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In-Depth Information
R"
R'
"R
R'
R'
R"
2
H
+
CH
2
N
2
−
2e
−
,
−
N
HN
N
N
NN
N
N
NN
N
N
Cl
R
R
R
9
13
6
Scheme 7.4
Collectively, the methods for making Kuhn verdazyls from formazans are broadly applicable, facilitated
in large part by the availability of a large number of the precursor formazans.
10
The R group in the 3-
position of the radical can be hydrogen, alkyl, aryl, cyano, nitro, sugars, carbonyl compounds, and so on.
The nitrogen substituents R
and R
are usually aromatic. A variety of Kuhn verdazyl-based di-, tri-, and
tetra-radicals have been prepared (Table 7.1) wherein the point of verdazyl attachment can be C3, C6 or
one of the nitrogen atoms. The formazan methodologies have also been used in the synthesis of polymers
containing verdazyls, either as pendant groups or incorporated into the polymer backbone (Figure 7.1).
7.2.1.2 Verdazyls from hydrazides and bis-hydrazides: 6-oxoverdazyls
Neugebauer developed the synthesis of so-called “6-oxoverdazyls”
7
in which the C6 carbon is part of
a carbonyl group (Scheme 7.5). 2,4-Disubstituted bis(carbohydrazides)
16
40
condense with aldehydes to
give 1,2,4,5-tetrazane-6-ones
17
which can then be oxidized to the 6-oxoverdazyls
7
.
41,42
Several oxidants
can be used in the final step, including silver(I) oxide (Ag
2
O), tripotassium hexacyanoferrate (K
3
Fe(CN)
6
),
lead(IV) oxide (PbO
2
), periodate, and benzoquinone.
Certain bis(hydrazides)
16
(R
methyl, CH
2
Ph) were later found to be accessible directly from the
monoalkylhydrazines (eliminating the need to prepare the hydrazone
14
and subsequent NH
2
deprotection
of bis-benzylidene intermediate
15
) either by performing the reaction at low temperatures
43
=
and/or by
using triphosgene (bis(trichloromethyl)carbonate) as a phosgene alternative (Scheme 7.6).
44
Neugebauer has made 6-thioxoverdazyls
8
by using thiophosgene in place of phosgene (Scheme 7.7).
43
Thus, dehydrogenation of the 6-thioxotetrazanes
18
gives 6-thioxoverdazyls
8
.
42
An additional feature of
the thioxotetrazanes
18
consists of their desulfurization to the corresponding tetrazanes
19
with a saturated
carbon at C6. Subsequent oxidation gives the only example of verdazyls of structure
6
in which the two
substituents on the nitrogen atoms are alkyl groups. Although radicals
6
are persistent they cannot be
isolated.
45
The main limitation of the syntheses described in Schemes 7.5 - 7.7 is the range of bis-hydrazide reagents,
which until recently was confined to R
=
methyl or benzyl. Brook has recently developed a method for
making 2,4-bis(isopropyl)hydrazide
23
as shown in Scheme 7.8, which provides entry into N,N-diisopropyl
substituted 6-oxoverdazyls.
46
Neugebauer
47
and Milcent
48
have expanded the repertoire of verdazyl N-substituents by constructing
the tetrazane ring in a step-wise manner (Scheme 7.9). Careful reaction of a hydrazone
24
with phosgene
or thiophosgene gives the N-chloroformylhydrazone
25
(X
O, S), which is subsequently treated with
a monosubstituted hydrazine to give tetrazane
26
; oxidation using standard protocols gives 6-oxo or 6-
thioxoverdazyls. This synthetic route considerably expands the possible derivatives of
7
and
8
by allowing
for (1) aromatic substituents and (2) differential substitution on the two nitrogen atoms.
Overall, a wide range of 6-oxo (and thioxo)verdazyl derivatives has been prepared. All three substituents
(R, R
,R
) can be alkyl or aryl and the Milcent procedure offers the opportunity to prepare radicals with
different N1/N5 substituents. Some diradicals based on 6-oxoverdazyls have been studied in which the
=
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