)-
•
OH-adduct is oxidized by Fe
3+
[reaction (89)] thereby regenerating Fe
2+
which give rise to further
•
OH upon its reaction with H
2
O
2
(Chap. 2.5).
The glycol is also the most important product when Ura is
-irradiated in aque-
ous solution in the presence of Fe
3+
(Bhattacharyya and Mandal 1983).
In
•
OH-induced reactions of Thy, Tg and 5-hydroxymethyluracil yields are
markedly enhanced in the presence of nitroxyl radicals such as TEMPO (Kagiya
et al. 1983). Adducts may be the intermediates. In the case of the nitroxyl radical
TAN and for the Thd system, such adducts have been characterized (Berger et
al. 1985).
The Tg yield is also substantially enhanced in the presence of electron-af-
finic sensitizers, and in the radiolysis of N
2
O-saturated solutions its yield has
been reported to increase from
G
= 0.32
γ
10
−7
mol J
−1
in the absence to
G
= 1.6
×
10
−7
mol J
−1
in the presence of misonidazole (Nishimoto et al. 1983a). It has
been emphasized that electron-afficinic sensitizers do not necessarily undergo
an outer-sphere ET, but may react by addition (Wardman 1987). This type of
reaction has been investigated in detail using nitroaromatics as oxidants (Jagan-
nadham and Steenken 1984, 1988a,b; Steenken and Jagannadham 1985). When
the adducts hydrolyze (possibly in competition with other reactions, since the Tg
yield depends on the oxidant; Nishimoto et al. 1983a), the products are identical
to the ones expected to be formed by ET [reactions (90) and (91)].
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