Chemistry Reference
In-Depth Information
R''''
R''''
R''''
R'''
R'''
R'''
R'''R''''CHX
base
R'
R"
R'
R"
R'
R"
R'
R"
[ox]
NH
NN
N
N
NN
N
N
N
N
N
NN
N
H
R
R
R
R
9
10
11
6
Scheme 7.1
H
H
NR'''
2
R'''
R'''CHO
base
H
2
CO
R'''
2
NH
"R
R'
"R
R'
R'
R"
N
N
N
N
HN
N
N
NN
N
N
N
R
R
R
5
9
5
Scheme 7.2
occasionally be isolated, and then heated to proceed to the leuco verdazyl and, ultimately (in the presence
of an oxidant), the verdazyl.
12,14,15
A variation on the formazan synthesis involves their reactions with forma1dehyde or aliphatic aldehy-
des under basic conditions (Scheme 7.2). This reaction presumably also proceeds via a leuco intermediate,
based on the observation that oxygen is required for radical formation. If formaldehyde is used with an
equivalent amount of a secondary amine (e.g., pyrrolidine, piperidine) then 6-aminoverdazyls are produced
(Scheme 7.2).
16,17
Reactions of formazans with formaldehyde under
acidic
conditions give cationic ver-
dazylium salts
12
, which can then be reduced to the radicals (Scheme 7.3).
15,18 - 20
The verdazylium salts
can be generated
in situ
12,21
and reduced or they can be isolated.
22
The reduction step can be carried
out with formaldehyde and base,
21,23
or with ascorbic acid
22
; the latter reagent can, depending on the
solvent system, reduce the cation either to the radical
5
or to the leuco compound
11
, which can then be
air-oxidized to the radical.
An alternative route to verdazyls from formazans was reported by Kuhn, Neugebauer
et al
.inwhich
triphenyltetrazolium cation
13
(R
R
R
phenyl; the product of oxidation of the correspond-
ing formazan
10
) was converted to triphenylverdazyl by reaction with diazomethane (Scheme 7.4).
16
=
=
=
This
interesting transformation has not been further exploited.
H
H
H
H
CH
2
O
H
+
R'
R"
R'
R"
R'
R"
NH
NN
N
N
NN
N
[red]
N
NN
N
R
R
R
9
12
5
Scheme 7.3
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