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In-Depth Information
efficient nitronyl nitroxide radicals to prepare metal - organic free radical complexes exhibiting interesting
ferromagnetic properties.
5.5 Synthesis of nitroxides
An impressive number of different stable nitroxides have been reported in the literature, and there are
several topics and reviews which cover early as well as more recent progress of the synthetic aspect of
nitroxide chemistry.
9,11a
-
d,117
The synthesis of nitroxides R
1
N(
•
O)R
2
are carried out through different
synthetic routes that depend on the nature of the targeted R
1
and R
2
groups. However, in almost all the
reported preparation of nitroxides, the aminoxyl group is introduced through an oxidation step involving
either an amine or a hydroxylamine.
5.5.1 Oxidation of amines
Peracids,
117,118,9
dimethyldioxirane,
119
oxone,
120
and hydrogen peroxide
121
are widely used to oxidize
secondary amines to nitroxides. Direct oxidation of tertiary amines to nitroxides
9,30a,122
is generally carried
out with mCPBA or H
2
O
2
/sodium tungstate (Na
2
WO
4
)
(Scheme 5.23).
5.5.2 Oxidation of hydroxylamines
Hydroxylamines can be easily oxidized to nitroxides. The main strategies to form an aminoxyl group
through the oxidation of an N-hydroxy intermediate are shown in Scheme 5.24.
123 - 125
When different
reactions must be carried out before the oxidation step, the hydroxylamine can be protected either by
O-acetylation
126
or O-silylation with
tert
-butylchlorodimethylsilane.
28c,127
5.5.3 Chiral nitroxides
Optically active
30,128
nitroxides and nitroxides for the control of free radical polymerization
22,23,129
have
been also intensively studied and some results concerning their synthesis are presented here.
Tamura
et al
.
130
developed a new synthesis of
α
-asymmetric bicyclic nitroxide radicals,
64
, via the
reduction with samarium diiodide (SmI
2
)
of homoallylic nitroenones and the subsequent addition of
electrophiles (Scheme 5.25). Rychnovsky
et al
.
131
prepared several chiral piperazine and morpholine
oxidation of secondary amines
R
1
R
1
[Ox]
N
H
N
O
oxidation of tertiary amines
R
2
R
2
[Ox] = MeCO
3
H, mCPBA, DMD, oxone, H
2
O
2
/Na
2
WO
4
O
2
N
O
2
N
mCPBA
N
Bn
N
O
CH
2
Cl
2
24h
O
O
Oxone, acetone / CH
2
Cl
2
Na
2
HPO
4
,nBu
4
N
+
HSO
4
−
pH 7.5
90 %
59
122a
−
8. (M. E. Brik
120
)
N
N
75 %
H
O
Scheme 5.23
Oxidation of amines to nitroxides
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