Preparation of Blatter Radicals via Aza-Wittig Chemistry: The Reaction of N-Aryliminophosphoranes with 1-(Het)aroyl-2-aryldiazenes

Article abstract of DOI:10.1021/acs.joc.7b01297

Reacting N-aryliminophosphoranes with 1-(het)aroyl-2-aryldiazenes in preheated diphenyl ether at ca. 150-250 °C for 5-25 min affords in most cases the 1,3-diaryl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yls (aka Blatter radicals) in moderate to good yields. Al

Full text of DOI:10.1021/acs.joc.7b01297

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Article
Preparation of Blatter Radicals via Aza-Wittig Chemistry: The Reaction
of N Aryliminophosphoranes with 1-(Het)aroyl-2-aryldiazenes
Anastasia C. Savva, Styliana I. Mirallai, Georgia A. Zissimou, Andrey A.
Berezin, Marina Demetriades, Andreas Kourtellaris, Christos P. Constantinides,
Constantinos Nicolaides, Theodossis Trypiniotis, and Panayiotis A. Koutentis
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017
Downloaded from http://pubs.acs.org on June 20, 2017
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Preparation of Blatter Radicals via Aza-Wittig Chemistry: The Reaction of
N-Aryliminophosphoranes with 1-(Het)aroyl-2-aryldiazenes
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Anastasia C. Savva,1 Styliana I. Mirallai,1 Georgia A. Zissimou,1 Andrey A. Berezin,1 Marina
Demetriades,1 Andreas Kourtellaris,1 Christos P. Constantinides,1 Constantinos Nicolaides,2
Theodossis Trypiniotis,2 Panayiotis A. Koutentis*,1
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1 Department of Chemistry, University of Cyprus, P. O. Box 20537, 1678 Nicosia, Cyprus
2 Department of Physics, University of Cyprus, P. O. Box 20537, 1678 Nicosia, Cyprus
eꢀmail: koutenti@ucy.ac.cy
Abstract. Reacting Nꢀaryliminophosphoranes with 1ꢀ(het)aroylꢀ2ꢀaryldiazenes in preheated
diphenyl ether at ca. 150ꢀ250 °C for 5ꢀ25 min, affords in most cases the 1,3ꢀdiarylꢀ1,4ꢀ
dihydrobenzo[e][1,2,4]triazinꢀ4ꢀyls (aka Blatter radicals) in moderate to good yields. All new
compounds are fully characterized, including for the radicals EPR and CV studies. Single
crystal xꢀray structures of 1ꢀbenzoylꢀ2ꢀ(perfluorophenyl)diazene and 1ꢀ(perfluorophenyl)ꢀ3ꢀ
phenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinyl are also presented.
Keywords: Stable organic radicals; AzaꢀWittig reaction; Nitrogen heterocycles; Blatter
radicals; Pentafluorophenyl.
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INTRODUCTION
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Blatter radical (1a), also known as 1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl, is an
air and moisture stable neutral organic radical, first reported in 1968, that shows reversible
redox behavior (Figure 1).1 In the presence of strongly oxidizing agents (e.g., MnO2 or
KMnO4) it gives the useful benzotriazinone 2,2 several analogues of which have interesting
biological properties,3 while with strongly reducing agents (e.g., Zn in AcOH) it gives 1,2ꢀ
diphenylbenzimidazole (3).4 Aside from an extensive EPR studies by Neugebauer,5 and later
by Kadirov,6 little else was reported until Wudl reignited interest by preparing a pressure senꢀ
sitive semiconducting charge transfer complex of Blatter radical 1a with 7,7,8,8ꢀ
tetracyanoquinodimethane (TCNQ)7 and the unusual zwitterionic tetraphenylhexaazaanthraꢀ
cene (TPHA) (Figure 1).8
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Figure 1. Structures of Blatter radical 1a with IUPAC numbering (in red), benzotriazinone 2,
1,2ꢀdiphenylbenzimidazole 3 and Wudl’s TPHA.
Since then, extensive studies have been reported on their interesting magnetic properties9 and
several new applications of Blatterꢀtype radicals have appeared, e.g., as ligands for metal coꢀ
ordination,10 as polymerization initiators,11 as pH sensors,12 as discotic liquid crystals,13 as
components of organic radical batteries,14 as highꢀspin diradicals15 and biradicaloids.16 Furꢀ
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thermore, stable thin films composed of Blatterꢀtype radicals have been prepared highlighting
their potential as new materials in electronic devices.17
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Future development of Blatter radical chemistry and applications will benefit from improved
syntheses. Up until recently, the most commonly used synthesis of Blatter radicals involved
the oxidative cyclisation of amidrazones 4 via the intermediate 1,2,4ꢀtriazabutadienes 5
(Scheme 1),5 however, yields are highly dependent on the purity of the starting amidrazone 4
which can often be difficult to access and purify.
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Scheme 1. Neugebauer’s Classical Synthesis of Blatter Radicals 1
Syntheses that avoid the need to prepare amidrazones 4 include: the cyclodehydration of N'ꢀ
(2ꢀaminophenyl)ꢀN'ꢀarylbenzohydrazides 6, prepared from Nꢀarylbenzohydrazides and either
1ꢀhaloꢀ2ꢀnitroarenes via nucleophilic aromatic substitution followed by reduction of the nitro
group18 or from 2ꢀhaloanilines via a Cuꢀmediated CꢀN coupling protocol9b and the
regioselective addition of aryllithium agents to preformed benzo[e][1,24]triazines 719
(Scheme 2).
Scheme 2. Routes to Blatter Radicals 1 that Avoid the Need to Prepare Amidrazones 4
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While these syntheses are advantageous as they avoid difficult to purify and oxidatively
unstable amidrazones 4, both require often costly starting 1ꢀhaloꢀ2ꢀnitroarenes, particularly
when R3 ≠ H, and the use of organolithium reagents can be problematic when sensitive
functional groups (e.g., halogens) are present. Recently, a fascinating ring transformation of
the analytical reagent Nitron to a 3ꢀamidoꢀsubstituted benzotriazinyl20 and a route to planar
Blatterꢀtype radicals21 have also been described, but these syntheses have been limited to just
a few analogues and the synthetic usefulness of these new routes remains to be tested.
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Considering the above, we continue to develop routes to Blatterꢀtype radicals that offer adꢀ
vantages such as ease of access to cheap starting materials, speed, overall yield and applicaꢀ
bility. Herein, we report an azaꢀWittigꢀmediated22 synthesis of Blatter radicals 1 starting from
Nꢀaryliminophosphoranes 8 and 1ꢀ(het)aroylꢀ2ꢀdiazenes 10 that was inspired by Huisgen’s
1969 low yielding (5.5%)23 synthesis of 7ꢀmethoxyꢀ1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]ꢀ
triazinꢀ4ꢀyl (1b) starting from Nꢀ(4ꢀmethoxyphenyl)iminophosphorane 8b and diphenylnitrylꢀ
imine 9 (Scheme 3).
Scheme 3. The Proposed Aza-Wittig-mediated Synthesis of Blatter Radicals 1 and
Huisgen’s Synthesis via N-(4-Methoxyphenyl)iminophosphorane 8b and Diphenyl-
nitrylimine 9
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2.
RESULTS AND DISCUSSION
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Initially, efforts focused on the azaꢀWittig coupling of N′ꢀphenylbenzohydrazide (11a) and
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Nꢀphenyliminophosphorane 8a but these failed. Amides are heavily deactivated towards azaꢀ
Wittig reactions owing to the strong electron release from the amide nitrogen to the carbonyl.
To the best of our knowledge, only a few examples of intramolecular azaꢀWittig reactions
have been reported.24 Nevertheless, α,βꢀunsaturated ketones or aldehydes can undergo interꢀ
molecular azaꢀWittig reactions with relative ease to give the α,βꢀunsaturated imines,25 and
based on this we considered oxidizing the N′ꢀaryl(het)arenohydrazides 11 to 1ꢀ(het)aroylꢀ2ꢀ
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aryldiazenes 10 in the hope that these would be more reactive.
The proposed azaꢀWittig reaction protocol for preparing Blatterꢀtype radicals required access
to Nꢀaryliminophosphoranes 8 and 1ꢀ(het)aroylꢀ2ꢀaryldiazenes 10. The former were readily
prepared in oneꢀpot either from the Staudinger reaction26a via the azide, or the Kirsanov reacꢀ
tion,26b that involves the in situ preparation of triphenyldibromophosphorane, or via direct
halogenation of Nꢀphenyliminophosphorane 8a.26c The latter were prepared via the mild oxiꢀ
dation of readily available N′ꢀaryl(het)arenohydrazides 11. Three methods are commonly
used for this oxidation and involve the use of HgO,27a NaNO2/Ac2O27b or NBS27c (Table 1).
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Table 1. Oxidation of N′-Aryl(het)arenohydrazides 11 to 1-(Het)aroyl-2-diazenes 10.
