Chemistry Reference
In-Depth Information
H
N
N
+2H
2
NNAr
2
HC(OEt
3
)
Ar
2
N
NAr
2
H
78
H
2
PbO
2
Ar
2
NH
+
∆
H
+
N
N
N
N
Ar
2
N
NAr
2
78
+
Ar
2
N
NAr
2
H
H
H
N
N
NAr
2
Ar
2
N
N
N
81
79
H
80
+???
Scheme 7.28
these radicals accelerates the decomposition process, affording diarylamines, azo compounds
80
, and other
unidentifiable products. Dissolving the radicals in formic acid causes disproproportionation to the parent
tetrazapentene
78
and a cyanine-like cation
81
(Scheme 7.28).
The EPR spectra of radicals
79
are quite complex; assignment of hyperfine coupling constants required
isotopically enriched (
2
H on some/all of the aryl positions,
15
N on the “inner” nitrogen atoms) samples.
141
These labeling studies, coupled with McLachlan molecular orbital calculations, permitted assignment of
the larger spin density values to the “inner” nitrogen atoms (
ρ
∼
0.26 each) and the smaller spin density
to the “outer” nitrogen atoms (
0.14 each); the outer nitrogen substituents possess a small amount of
spin and the central methine carbon carries a small
negative
spin density (
ρ
∼
ρ
∼−
0.08) which arises from
spin polarization effects.
7.4 Tetrazolinyl radicals
Photolysis of 2,3-diaryltetrazolium cations (
13
) causes an intramolecular aryl - aryl coupling reaction to
occur, resulting in the fused “phototetrazolium” cations
82
in high yield (Scheme 7.29).
142
The aromatic
groups include substituted phenyl
143,144
and pyridyl,
145
the latter leading to azabiphenylene - tetrazolium
salts. These cations can be reduced to stable radicals
83
(“Kuhn - Jerchel” radicals
146
) with
sodium/ammonia, sodium amalgam, dithionite, hypophosphite, or tin(II) chloride (SnCl
2
).
143 - 145
Harsher
reducing agents (e.g., Raney nickel) or acid fragments the radical to a diazaphenanthrene
84
, but otherwise
many of these fused tetrazolinyl radicals are stable and isolable as crystalline compounds.
Kuhn - Jerchel radicals
83
are a subset of the more general class of tetrazolinyl radicals
85
whose obser-
vation and elucidation were developed independently. The formazan - tetrazolium redox couple
10
has found
widespread use as a redox indicator in cell biology.
11
Both species can give rise to unusually strong EPR
signals, in solid form as well as in solution, which were shown to arise from a paramagnetic impurity.
147
The EPR active species was later identified as a tetrazolinyl radical
85
, essentially a redox form intermedi-
ate between formazan and tetrazolium salt.
148
Accordingly, tetrazolinyls
85
can be made by (i) reduction
of the corresponding tetrazolium salts (by analogy to Kuhn - Jerchel radicals
83
) (Scheme 7.30),
148 - 150
(ii) oxidation of formazans (oxygen and base; diarylaminyl radicals generated from a tetraarylhydrazine;
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