Chemistry Reference
In-Depth Information
10.2.3
Reactions of SO
4
•
−
/PO
4
•
2
−
-Adduct Radicals, Radical Cations
and Heteroatom-Centered Radicals
Radical cations are strongly oxidizing intermediates, but also after deproton-
ation at a heteroatom (in the present systems at nitrogen) some of this oxidizing
property remains. Thus a common feature of these intermediates is that they are
readily reduced by good electron donors. Since the heteroatom-centered radi-
cals and the radical cations are always in equilibrium, it is, at least in principle,
possible that such intermediates react with water at another site (canonical me-
someric form), that is
at carbon. This reaction leads to
•
OH-adduct radicals. Al-
though deprotonation at a heteroatom is usually faster (but also reversible) than
deprotonation at carbon, the latter reaction is typically "irreversible". This also
holds for a deprotonation at methyl (in Thy).
Pyrimidines.
Reaction of Thy with photoexited menadione or its electrochemi-
cal oxidation yields mainly to the
N
(1)
C
(5)-linked dimer
(Hatta et al. 2001).
This can be accounted for if the precursor radical cation deprotonates at
N
(1)
(see above). For this
N
(1)-centered radical a second mesomeric form with the
spin at
C
(5) can be written. Head-to-tail recombination leads to the isopyrimi-
dine-type dimer [reaction (12)]. Isopyrimdines are unstable (see below) and
add rapidly water [reaction (13)]. This dimer is also formed in the reaction with
SO
4
•
−
, albeit with a lower yield.
−
Upon electrochemical oxidation, dimers resulting from a recombination at
C
(5)
are formed as well. Whether this route is mediated by the electrode surface or
due to a recombination of a radical cation with an
N
-centered radical (note that
there is a high radical density at the electrode surface and that the radical cat-
ion (p
K
a
= 3.2) has a lifetime of
0.5 µs), must remain speculation. To a smaller
extent, a further dimer is also observed. It may arise by an addition/oxidation
∼
Search WWH ::
Custom Search