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entry
R1
R2
reagents
(equiv)
HgO (1)
solvent
temp
(°C)
20
20
0ꢀ20
20
20
0ꢀ20
20
20
20
time
(h)
3
5
0.5
3
3
0.5
5
5
5
yields 10
(%)
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9
Ph
Ph
Ph
Ph
Ph
Ph
Ph
C6F5
C6F5
Ph
Ph
Ph
nꢀhexane
acetone
CH2Cl2
nꢀhexane
nꢀhexane
CH2Cl2
acetone
acetone
acetone
10a (93)
10a (82)
10a (84)
10b (99)
10c (96)
10d (94)
10e (95)
10f (73)
10g (52)
NaNO2 (3)/Ac2O (3)
NBS (1)/pyridine (1.1)
HgO (1)
4ꢀTol
4ꢀFC6H4
4ꢀO2NC6H4
thienꢀ2ꢀyl
Ph
HgO (1)
NBS (1)/pyridine (1.1)
NaNO2 (3)/Ac2O (3)
NaNO2 (3)/Ac2O (3)
NaNO2 (3)/Ac2O (3)
thienꢀ2ꢀyl
Initially, all three oxidation protocols were screened for the conversion of N′ꢀphenylꢀ
benzohydrazide (11a) into 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (Table 1, entries 1ꢀ3), and in our
hands, the use of powdered yellow HgO (1 equiv) in nꢀhexane at ca. 20 °C for 3 h gave the
desired diazene 10a in the highest yield of 93% (Table 1, entry 1). These conditions also
worked well for the 4ꢀtolyl and 4ꢀfluorophenyl analogues affording the diazenes 10b and 10c
in 99 and 96% yields, respectively (Table 1, entries 4 & 5). However, for hydrazide 11d (R2
= 4ꢀO2NC6H4) the use of either HgO (1 equiv) or NaNO2 (3 equiv)/Ac2O (3 equiv) failed.
Fortunately, oxidation of this hydrazide was achieved using NBS (1 equiv) and pyridine (1.1
equiv) in CH2Cl2 at ca. 0ꢀ20 °C in 0.5ꢀ2 h to afford the desired diazene 10d in 94% yield
(Table 1, entry 6). The use of HgO (1 equiv) also failed to oxidize hydrazides bearing either
thienꢀ2ꢀyl groups at the carbonyl (e.g., hydrazides 11e & 11g) or Nꢀpentafluorophenyl groups
(e.g., hydrazides 11f & 11g) but, in these cases, oxidation with NaNO2 (3 equiv) in Ac2O (3
equiv) in acetone at 20 °C for 5 h gave the desired 1ꢀhetaroylꢀ2ꢀaryldiazenes 10e-g in 95, 73
and 52% yield, respectively (Table 1, entries 7ꢀ9). Attempts to oxidize hydrazides bearing
methyl, trifluoromethyl or pyridꢀ2ꢀyl groups at the carbonyl using these three protocols led to
complex reaction mixtures from which no diazene could be isolated (data not shown).
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Interestingly, a single crystal Xꢀray structure was obtained for one of the crystalline anaꢀ
logues 1ꢀbenzoylꢀ2ꢀ(perfluorophenyl)diazene (10f) (Figure 2). For the following discussion
on the geometry the crystallographic numbering is used.
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Figure 2. ORTEP view of 1ꢀbenzoylꢀ2ꢀ(perfluorophenyl)diazene (10f) (CCDC 1551626).
Crystallographic atom numbering showing. Hydrogens omitted for clarity. Ellipsoids at 50%
probability.
The structure, which was surprisingly non planar (torsion angle C6ꢀC7ꢀN2ꢀN1 = 153.8°),
supported a trans (E) geometry for the diazene [d(N=N) 1.238(2) Å] and a short carbonyl bond
of d(C=O) 1.199(3) Å which was typical of highly electrophilic non resonance stabilized carꢀ
bonyls [cf. cyclobutanone d(C=O) 1.198(1) Å]28 and dissimilar to typical α,βꢀunsaturated carꢀ
bonyl compounds [cf. C=CꢀC=O d(C=O) 1.222 Å].28 The short C=O bond length suggested the
diazenes 10 would be suitable partners for the proposed azaꢀWittig reaction.
The anticipated reaction sequence for the azaꢀWittigꢀmediated synthesis of Blatter radicals 1
was expected to proceed via an initial azaꢀWittig reaction to give the iminodiazenes 5, which
under the high reaction temperatures cyclize and prototautomerize to afford the benzotriaꢀ
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zines 12. These, on air oxidation or on treatment with alkali in air, should afford the desired
radicals 1 (Scheme 4).
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Scheme 4. Proposed Reaction Sequence for the Aza-Wittig-mediated Synthesis of Blat-
ter Radicals 1
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As predicted, the treatment of 1ꢀ(het)aroylꢀ2ꢀaryldiazenes 10 (1 equiv) with the
corresponding Nꢀaryliminophosphorane 8 (1 equiv) in preheated Ph2O (ca. 150ꢀ200 °C) for
5ꢀ25 min led in most cases to the desired benzotriazinyls 1 (Table 2).
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Table 2. Preparation of Blatter Radicals 1 via the Aza-Wittig Reaction.
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entry
R1
R2
R3
8/10
mol ratio
1:1
temp.
(°C)
200
200
200
200
200
200
200
200
200
160
200
200
160
250
160
200
200
200
160
150
150
150
time
(min)
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10
10
20
25
10
15
10
5
yields 1
(%)
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2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
MeO
Me
F3C
NC
O2N
F
Cl
Br
I
H
1a (80)
1b (ꢀ)a,b
1c (84)
1d (76)
1e (48)
1f (ꢀ)a,b
1g (71)a
1h (62)a
1i (57)a
1j (52)a
1k (75)a
1l (77)
2:1
1:1
1:1
1:1
2:1
2:1
2:1
2:1
2:1
2:1
1:1
2:1
1:1
2:1
1:1
1:1
Ph
5
4ꢀO2NC6H4
4ꢀTol
4ꢀTol
4ꢀFC6H4
4ꢀFC6H4
thienꢀ2ꢀyl
thienꢀ2ꢀyl
thienꢀ2ꢀyl
thienꢀ2ꢀyl
Ph
10
10
10
10
10
20
5
20
10
10
5
F3C
I
F3C
I
1m (57)a
1n (74)
1o (55)a
1p (65)
1q (82)
1r (81)a
1s (54)a
1t (61)
1u (70)
1v (30)
H
F3C
CN
I
H
F3C
F3C
Ph
Ph
C6F5
C6F5
C6F5
2:1
2:1
2:1
2:1
Ph
thienꢀ2ꢀyl
2:1
5
a Under Ar atmosphere. b Decomposition; mainly 1,3ꢀdiphenylꢀbenzo[e][1,2,4]triazinꢀ7(H)ꢀone (2) observed by
TLC.
Several attempts were made to improve the product yields. According to recent work by
Kaszynski et al.,29 the ring closure should be facilitated by acid catalysis, as such, we carried
out the reaction in the presence of both Brønsted e.g., AcOH, TsOH and Lewis acids e.g.,
Cu(OTf)2 in catalytic (5 mol %) and stoichiometric (100 mol %) amounts but in our hands,
there was no improvement in either yield or reaction time. In several cases, minor
modifications of the reaction conditions did improve the yields but these were substrate
specific. For example, in most cases where the yields of radical were low owing to the
formation of over oxidation products such as the purple colored benzotriazinone 2,2 the yields
of radicals were improved by performing the reactions under an argon atmosphere (e.g., the
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7ꢀhalobenzotriazinyls 1gꢀj, 1m, 1o, and 1s; Table 2, entries 7ꢀ10, 13, 15 and 19). Exceptions
included the reaction of 4ꢀmethoxyphenylꢀ (8b) and 4ꢀnitrophenyliminophosphoranes (8f)
with 1ꢀbenzoylꢀ2ꢀphenyldiazene 10a (Table 2, entries 2 and 6, respectively) that gave no
radical; mainly benzotriazinone 2 (by TLC). A further modification that worked was to
increase the equivalents of iminophosphorane 8 (from 1ꢀ2 equiv). Tentatively, in these cases,
decomposition of the diazene 10 under the reaction conditions competed with the desired azaꢀ
Wittig reaction; evidenced by the disappearance of diazene by TLC (e.g., the
pentafluorophenylꢀsubstituted diazene, Table 2, entries 20ꢀ22). Worthy of note was that
differential scanning colorimetry (DSC) studies on selected diazenes 10 indicated
decomposition onset temperatures at low as 189.3 °C (e.g., diazene 10g) and as high as 251.7
°C (e.g., diazene 10c). Increasing the equivalents of the iminophosphorane 8 (up to 2 equiv)
enabled a better consumption of the diazene 10 and improved the yield of desired radicals.
Interestingly, this was more successful than increasing the equivalents of diazene 10 and
fortunately, unlike the diazene, unreacted iminophosphorane 8 was also recoverable (by
chromatography) after the reaction. Furthermore, attempts to use microwave irradiation to
facilitate the heating rate and shorten the reaction times were less successful. Finally, the
above modifications could be combined to enable the yield of radical to be maximized.
