Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

200 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
R
N
H
N
+
O
228 229 230
R
−
N
R
N
NO
+
O
N
N
CH3
−
O
231 232
+
Fig. 2.19
R
−
N
R
N
NO 2
+
O
N
N
+
OCH3
has not been detected (428). In this paper, electrochemical oxidation potentials
are reported of 31 cyclic nitrones, derived from 4-R-3-imidazoline-3-oxide
(228), (230–232) (Fig. 2.19), in which Ep/2 undergoes wide-range changes from
1.36 to 2.58 V (ca Ep/2 DMPO 1.57 V, PBN 1.41 V) depending on substituents
R (Table 2.8). The effect of substituents R on Ep/2 can be described
by two-parameter correlation Equations 2.1 and 2.2 for N-NO (231) and N-NO2
(232) derivatives, respectively. In the 1-methyl-3-imidazoline-3-oxides (229) the
tert-amino group, in position 1, oxidizes rather than the nitrone group (428).
Ep/2 = 1,944 + 0,251 σI + 1,404 σR
Ep/2 = 2,009 + 0,829 σI + 0,768 σR
In the series of α-substituted nitrones, the α-methoxy nitrones are the most
easily oxidized nitrone derivatives. The study of electrochemical behavior of
acyclic α-methoxy-, α-amino-, α-cyano- and α-mercapto-nitrones has shown an
irreversible one-electron oxidation of the nitrone group (429).
Determination of electrochemical oxidation potentials and electrochemical
reduction of 13 β-phosphorylated acyclic nitrones shows that phosphorylated
compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40
to 2.00 V versus SCE in CH3CN) and reduction (Ep−0.94 to −2.06 V). This is
caused by a strong electron-acceptor inßuence of the diethoxyphosphoryl group
(430). In contrast, a reversible one-electron oxidation of azulene nitrones (233)
(Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value
one observes the oxidation of 4H -imidazole-1,3-dioxides (219) (428, 429). In
other words, the corresponding RC (234) is 14 kcal more stable than the RC of
PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from
dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170).
−
O
−