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The azaꢀWittig reaction sequence for preparing Blatter radicals is simple, cheap and fast
affording radicals in moderate to good yields. Limitations include: the difficulty in preparing
1ꢀacylꢀ2ꢀaryldiazenes which prevent the preparation of 3ꢀalkylꢀsubstituted radicals, and the
poor stability at the elevated reaction temperatures of Blatter radicals that have labile
substituents such as methoxy or nitro groups at the C7 position.
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EPR Spectroscopy
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EPR studies on the new benzo[e][1,2,4]triazinꢀ4ꢀyls (1e,g,k-o,q-v) show that the unpaired
electron was mainly delocalized on the amidrazonyl moiety.5,6,18,23 Most of the spin density is
located on the N1 atom (aN1 ~7.7 G), with N2 and N4 bearing smaller but similar spin
densities (aN2 ~4.4 G and aN4 ~5.3 G) (Table 3). Elegant EPR and ENDOR studies by
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15
Neugebauer on Nꢀlabelled derivatives reveal the coupling constants to be 4.9 G for N2 and
5.2 G for N4 (aN1 >> aN4 > aN2).5c
Table 3. Fitting Parameters of Simulated Spectra for the New Radicals 1e,g,k-o,q-v.
entry
radical
g
α (G)
/X
0.98/N
N1
N2
N4
/X
/X
/X
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1ea
1g
2.0036
2.0041
2.0048
2.0077
2.0078
2.0042
2.0036
2.0038
2.0041
2.0037
2.0038
2.0037
2.0043
2.0039
2.0056
2.0057
2.0040
2.0041
8.26
7.67
7.63
8.05
7.80
7.62
7.84
7.84
7.74
8.65
8.24
8.19
7.48
6.18
7.37
7.15
7.27
7.22
4.53
4.53
4.46
3.52
3.52
4.91
3.36
3.38
4.75
3.81
3.62
4.49
5.07
4.79
4.85
4.74
5.32
5.26
5.29
4.94
5.19
5.10
4.94
5.02
5.22
5.21
5.07
5.59
5.42
5.16
5.30
5.68
5.34
5.36
5.44
5.50
1k
1lb
1lc
3.70/F
3.70/F
1m
1nb
1nc
1o
3.27/F
3.26/F
1qb
1qc
1r
3.66/F
3.46/F
1.02/N
1s
1t
1ub
1uc
1vb
1vc
4.90/F
4.95/F
4.96/F
5.02/F
4.25/F
4.22/F
4.24/F
4.26/F
3.18/F
3.22/F
3.18/F
3.12/F
1.91/F
2.04/F
1.96/F
2.02/F
a Radical 1e is known (ref 11b), but no EPR data has been reported, as such, it is included. b First derivative. c
Second derivative.
Solution EPR spectra of radicals 1e,g,k-o,q-v [Sect. S1. in the Supporting Information (SI)]
are typical of benzo[e][1,2,4]triazinꢀ4ꢀyls,5,6,18,23 and exhibit a sevenꢀline spectrum arising
from the coupling of the unpaired electron with three similar but slightly nonequivalent 14N
nuclei. EPR spectra of F3C containing radicals (1l, 1n and 1q) show additional coupling to
19F atoms (Figures S1.4., S1.6. and S1.8. in the SI). In these radicals, the F3C group causes
further splitting of the EPR signals into quartets, implying rotational averaging of the three
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fluorine nuclei. The hyperfine coupling constants, in radicals with F3C group attached on C7,
are slightly larger because of the mesomeric influence of F lone pairs.
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Table 4. Spin Densities (
ρ) of New Radicals 1e,g,k-o,q-v Estimated from Hyperfine
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60
Coupling Constants using McConnell’s equation.30
entry
4
5
6
7
radical
1ea
1g
N1
N2
N4
0.330
0.305
0.305
0.322
0.312
0.305
0.314
0.314
0.310
0.346
0.330
0.328
0.299
0.277
0.295
0.286
0.291
0.289
0.214
0.214
0.210
0.166
0.166
0.232
0.158
0.159
0.224
0.180
0.171
0.212
0.239
0.238
0.229
0.226
0.251
0.248
0.250
0.233
0.245
0.240
0.233
0.237
0.246
0.246
0.239
0.264
0.256
0.243
0.250
0.299
0.252
0.253
0.257
0.259
1k
1lb
8
9
1lc
1m
1nb
1nc
1o
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13
14
15
16
17
18
19
20
21
1qb
1qc
1r
1s
1t
1ub
1uc
1vb
1vc
a Radical 1e is known (ref 11b), but no EPR data has been reported, as such, it is included. b First derivative.
c
Second derivative. AN = QN. ρN where AN is the hyperfine coupling constant in Gauss; QN = 21.2 G (for N2, N4)
and QN = 25 G (for N1).31
The McConnell equation was used to calculate the spin density distribution based on the
estimated hyperfine coupling constants (Table 4). Spin density on F atoms is small due to the
large atomic hyperfine parameter of fluorine.32 The F3C group leads to a noticeable consistent
decrease of the spin density on N2 to ~0.17 in radicals 1l, 1n and 1q. The perfluorophenyl
substituent in radicals 1t-v decreases slightly the spin density on N1 to ~0.29. Despite these
subtle changes in the spin density distribution, our study supports that most of the spin
density remains mainly delocalized over the 1,2,4ꢀtriazinyl ring denoting its importance in the
transport and magnetic properties of these radicals.
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Cyclic Voltammetry
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In most cases, the cyclic voltammograms of radicals 1a,c-e,g-v have one reversible reduction
and one reversible oxidation, typical of 1,2,4ꢀbenzotriazinyl radicals (see SI, Sect. S2, Figures
S2.1ꢀS2.20).18 The exception was the 3ꢀ(4ꢀnitrophenyl)ꢀsubstituted radical 1k, which presents
two reversible reductions and one reversible oxidation (Table 5, entry 9; SI, Figure S2.9). We
have seen in only one other case an unusual deviation in the redox behavior which was
attributed to an oxidatively dimerized Blatter radical that gave two reversible oxidations,18,33
as such, to remove any ambiguity in the structure of radical 1k it was reduced in situ to the
corresponding 1,4ꢀdihydrobenzotriazine (12a) with ascorbic acid (1 equiv) in DMSOꢀd6 and
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1
had its HꢀNMR spectrum recorded (see SI, Sect. S3), that gratifyingly supported the
monomeric structure. The second reversible reduction (ꢀꢁ ꢂ ≈ −1.185 V ꢃꢄ Fc) was
⁄
therefore attributed to the reduction of the nitro group34 on the C3ꢀphenyl.
Table 5. Overview of Electrochemical Characteristics of Radicals 1a,c-e,g-v.
ꢇꢈꢉ
⁄
ꢁ ꢂ
ꢅꢆ
⁄
ꢇꢈꢉ
⁄
ꢊ
ꢀ
ꢀ
ꢀ
ꢀꢋꢈꢌꢌ
ꢀꢍꢎꢏꢎ
ꢀꢐꢑꢏꢎ
ꢁ ꢂ
ꢁ ꢂ
entry
radical
(V)
(eV)
(eV)
(V)
(V)
(V)
1
2
0.288
0.193
0.476
0.543
0.281
0.361
0.380
0.366
0.365
0.462
0.350
0.474
0.356
0.321
0.488
0.555
0.420
0.565
0.796
0.817
ꢀ0.864
ꢀ0.936
ꢀ0.700
ꢀ0.627
ꢀ0.851
ꢀ0.799
ꢀ0.772
ꢀ0.767
ꢀ0.811
ꢀ0.710
ꢀ0.783
ꢀ0.704
ꢀ0.781
ꢀ0.823
ꢀ0.680
ꢀ0.598
ꢀ0.703
ꢀ0.757
ꢀ0.522
ꢀ0.480
1.152
1.129
1.176
1.170
1.132
1.160
1.152
1.133
1.176
1.172
1.133
1.178
1.137
1.144
1.153
1.168
1.123
1.322
1.318
1.297
ꢀ5.272
ꢀ5.367
ꢀ5.084
ꢀ5.017
ꢀ5.279
ꢀ5.199
ꢀ5.180
ꢀ5.194
ꢀ5.195
ꢀ5.098
ꢀ5.210
ꢀ5.086
ꢀ5.204
ꢀ5.239
ꢀ5.005
ꢀ5.072
ꢀ5.140
ꢀ4.995
ꢀ4.764
ꢀ4.743
ꢀ6.424
ꢀ6.496
ꢀ6.260
ꢀ6.187
ꢀ6.411
ꢀ6.359
ꢀ6.332
ꢀ6.327
ꢀ6.371
ꢀ6.270
ꢀ6.343
ꢀ6.264
ꢀ6.341
ꢀ6.383
ꢀ6.158
ꢀ6.240
ꢀ6.263
ꢀ6.317
ꢀ6.082
ꢀ6.040
1a
1c
1d
1e
1g
1h
1i
3
4
5
6
7
8
1j
9
ꢀ1.185
1k
1l
10
11
12
13
14
15
16
17
18
19
20
1m
1n
1o
1p
1q
1r
1s
1t
1u
1v
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Strong electron withdrawing groups, such as –CF3 (σmeta 0.43; σpara 0.54),35 and –C≡N (σmeta
0.56; σpara 0.66),35 at the C7 position of the Blatter radical, increase both the oxidation and the
reduction halfꢀwave potentials, by ~200 mV (Table 5, entries 3ꢀ4). While the introduction of
either electron withdrawing or donating groups at the benzotriazinyl C3 position did not
significantly alter the redox potentials, as has been observed previously.18 Notable changes
occur when the pentafluorophenyl (ꢀC6F5) group (σmeta 0.26; σpara 0.27)35 occupies the N1
position (Table 5, entries 18ꢀ20). While the C6F5 is clearly not as powerful at electron
withdrawing group as either CF3 and C≡N groups, it nevertheless resides on the spin rich N1
position (see Table 3, aN1 ~7.7 G). As such, its ability to influence the radical’s redox
properties is enhanced, increasing (more +ve) both the oxidation and reduction potentials.
Differences up to ~530 mV are observed for the oxidation halfꢀwave potentials and up to
~380 mV for the reduction halfꢀwave potentials, when synergistic effects with strong electron
withdrawing groups at the C7 position are in place (Table 5, entries 19ꢀ20).
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The above data supports the use of pentafluorophenyl at N1 and/or trifluoromethyl and cyano
groups at C7 to significantly moderate the redox potentials of 1,2,4ꢀbenzotriazinyls. The
effects of multiple substitutions appear to be somewhat additive further suggesting that
customizing the redox potentials of 1,2,4ꢀbenzotriazinyls is possible and can enable their
broader application in the material sciences
Crystallography
In light of the electron withdrawing influence of the pentafluorophenyl group at N1 which
leads to a significant shift in the redox behaviors of radicals 1t-v, we looked at the geometry
on one of these Blatter radicals. Single crystals of 1ꢀpentafluorophenyl radical 1t were grown
via bulk recrystallization from cꢀhexane and Xꢀray crystallography was carried out to better
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understand the substituents effect on the geometry of the benzotriazinyl moiety (Figure 3);
the crystallographic numbering shown is used for the subsequent discussion.
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Figure 3. Intramolecular geometry of radical 1t (CCDC 1551625) along with the
crystallographic atom numbering (hydrogens are omitted for clarity reasons and ellipsoid at
50% probability).
Radical 1t crystalized in the monoclinic P 21/c space group with one molecule in the
asymmetric unit cell. The C7ꢀN2 and C7ꢀN3 bond lengths [1.333(2) Å and 1.334(2) Å] are
intermediate of single and double CꢀN bonds indicating strong conjugation in the
amidrazonyl moiety of the heterocycle. This was further supported by the decrease in the C7ꢀ
N3ꢀC5 and C7ꢀN2ꢀN1 bond angles [115.9(2)° and 115.5(1)°, respectively] typical of sp2ꢀ
hybridized pyridine nitrogen atoms coordinated to metals.36 Overall the benzotriazinyl moiety
was planar, with a maximum deviation from the plane no greater than 0.047 Å. The phenyl
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substituent at C7 is almost coplanar with the benzotriazinyl moiety as the torsion angle about
the C7ꢀC8 bond is only 7.4° (cf. for Blatter radical 1a the analogous torsion37 is 10.4°). The
N1 perfluorophenyl substituent is comfortably out of the benzotriazinyl plane with a torsion
angle of 52.6° (defined by the angle between the mean planes of the benzotriazine and the N1
aryl ring), but it is not significantly different from the analogous torsion angle of the parent
Blatter radical 1a (53.8°)37 suggesting a negligible steric effect owing to the presence of the
fluorine atoms. This torsion angle also falls comfortably within the range of N1ꢀAr torsion
angles of 38–82° observed in other benzo[e][1,2,4]triazinꢀ4ꢀyls.9c,38 The N1ꢀC14 bond length
of 1.418(2) Å, however, was marginally shorter by ~0.01 Å than the analogous bond in
Blatter radical 1a [1.427(2) Å],37 tentatively suggesting a small degree additional electron
release from N1 (or the triazine) to the pentafluorophenyl group.
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3.
CONCLUSION
A short synthesis of Blatter radicals has been developed that invokes the azaꢀWittig reaction
between readily available Nꢀaryliminophosphoranes and 1ꢀ(het)aroylꢀ2ꢀaryldiazenes. The low
cost synthesis avoids the formation of either moisture sensitive imidoyl chlorides, oxidatively
unstable amidrazones, expensive reagents or organolithium bases. As such, it offers the above
advantages to the known syntheses of Blatter radicals.
4
EXPERIMENTAL SECTION
General methods and materials
4.1
All chemicals were commercially sourced except those whose synthesis is described. CH2Cl2
was freshly distilled from CaH2 under argon. Reactions were protected from atmospheric
moisture by CaCl2 drying tubes or argon atmosphere was used where stated. Anhydrous
Na2SO4 was used to dry organic extracts, and all volatiles were removed under reduced presꢀ
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sure. All reaction mixtures and column eluents were monitored by thinꢀlayer chromatography
(TLC) using commercial aluminumꢀbacked TLC plates (Merck Kieselgel 60 F254 or, where
stated, aluminum oxide 60 F254 neutral). TLC plates were observed under UV light at 254 and
365 nm. The technique of dry flash chromatography39 was used throughout for all nonꢀTLCꢀ
scale chromatographic separations and employed either silica gel 60 (<0.063 mm) or where
stated, aluminum oxide 60 G neutral type (type E). Melting and decomposition points were
determined using either a PolyThermꢀA, Wagner & Munz, Kofler hotꢀstage microscope appaꢀ
ratus or a TA Instruments DSC Q1000 differential scanning calorimeter (DSC) with samples
hermetically sealed in aluminum pans under an argon atmosphere, using heating rates of 5
°C/min (DSC mp and decomp. points are listed by onset and peak max values). The solvent
used for recrystallization is indicated after each melting point. UV/vis spectra were obtained
using a PerkinꢀElmer Lambdaꢀ25 UV/vis spectrophotometer, and inflections are identified by
the abbreviation “inf”. IR spectra were recorded on Shimadzu FTIRꢀNIR Prestigeꢀ21 specꢀ
trometer with a Pike Miracle Ge ATR accessory; strong, medium, and weak peaks are repreꢀ
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1
sented by “s”, “m”, and “w”, respectively. H and 13C NMR spectra were recorded on a
Bruker Avance III 500 machine at 500 and 125 MHz, respectively. Deuterated solvents were
used for homonuclear lock, and the signals are referenced to the deuterated solvent peaks.
APT NMR studies identified quaternary, tertiary, secondary and primary carbons, which are
indicated by (s), (d), (t) and (q) notations, respectively. Lowꢀresolution (EI) mass spectra
were recorded on a Shimadzu GCꢀMS QP2010 with direct inlet probe. MALDIꢀTOF mass
spectra were recorded on a on a Bruker Autoflex III Smartbeam instrument while ESIꢀAPCI+
mass spectra were recorded on a Model 6110 Quadrupole MSD, Agilent Technologies. Eleꢀ
mental analysis was performed on a Perkin Elmer 2400 Series Elemental Analyzer by Steꢀ
phen Boyer of London Metropolitan University. NꢀPhenyl(triphenyl)iminophosphorane
(8a),40 Nꢀ(4ꢀmethoxyphenyl)(triphenyl)iminophosphorane (8b),41 Nꢀ(4ꢀtolyl)(triphenyl)ꢀ
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iminophosphorane (8c),40 Nꢀ(4ꢀtrifluoromethylphenyl)(triphenyl)iminophosphorane (8d),42
Nꢀ(4ꢀcyanophenyl)(triphenyl)iminophosphorane (8e),43 Nꢀ(4ꢀnitrophenyl)(triphenyl)iminoꢀ
phosphorane (8f),41 Nꢀ(4ꢀfluorophenyl)(triphenyl)iminophosphorane (8g),43 Nꢀ(4ꢀchloroꢀ
phenyl)(triphenyl)iminophosphorane (8h),40 Nꢀ(4ꢀbromophenyl)(triphenyliminophosphorane
(8i),44 Nꢀ(4ꢀiodophenyl)(triphenyl)iminophosphorane (8j)45 N′ꢀphenylbenzohydrazide (11a),46
4ꢀmethylꢀN'ꢀphenylbenzohydrazide (11b),47 4ꢀfluoroꢀN'ꢀphenylbenzohydrazide (11c),46 4ꢀ
nitroꢀN'ꢀphenylbenzohydrazide (11d),48 N'ꢀphenylthiopheneꢀ2ꢀcarbohydrazide (11e),46 and
N'ꢀpentafluorophenylbenzohydrazide (11f)49 were prepared according to the literature.
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4.2
EPR Spectroscopy
EPR spectra were recorded on an Xꢀband EPR spectrometer (Adani CMS 8400) at room
temperature on dilute solutions of the benzotriazinyls 1 in CH2Cl2. For the dilute solution
spectra, the microwave power was in the region 0.5−7.0 mW with a modulation frequency of
100 kHz and a modulation amplitude of 1.0 Gpp. Simulations of the solution spectra were
made using EasySpin.50 The nearly isotropic nature of most of the benzotriazinyl radical
samples meant that most of the solidꢀstate samples could be initially modeled with an
isotropic spectrum using EasySpin.
4.3
Cyclic Voltammetry
Cyclic voltammetry studies were performed on a Princeton Applied Research
Potentiostat/Galvanostat 263A. The concentrations of radicals 1 used were 1 mM in CH2Cl2
containing nꢀBu4NPF6 (0.1 M) as an electrolyte. A three electrode electrochemical cell was
used with glassy carbon disk as working electrode, Pt wire as counter electron and Ag/AgCl
(1 M KCl) as reference electrode. Scan rate 50 mV s−1. Temperature = 20 °C. Fc/Fc+ (ꢀꢒꢓ ꢒꢓ
ꢔ
⁄
= 0.460 V vs SCE) was used as an internal reference.51
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CH2Cl2 was distilled over CaH2. Samples were deaerated by passing argon through the
solvent, prior to measuring the cyclic voltammogram of each radical. For all measurements,
blank samples (only electrolyte in the system) were taken to ensure the correct operation of
the electrochemical cell. Upon each measurement, ferrocene was added and the cyclic
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voltammogram was taken again. All redox potentials (
ꢀ
ꢁ ꢂ) are referenced according to the
⁄
ꢔ
ferrocene’s value (ꢀꢒꢓ ꢒꢓ = 0.460 ꢕ vs SCE).
⁄
The following equations were used to calculate the ꢀꢋꢈꢌꢌ
,
ꢀꢍꢎꢏꢎ and ꢀꢐꢑꢏꢎ:
ꢀꢋꢈꢌꢌ = ꢀꢁꢅꢆꢂ − ꢀꢇꢈꢉ
⁄
⁄
ꢁ ꢂ
ꢀꢍꢎꢏꢎ = −ꢖꢗꢀꢅꢆ − ꢀꢒꢓ ꢒꢓ ꢘ − 5.1ꢙeV
ꢔ
⁄
⁄
ꢁ ꢂ
ꢀꢐꢑꢏꢎ = −ꢖꢗꢀꢇꢈꢉ − ꢀꢒꢓ ꢒꢓ ꢘ − 5.1ꢙeV
ꢔ
⁄
⁄
ꢁ ꢂ
4.4
X-Ray Crystallography
Data were collected on an OxfordꢀDiffraction Supernova diffractometer, equipped with a
CCD area detector utilizing MoꢀKα radiation (λ = 0.7103 Å). A suitable crystal was attached
to glass fibers using paratoneꢀN oil and transferred to a goniostat where they were cooled for
data collection. Unit cell dimensions were determined and refined by using 1464 (3.76
≤ θ ≤
28.77°) and 2149 (3.03 24.99°) reflections for 10f and 1t, respectively. Empirical
≤
θ ≤
absorption corrections (multiꢀscan based on symmetryꢀrelated measurements) were applied
using CrysAlis RED software.52 The structures were solved by direct methodand refined on
F2 using fullꢀmatrix least squares using SHELXL97.53 Software packages used: CrysAlis
CCD52 for data collection, CrysAlis RED52 for cell refinement and data reduction, WINGX
for geometric calculations,54 and DIAMOND55 for molecular graphics. The nonꢀH atoms
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were treated anisotropically. The hydrogen atoms were placed in calculated, ideal positions
and refined as riding on their respective carbon atoms.
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Crystal refinement data for compound 10f (CCDC 1551626): C13H5F5N2O, M = 300.19,
triclinic, space group Pꢀ1, a = 7.3520(5) Å, b = 7.894(5) Å, c = 10.285(5) Å, α = 80.949(5)°,
β = 79.134(5)°, γ = 78.079(5)°, V = 567.60(7) Å3, Z = 2, T = 100(2) K, ρcalcd = 1.747 g cmꢀ3,
2θmax = 25. Refinement of 190 parameters on 2008 independent reflections out of 4627
measured reflections (Rint = 0.0398) led to R1 = 0.0484 [I > 2s(I)], wR2 = 0.1243 (all data),
and S = 1.042 with the largest difference peak and hole of 0.245 and ꢀ0.313 eꢀ3, respectively.
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Crystal refinement data for compound 1t (CCDC 1551625): C19H9F5N3, M = 374.29,
monoclinic, space group P21c, a = 15.6557(10) Å, b = 7.5132(4) Å, c = 13.4325(7) Å, α =
90°, β = 105.036(6)°, γ = 90°, V = 1525.90(15) Å3, Z = 4, T = 100(2) K, ρcalcd = 1.629 gcmꢀ3,
2θmax = 25. Refinement of 244 parameters on 2673 independent reflections out of 3230
measured reflections (Rint = 0.0474) led to R1 = 0.0426 [I > 2s(I)], wR2 = 0.1146 (all data),
and S = 1.073 with the largest difference peak and hole of 0.218 and ꢀ0.227 eꢀ3, respectively.
4.5
Synthesis of Hydrazides
4.5.1
N'ꢀ(Perfluorophenyl)thiopheneꢀ2ꢀcarbohydrazide (11g): To a stirred solution of
(perfluorophenyl)hydrazine (3.011 g, 15.20 mmol) in pyridine (10 mL) at ca. 0 °C, was
added dropwise 2ꢀthiophenecarbonyl chloride (1.63 mL, 15.20 mmol). The reaction mixture
was then allowed to warm to ca. 20 °C and left to stir for 16 h, after which time it was poured
into a stirred aqueous solution of 4.3 M H2SO4 (50 mL) and left to stir for an additional 2 h.
The formed precipitate was collected by filtration, washed (H2O) and recrystallized to afford
the title compound 11g (2.910 g, 62%) as colorless needles, mp (hotꢀstage) 181.4ꢀ183.8 °C
(EtOH), mp (DSC) onset: 185.3 °C, peak max: 186.4 °C; decomp. onset: 239.7 °C, peak max:
255.5 °C (EtOH); Rf 0.40 (nꢀhexane/Et2O, 50:50); Anal. Calcd for C11H5F5N2OS: C, 42.86;
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H, 1.64; N, 9.09. Found: C, 42.79; H, 1.57; N, 8.91; λmax(CH2Cl2)/nm 271 inf (log ε 3.49),
317 inf (2.75), 364 (2.57); νmax/cmꢀ1 3285w (NꢀH), 1624m (C=O), 1518s, 1503w, 1464w,
1418w, 1356w, 1321w, 1302w, 1237w, 1200w, 1128w, 1088w, 1067w, 1022m, 962m, 880w,
858m, 806m, 719s; δH(500 MHz, DMSOꢀd6) 10.75 (1H, d, J 1.5, NH), 8.24 (1H, d, J 1.5,
NH), 7.84ꢀ7.83 (2H, m, thienyl H), 7.19 (1H, dd, J 4.5, 4.5, thienyl H); δC(125 MHz, CDCl3)
one C (s) resonance missing, 161.4 (s), 137.4 (dm 1JCF 242.5, overlapping Ar CFs), 136.7 (s),
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133.7 (dt, 1JCF 241.3, 2JCF 13.8, Ar CF), 131.6 (d), 128.9 (d), 128.1 (d), 124.6 (t, JCF 9.4, Ar
C); m/z (MALDIꢀTOF) 309 (MH+, 14%), 111 (100).
4.6
Synthesis of 1-(Het)aroyl-2-aryldiazenes 10 (see Table 1)
4.6.1
1ꢀBenzoylꢀ2ꢀphenyldiazene (10a) (Method A, Typical Procedure): To a stirred
solution of N'ꢀphenylbenzohydrazide (11a) (1.061 g, 5.00 mmol) in nꢀhexane (30 mL) at ca.
20 °C was added yellow HgO (1.083 g, 5.00 mmol). After 3 h the mixture was passed
through thin layer of silica, which was subsequently rinsed with CH2Cl2 (50 mL). The
combined filtrate was concentrated in vacuo at ca. 20 °C and recrystallized at ca. ꢀ20 °C to
give the title compound 10a (1.130 g, 93%) as red needles, mp (hotꢀstage) 23.5ꢀ25.4 °C (nꢀ
pentane/CH2Cl2, 90:10 at ca. ꢀ20 °C), (lit.,27a 28ꢀ29 °C); Rf 0.61 (nꢀhexane/CH2Cl2, 50:50);
λmax(CH2Cl2)/nm 297 (log ε 3.83), 450 (2.47); νmax/cmꢀ1 3065w (aryl CꢀH), 1709s (C=O),
1599m, 1582w, 1501s, 1450s, 1314m, 1256s, 1194m, 1177m, 1148s, 1072w, 1032m, 1007s,
997s, 928w, 787w, 762s, 712s; δH(500 MHz, CDCl3) 8.07 (2H, dd, J 8.3, 1.3, Ar H), 8.01
(2H, dd, J 8.3, 1.8, Ar H), 7.67 (1H, dd, J 7.5, 7.5, Ar H), 7.61ꢀ7.56 (3H, m, Ar H), 7.53 (2H,
dd, J 7.8, 7.8, Ar H); δC(125 MHz, CDCl3) 182.0 (s), 152.0 (s), 134.5 (d), 133.4 (d), 130.8
(s), 130.5 (d), 129.3 (d), 128.9 (d), 123.6 (d); m/z (APCI) 211 (MH+, 30%), 168 (13), 105
(13).
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4.6.2
1ꢀ(4ꢀMethylbenzoyl)ꢀ2ꢀphenyldiazene (10b) (Method A): Similar treatment of 4ꢀ
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methylꢀN'ꢀphenylbenzohydrazide (11b) (1.131 g, 5.00 mmol) with yellow HgO (1.083 g,
5.00 mmol) gave the title compound 10b (1.110 g, 99%) as red needles, mp (hotꢀstage) 38.2ꢀ
40.1 °C (nꢀpentane/CH2Cl2, 90:10 at ca. ꢀ20 °C), (lit.,56 bp 130ꢀ133 °C at 0.2 Torr); mp
(DSC) onset: 40.6 °C, peak max: 42.9 °C; decomp. onset: 238.5 °C, peak max: 255.7 °C (nꢀ
pentane/CH2Cl2, 90:10 at ca. ꢀ20 °C); Rf 0.62 (nꢀhexane/Et2O, 50:50); λmax(CH2Cl2)/nm 246
(log ε 4.21), 289 (4.37), 444 (2.59); νmax/cmꢀ1 3065w (aryl CꢀH), 2934w (alkyl CꢀH), 1701s
(C=O), 1605m, 1503m, 1452m, 1310w, 1296w, 1261s, 1244m, 1209w, 1194w, 1179m,
1146m, 1119w, 1072w, 1026m, 1009s, 999m, 930w, 912w, 829m, 775s, 760s, 733m; δH(500
MHz, CDCl3) 7.99 (2H, dd, J 8.5, 1.5, Ar H), 7.95 (2H, d, J 8.5, Ar H), 7.62ꢀ7.55 (3H, m, Ar
H), 7.32 (2H, d, J 8.0, Ar H), 2.45 (3H, s, CH3); δC(125 MHz, CDCl3) 181.9 (s), 152.1 (s),
145.7 (s), 133.3 (d), 130.6 (d), 129.6 (d), 129.3 (d), 128.2 (s), 123.6 (d), 21.9 (q); m/z (APCI)
225 (MH+, 100%), 215 (11), 119 (28).
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4.6.3
1ꢀ(4ꢀFluorobenzoyl)ꢀ2ꢀphenyldiazene (10c) (Method A): Similar treatment of 4ꢀfluoꢀ
roꢀN'ꢀphenylbenzohydrazide (11c) (1.151 g, 5.00 mmol) with yellow HgO (1.083 g, 5.00
mmol) gave the title compound 10c (1.095 g, 96%) as red needles, mp (hotꢀstage) 32.1ꢀ34.9
°C (nꢀpentane/CH2Cl2, 90:10 at ca. ꢀ20 °C), mp (DSC) onset: 36.6 °C, peak max: 39.1 °C;
decomp. onset: 251.7 °C, peak max: 262.7 °C (nꢀpentane/CH2Cl2, 90:10 at ca. ꢀ20 °C); Rf
0.68 (nꢀhexane/Et2O, 50:50); Anal. Calcd for C13H9FN2O: C, 68.42; H, 3.98; N, 12.27.
Found: C, 68.35; H, 3.89; N, 12.14; λmax(CH2Cl2)/nm 294 (log ε 4.04), 450 (2.30); νmax/cmꢀ1
3065w (aryl CꢀH), 1707s (C=O), 1595m, 1503m, 1497m, 1452m, 1410w, 1312w, 1302w,
1290w, 1261s, 1238m, 1223m, 1198m, 1148s, 1094w, 1072w, 1009s, 997s, 928w, 918w,
847m, 820w, 779m, 770s, 743m; δH(500 MHz, CDCl3) 8.14ꢀ8.10 (2H, m, Ar H), 8.02ꢀ7.99
(2H, m, Ar H), 7.64ꢀ7.61 (1H, m, Ar H), 7.60ꢀ7.56 (2H, m, Ar H), 7.20 (2H, dd, J 8.5, 8.5, Ar
1
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H); δC(125 MHz, CDCl3) 180.5 (s), 166.6 (d, JCF 256.3), 152.0 (s), 133.6 (d), 133.4 (d, JCF
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100%), 123 (30).
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4.6.4
1ꢀ(4ꢀNitrobenzoyl)ꢀ2ꢀphenyldiazene (10d) (Method C, Typical Procedure): To a
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stirred solution of 4ꢀnitroꢀN'ꢀphenylbenzohydrazide (11d) (1.286 g, 5.00 mmol) in CH2Cl2
(20 mL) at ca. 20 °C was added pyridine (0.443 mL, 5.50 mmol). The reaction mixture was
then cooled to ca. 0 °C and portionwise was added NBS (0.890 g, 5.00 mmol). After the
addition, the mixture was allowed to warm to ca. 20 °C and left to stir for 0.5 h. The reaction
mixture was then extracted with 1.65 M HCl (2 × 20 mL). The organic phase was separated
and sequentially washed first with 0.1 M aq. Na2S2O3·5H2O (20 mL), then with saturated aq.
NaHCO3 (20 mL) and finally with brine (20 mL), dried (Na2SO4) and filtered. The volatiles
were then removed in vacuo at ca. 20 °C to give the title compound 10d (1.200 g, 94%) as
red needles, mp (hotꢀstage) 128.9ꢀ132.1 °C (CH2Cl2), (lit.,57 136 °C); mp (DSC) onset: 131.1
°C, peak max: 134.5 °C; decomp. onset: 191.7 °C, peak max: 226.1 °C (CH2Cl2); Rf 0.72 (nꢀ
hexane/Et2O, 50:50); λmax(CH2Cl2)/nm 264 (log ε 4.15), 300 (4.20), 446 (3.02); νmax/cmꢀ1
3057w (aryl CꢀH), 1709s (C=O), 1703m, 1605m, 1524s, 1499m, 1450m, 1346s, 1321w,
1302w, 1248m, 1240m, 1229m, 1190m, 1180m, 1148m, 1111w, 1074w, 1020m, 1008s,
997s, 935w, 922w, 880w, 860w, 854s, 816w, 783s, 758s, 720s; δH(500 MHz, CDCl3) 8.38
(2H, d, J 8.5, Ar H), 8.28 (2H, d, J 9.0, Ar H), 8.04ꢀ8.02 (2H, m, Ar H), 7.66 (1H, dd, J 7.3,
7.3, Ar H), 7.60 (2H, dd, J 7.3, 7.3, Ar H); δC(125 MHz, CDCl3) 179.7 (s), 152.0 (s), 151.1
(s), 136.0 (s), 134.3 (d), 131.6 (d), 129.5 (d), 124.0 (d), 123.9 (d); m/z (APCI) 256 (MH+,
100%), 150 (20).
4.6.5
1ꢀPhenylꢀ2ꢀ(2ꢀthienylcarbonyl)diazene (10e) (Method B, Typical Procedure): To a
stirred solution of N'ꢀphenylthiopheneꢀ2ꢀcarbohydrazide (11e) (1.091 g, 5.00 mmol) in
acetone (50 mL) at ca. 20 °C was added acetic anhydride (1.42 mL, 15.00 mmol) and sodium
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nitrite (1.035 g, 15.00 mmol). After 5 h the reaction mixture was diluted with H2O (100 mL)
and extracted with CH2Cl2 (2 × 50 mL). The organic phase was then washed with 1% aq.
Na2CO3 (50 mL), dried (Na2SO4), filtered and concentrated in vacuo at ca. 20 °C to give the
title compound 10e (1.027 g, 95%) as a red oil, (lit.,58 no data provided); Rf 0.79 (nꢀ
hexane/Et2O, 50:50); Anal. Calcd for C11H8N2OS: C, 61.09; H, 3.73; N: 12.95. Found: C,
61.19; H, 3.70; N, 12.86; λmax(CH2Cl2)/nm 237 (log ε 4.18), 312 (3.75), 442 inf (2.82);
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max/cmꢀ1 3096w (aryl CꢀH), 1788w, 1692s (C=O), 1585w, 1514m, 1497s, 1474w, 1452m,
1408s, 1358s, 1311m, 1250s, 1192s, 1148s, 1082m, 1070w, 1045m, 1020m, 976m, 952m,
928m, 858s, 775s, 754w, 725s; δH(500 MHz, CDCl3) 8.05ꢀ8.02 (2H, m, Ar H), 7.95 (1H, dd,
J 3.8, 1.3, thienyl H), 7.85 (1H, dd, J 5.0, 1.0, thienyl H), 7.63 (1H, dd, J 7.8, 7.8, Ar H), 7.58
(2H, dd, J 7.6, 7.6, Ar H), 7.24 (1H, dd, J 4.8, 3.8, thienyl H); δC(125 MHz, CDCl3) 174.7 (s),
151.9 (s), 136.5 (d), 136.3 (d), 135.8 (s), 133.8 (d), 129.4 (d), 128.7 (d), 123.9 (d).
4.6.6
1ꢀBenzoylꢀ2ꢀ(perfluorophenyl)diazene (10f) (Method B): Similar treatment of N'ꢀ
perfluorophenylbenzohydrazide (11f) (1.511 g, 5.00 mmol) with acetic anhydride (1.42 mL,
15.0 mmol) and sodium nitrite (1.035 g, 15.00 mmol) gave the title compound 10f (1.096 g,
73%) as orange microneedles, mp (hotꢀstage) 118.6ꢀ121.2 °C (nꢀpentane/CH2Cl2, 90:10), mp
(DSC) onset: 121.0 °C, peak max: 121.9 °C; decomp. onset: 200.1 °C, peak max: 225.2 °C
(nꢀpentane/CH2Cl2, 90:10); Rf 0.77 (nꢀhexane/Et2O, 50:50); Anal. Calcd for C13H5F5N2O: C,
52.02; H, 1.68; N, 9.33. Found: C, 52.11; H, 1.54; N, 9.46; λmax(CH2Cl2)/nm 240 (log ε 4.29),
282 (4.37), 454 (2.13); νmax/cmꢀ1 3071w (aryl CꢀH), 1721s (C=O), 1645w, 1595w, 1518s,
1514s, 1497m, 1487m, 1450m, 1406w, 1314m, 1244s, 1177w, 1150m, 1136m, 1020s, 1003s,
997s, 955m, 883w, 800w, 735m, 708s; δH(500 MHz, CDCl3) 8.05 (2H, dd, J 8.3, 1.3, Ar H),
7.71 (1H, dd, J 7.5, 7.5, Ar H), 7.56 (2H, dd, J 7.8, 7.8, Ar H); δC(125 MHz, CDCl3) one C
(s) resonance is missing, 180.1 (s), 144.4ꢀ144.2 and 142.4ꢀ142.1 (m, CF), 142.9ꢀ142.8 and
140.8ꢀ140.6 (m, CF), 139.2ꢀ138.9 and 137.1ꢀ136.9 (m, CF), 135.1 (d), 130.7 (d), 129.8 (s),
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4.6.7
1ꢀ(Perfluorophenyl)ꢀ2ꢀ(2ꢀthienylcarbonyl)diazene (10g) (Method B): Similar treatꢀ
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ment of N'ꢀ(perfluorophenyl)thiopheneꢀ3ꢀcarbohydrazide (11g) (1.541 g, 5.00 mmol) with
acetic anhydride (1.42 mL, 15.0 mmol) and sodium nitrite (1.035 g, 15.00 mmol) gave the
title compound 10g (0.796 g, 52%) as orange needles, mp (hotꢀstage) 78.8ꢀ80.5 °C (nꢀ
pentane/CH2Cl2, 90:10), mp (DSC) onset: 85.4 °C, peak max: 89.2 °C; decomp. onset: 189.3
°C, peak max: 237.6 °C (nꢀpentane/CH2Cl2, 90:10); Rf 0.80 (nꢀhexane/Et2O, 50:50); Anal.
Calcd for C11H3F5N2OS: C, 43.15; H, 0.99; N, 9.15. Found: C, 43.10; H, 1.08; N, 9.11;
λmax(CH2Cl2)/nm 259 inf (log ε 4.42), 288 (4.51), 465 (2.56); νmax/cmꢀ1 3119w (aryl CꢀH),
1792w, 1697s (C=O), 1643w, 1512s, 1487m, 1408m, 1356m, 1317w, 1258m, 1248m,
1182w, 1155w, 1138m, 1084w, 1059m, 1047w, 1024s, 984m, 951m, 922w, 862w, 845w,
756m, 712w; δH(500 MHz, CDCl3) 7.95 (1H, dd, J 4.0, 1.0, thienyl H), 7.91 (1H, dd, J 4.8,
1.3, thienyl H), 7.24 (1H, dd, J 4.5, 4.5, thienyl H); δC(125 MHz, CDCl3) one C (s) resonance
is missing, 172.8 (s), 144.6ꢀ144.4 and 143.0ꢀ142.9 (m, CF), 142.5ꢀ142.3 (m, CF), 140.9ꢀ
140.7 and 139.2ꢀ139.0 (m, CF), 137.5 (d), 137.1 (d), 134.8 (s), 129.0 (d); m/z (APCI) 307
(MH+, 100%), 285 (18), 143 (38), 129 (11), 111 (53).
4.7
Synthesis of Benzotriazinyl Radicals
4.7.1
1,3ꢀDiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1a) (Typical Procedure): To a
stirred solution of 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (105.0 mg, 0.50 mmol) in diphenyl ether
(1 mL) at ca. 20 °C was added Nꢀphenyl(triphenyl)iminophosphorane (8a) (176.7 mg, 0.50
mmol). The stirred reaction mixture was then immersed into a preheated (ca. 200 °C) Wood’s
metal bath for 10 min and then cooled to ca. 20 °C. Dilution of the reaction mixture in
CH2Cl2 (2 mL), followed by chromatography (neutral Al2O3, nꢀhexane/CH2Cl2, 50:50) gave
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the title compound 1a (113.7 mg, 80%) as black needles, mp (hotꢀstage) 109.2ꢀ111.3 °C
(EtOH) (lit.,18 109ꢀ111 °C); Rf 0.67 (neutral Al2O3, nꢀhexane/CH2Cl2, 50:50);
λmax(CH2Cl2)/nm 271 (log ε 3.63), 322 (2.93), 372 (2.82), 429 (2.56), 494 (2.17); νmax/cm−1
3061w and 3003w (aryl CꢀH), 1585w, 1481w, 1450m, 1395s, 1317w, 1252w, 1206w, 1175w,
1082w, 1065w, 1024w, 984w, 916w, 880w, 841w, 785m, 750s; identical to an authentic
sample.
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4.7.2
7ꢀMethylꢀ1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1c): Similar treatꢀ
ment of 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (105.0 mg, 0.50 mmol) in diphenyl ether (1 mL)
with Nꢀ(4ꢀtolyl)(triphenyl)iminophosphorane (8c) (183.7 mg, 0.50 mmol) at ca. 200 °C for
10 min gave on chromatography (neutral Al2O3, nꢀhexane/CH2Cl2, 50:50) the title compound
1c (125.0 mg, 84%) as black needles, mp (hotꢀstage) 159.8ꢀ163.2 °C (cꢀhexane) (lit.,59 160ꢀ
163 °C); Rf 0.42 (neutral Al2O3, nꢀhexane/CH2Cl2, 50:50); λmax(CH2Cl2)/nm 275 (log ε 4.21),
283 inf (4.10), 322 (3.55), 373 (3.35), 433 (2.95), 481 inf (1.58); νmax/cm−1 3063w (aryl CꢀH),
2920w (alkyl CꢀH), 1684w, 1653w, 1592m, 1559w, 1539w, 1503m, 1490m, 1459w, 1452m,
1420w, 1394s, 1327m, 1279w, 1257w, 1170m, 1087w, 1067w, 1024m, 1002w, 929w, 917w,
862w, 849w, 844w, 803s, 780s, 759s, 712m; identical to an authentic sample.
4.7.3
1,3ꢀDiphenylꢀ7ꢀ(trifluoromethyl)ꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl
(1d):
Similar treatment of 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (105.0 mg, 0.50 mmol) in diphenyl
ether (1 mL) with Nꢀ(4ꢀtrifluoromethylphenyl)(triphenyl)iminophosphorane (8d) (210.7 mg,
0.50 mmol) at ca. 200 °C for 20 min gave on chromatography (neutral Al2O3, nꢀ
hexane/CH2Cl2, 50:50) the title compound 1d (133.9 mg, 76%) as black needles, mp (hotꢀ
stage) 148.7ꢀ150.3 °C (cꢀhexane) (lit.,59 149ꢀ153 °C); Rf 0.78 (neutral Al2O3, nꢀ
hexane/CH2Cl2, 50:50); λmax(CH2Cl2)/nm 259 inf (log ε 4.04), 273 (4.21), 284 inf (4.03), 323
(3.55), 373 (3.40), 431 (3.17), 495 (284); νmax/cm−1 1593w, 1506w, 1489m, 1452w, 1422m,
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1395m, 1356m, 1337w, 1314m, 1281w, 1261m, 1248w, 1204w, 1150m, 1117s, 1063m,
1024w, 905m, 870m, 841m, 793w, 781m, 768m; identical to an authentic sample.
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4.7.4
7ꢀCyanoꢀ1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1e): Similar treatꢀ
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ment of 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (105.0 mg, 0.50 mmol) in diphenyl ether (1 mL)
with Nꢀ(4ꢀcyanophenyl)(triphenyl)iminophosphorane (8e) (189.2 mg, 0.50 mmol) at ca. 200
°C, stirred for 25 min, gave on chromatography (neutral Al2O3, nꢀhexane/Et2O, 50:50) the
title compound 1e as black needles (74.2 mg, 48%), mp (hotꢀstage) 213.7ꢀ216.6 °C (cꢀ
hexane) (lit.,11b no data provided); mp (DSC) onset: 219.2 °C, peak max: 220.9 °C; decomp.
onset: 276.1 °C, peak max: 337.2 °C (cꢀhexane); Rf 0.83 (neutral Al2O3, nꢀhexane/tꢀBuOMe,
50:50); Anal. Calcd for C20H13N4: C, 77.65; H, 4.24; N, 18.11. Found: C, 77.43; H, 4.46; N,
17.92; λmax(CH2Cl2)/nm 262 (log ε 4.03), 294 (4.20), 323 (3.60), 378 (3.55), 441 (3.38), 461
inf (3.30), 510 (3.18); νmax/cmꢀ1 3082w (aryl CꢀH), 2222m (C≡N), 1587w, 1485m, 1450w,
1400s, 1313,, 1277w, 1254w, 1194m, 1179w, 1142w, 1126w, 1065w, 1028m, 916w, 887m,
847w, 829s, 754s, 702m; m/z (MALDIꢀTOF) 310 (MH+, 24%), 309 (M+, 100).
4.7.5
7ꢀFluoroꢀ1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1g): Similar treatꢀ
ment of 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (105.0 mg, 0.50 mmol) in diphenyl ether (1 mL)
with Nꢀ(4ꢀfluorophenyl)(triphenyl)iminophosphorane (8g) (371.4 mg, 1.00 mmol), deaerated
and under Ar atmosphere at ca. 20 °C, and immersed into a preheated Wood’s metal bath at
ca. 200 °C for 15 min gave on chromatography (neutral Al2O3, nꢀhexane/Et2O, 50:50) the
title compound 1g (107.3 mg, 71%) as olive green needles, mp (hotꢀstage) 119.0ꢀ123.7 °C (nꢀ
pentane/CH2Cl2, 90:10); Rf 0.60 (nꢀhexane/tꢀBuOMe, 50:50); Anal. Calcd for C19H13FN3: C,
75.48; H, 4.33; N, 13.90. Found: C, 75.31; H, 4.26; N, 13.97; λmax(CH2Cl2)/nm 272 (log ε
3.66), 284 inf (3.43), 322 (2.98), 372 (2.87), 427 (2.61), 493 (2.28), 515 inf (3.63); νmax/cmꢀ1
3065w (aryl CꢀH), 1599w, 1479m, 1450w, 1414w, 1395s, 1325w, 1292w, 1246w, 1217m,
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1155m, 1096w, 1082w, 1065w, 1016w, 928w, 922w, 910w, 881w, 841m, 812w, 762s, 746m,
729s, 714m, 706m; m/z (MALDIꢀTOF) 302 (M+, 21%), 301 (100), 279 (18).
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7ꢀChloroꢀ1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1h): Similar treatꢀ
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with Nꢀ(4ꢀchlorophenyl)(triphenyl)iminophosphorane (8h) (387.8 mg, 1.00 mmol), deaerated
under Ar atmosphere at ca. 20 °C, and immersed into a preheated Wood’s metal bath at ca.
200 °C for 10 min gave on chromatography (silica, nꢀhexane/CH2Cl2, 50:50) the title
compound 1h (99.0 mg, 62%) as black needles, mp (hotꢀstage) 149.0−151.2 °C (cꢀhexane)
(lit.,59 149–152 °C); Rf 0.24 (silica, nꢀhexane/CH2Cl2, 50:50); λmax(CH2Cl2)/nm 275 (log ε
3.55), 322 (2.81), 374 (2.72), 436 (2.48), 480 inf (2.05); νmax/cm−1 3069w (aryl CꢀH), 1614w,
1591w, 1474s, 1449w, 1394m, 1356w, 1317w, 1294w, 1265w, 1242w, 1192w, 1150w,
1092w, 1082w, 1024w, 897m, 846w, 829m, 779m; identical to an authentic sample.
4.7.7
7ꢀBromoꢀ1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1i): Similar treatment
of 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (105.0 mg, 0.50 mmol) in diphenyl ether (1 mL) with Nꢀ
(4ꢀbromophenyl)(triphenyl)iminophosphorane (8i) (432.3 mg, 1.00 mmol), deaerated under
Ar atmosphere at ca. 20 °C, and immersed into a preheated Wood’s metal bath at ca. 200 °C
for 5 min gave on chromatography (silica, nꢀhexane/CH2Cl2, 50:50) the title compound 1i
(103.4 mg, 57%) as black needles, mp (hotꢀstage) 160.1ꢀ162.3 °C (cꢀhexane) (lit.,59 160ꢀ162
°C); Rf 0.26 (silica, nꢀhexane/CH2Cl2, 50:50); λmax(CH2Cl2)/nm 277 (log ε 3.62), 322 (2.87),
376 (2.84), 434 (2.59), 498 (2.08); νmax/cm−1 3069w (aryl CꢀH), 1591w, 1566w, 1476s,
1449m, 1393s, 1317w, 1265w, 1242w, 1196w, 1130w, 1070w, 1061w, 1022w, 926w, 889m,
827m, 777s, 758w, 740w; identical to an authentic sample.
4.7.8
7ꢀIodoꢀ1,3ꢀdiphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1j): Similar treatment
of 1ꢀbenzoylꢀ2ꢀphenyldiazene (10a) (105.0 mg, 0.50 mmol) in diphenyl ether (1 mL) with Nꢀ
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(4ꢀiodophenyl)(triphenyl)iminophosphorane (8j) (479.3 mg, 1.00 mmol), deaerated under Ar
atmosphere at ca. 20 °C and immersed into a preheated Wood’s metal bath at ca.at ca. 160
°C for 5 min gave upon chromatography (silica, nꢀhexane/CH2Cl2, 50:50) the title compound
1j (106.7 mg, 52%) as black needles, mp (hotꢀstage) 150.4ꢀ151.8 °C (cꢀhexane) (lit.,59 149–
152 °C); Rf 0.26 (silica, nꢀhexane/CH2Cl2, 50:50); λmax(CH2Cl2)/nm 280 (log ε 3.37), 323
(2.77), 377 (2.71), 438 (2.37), 481 inf (1.72); νmax/cm−1 3065w (aryl CꢀH), 1591w, 1487m,
1477s, 1458w, 1450m, 1393s, 1315m, 1267w, 1246w, 1194w, 1175w, 1067w, 1057w,
1024m, 883s, 827s, 781s, 773s, 739m; identical to an authentic sample.
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4.7.9
3ꢀ(4ꢀNitrophenyl)ꢀ1ꢀphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl
(1k): Similar
treatment of 1ꢀ(4ꢀnitrobenzoyl)ꢀ2ꢀphenyldiazene (10d) (127.6 mg, 0.50 mmol) in diphenyl
ether (1 mL) with Nꢀphenyl(triphenyl)iminophosphorane (8a) (353.4 mg, 1.00 mmol),
deaerated under Ar atmosphere at ca. 20 °C and immersed into a preheated Wood’s metal
bath at ca. 200 °C for 10 min gave on chromatography (silica, nꢀhexane/Et2O, 50:50) the title
compound 1k (123.5 mg, 75%) as dark brown needles, mp (hotstage) 195.9ꢀ197.8 °C (cꢀ
hexane), mp (DSC) onset: 189.5 °C, peak max: 192.7 °C; decomp. onset: 272.0 °C, peak
max: 317.5 °C (cꢀhexane); Rf 0.60 (nꢀhexane/Et2O, 50:50); Anal. Calcd for C19H13N4O2: C,
69.29; H, 3.98; N, 17.01. Found: C, 69.12; H, 4.04; N, 16.98; λmax(CH2Cl2)/nm 244 (log ε
4.28), 311 (4.43), 353 inf (3.82), 425 (3.44), 460 inf (3.24), 509 (3.14); νmax/cmꢀ1 1599w,
1522s, 1483m, 1450w, 1383w, 1344s, 1323w, 1314w, 1302w, 1252w, 1206w, 1175w,
1161w, 1152w, 1107w, 1099w, 1080w, 1067w, 1026w, 1013w, 986w, 864m, 850m, 841w,
756s, 737m, 702m; m/z (MALDIꢀTOF) 330 (MH+, 19%), 329 (M+, 100), 284 (7). In situ
reduction of 3ꢀ(4ꢀnitrophenyl)ꢀ1ꢀphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazinꢀ4ꢀyl (1k) using Lꢀ
ascorbic acid (1.8 mg, 0.01 mmol) in DMSOꢀd6 (0.6 mL) at ca. 20 °C for 2 min, gave the
NMR spectra of 3ꢀ(4ꢀnitrophenyl)ꢀ1ꢀphenylꢀ1,4ꢀdihydrobenzo[e][1,2,4]triazine (12a): δH(500
MHz, DMSOꢀd6) 8.98 (1H, s, NH), 8.32 (2H, dd, J 7.0, 2.0, Ar H), 8.10 (2H, dd, J 7.0, 2.0 Ar
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