Selective and Scalable Electrosynthesis of 2H-2-(Aryl)-benzo[d]-1,2,3-triazoles and Their N-Oxides by Using Leaded Bronze Cathodes

Article abstract of DOI:10.1002/chem.201905874

Electrosynthesis of 2H-2-(aryl)benzo[d]-1,2,3-triazoles and their N-oxides from 2-nitroazobenzene derivatives is reported. The electrolysis is conducted in a very simple undivided cell under constant current conditions with a leaded bronze cathode and a g

Full text of DOI:10.1002/chem.201905874

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Accepted Article
Title: Selective and Scalable Electrosynthesis of 2H-2-(Aryl)-
benzo[d]-1,2,3-triazoles and Their N-oxides Using Leaded Bronze
Cathodes
Authors: Siegfried R Waldvogel, Tom Wirtanen, and Eduardo Rodrigo
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To be cited as: Chem. Eur. J. 10.1002/chem.201905874
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Selective and Scalable Electrosynthesis of 2H-2-(Aryl)-benzo[d]-
1,2,3-triazoles and Their N-oxides Using Leaded Bronze Cathodes
Tom Wirtanen[a], Eduardo Rodrigo[a], and Siegfried R. Waldvogel*[a]
In the memory of Prof. Jun-ichi Yoshida
Abstract: Electrosynthesis of 2H-2-(aryl)benzo[d]-1,2,3-triazoles and
their N-oxides from 2-nitroazobenzene derivatives is reported. The
electrolysis is conducted in a very simple undivided cell under
constant current conditions with a leaded bronze cathode and a
glassy carbon anode. The product distribution between 2H-2-
(aryl)benzo[d]-1,2,3-triazoles and their N-oxides can be guided by
simply controlling the current density and the amount of the charge
applied. The reaction tolerates several sensitive functional groups in
reductive electrochemistry. The usefulness and the applicability of the
synthetic method is demonstrated by a formal synthesis of an antiviral
compound.
The intriguing and versatile chemical and physical properties of
2H-2-(aryl)benzo[d]-1,2,3-triazoles have made them a frequently
used substructure in many different functional molecules and
commercial products. Just to name a few examples, 2H-2-
(aryl)benzo[d]-1,2,3-triazole containing molecules have been
studied thus far as novel ligands[1] (Scheme 1), as potential
treatment to Duchenne Muscular Dystrophy[2] and as antivirals
with either a broad inhibitory spectrum or a high selectivity
towards Human Poliovirus Type-1 Sabin Strain (HPV-1, Sb-1).[3]
In technical products, the motif has found use in UV-absorbers
(BASF, Tinuvin©) and its use as fluorescent light emitting element
has been patented.[4] Furthermore, also 2H-2-(aryl)benzo[d]-
1,2,3-triazole-1-oxides are versatile synthetic intermediates as
the oxygen can be exploited to tune the reactivity in C-H
functionalizations.[5] Hence, developing new sustainable synthetic
methods for both 2H-2-(aryl)benzo[d]-1,2,3-triazoles and to their
N-oxides preferably from a single starting material is of very high
importance.
Scheme 2. Synthetic approaches to 2H-2-(aryl)benzo[d]-1,2,3-triazoles and 2H-
2-(aryl)benzo[d]-1,2,3-triazole-1-oxides
Broadly, 2H-2-(aryl)benzo[d]-1,2,3-triazoles are synthesized
either with an arylation of N-unsubstituted benzo[d]-1,2,3-
triazoles or by cyclizing 2-substituted azobenzenes either
oxidatively or reductively (Scheme 2). In the former, benzo[d]-
1,2,3-triazoles are N2-arylated with aryl halogens catalyzed by
Pd,[6] Cu[7] or Fe/Cu,[8] with iodine(III) compounds with[9] or
without[10] transition metal catalysts, with silyl aryl triflates,[11] and
in SNAr reactions with electron-deficient fluoroaryls.[12]
In the second strategy, 2-substituted azobenzenes or
azoxybenzenes are either oxidatively or reductively cyclized.
Oxidatively, this sequence has been realized with (2-
tosylamino)azobenzenes[13] and 2-aminoazoxybenzenes[14] using
PhI(OAc)2 as oxidant, or by oxidizing 2-aminoazobenzenes with
Scheme 1. Various applications of the 2H-2-(aryl)benzo[d]-1,2,3-triazole motif
[17]
Cu(II),[15] Cu/Py/O2,[16] PhI(OAc)2 or NaOCl.[18] Substrates with
ortho-N3-moiety, that can also be prepared in-situ from suitable
starting materials, are cyclized thermally[19] or by various different
reagents such as BCl3,[20] BF3,[20] Pd/TBHP,[21] and
CuI/TMEDA.[22] In addition, reduction of 2-nitroazobenzenes can
also give access to 2-substituted benzo[d]-1,2,3-triazoles and this
has been realized using P(III/V)/PhSiH3,[23] Zn,[24] SmI2,[25]
hydroxylamine,[26] thiourea S,S-dioxide,[27] baker´s yeast,[28]
[a]
Dr. T. Wirtanen, Dr. E. Rodrigo, Prof. Dr. S. R. Waldvogel,
Department Chemie
Johannes Gutenber-Universität Mainz
Duesbergweg 10-14, 55128 Mainz (Germany)
E-mail: waldvogel@uni-mainz.de
Homepage: http://www.chemie.uni-mainz.de/OC/AK-Waldvogel/
Supporting information for this article is given via a link at the end of
the document.
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Ru3(CO)12/CO[29] or Ag.[30,31] Furthermore, also galvanostatic[32]
and potentiostatic[33] electroreductive conditions have been
reported in 1903 and 2000, respectively. Notably, many of these
reductive protocols focus only on the synthesis of 2H-2-(2-
hydroxyphenyl)benzo[d]-1,2,3-triazole derivatives and synthetic
methods with a broader scope and a more diverse variation in the
aryl rings have not been elaborated. Beyond these described
oxide (3a) as the test reaction for the screening together with
modified reaction conditions that we have previously found highly
suitable for nitroarenes.[37] We first explored different cathode
materials such as lead, boron-doped diamond (BDD), glassy
carbon (GC), CuSn10Pb10 and CuSn5Pb20 leaded bronzes in a
mixture of MeCN / H2O (3:1) with AcONa:AcOH (pH 3.7, 0.1 M)
buffer additive using GC as an anode with a 12 mA cm-2 current
density (Entries 1-5). Out of these materials, CuSn5Pb20 gave
the best result (Entry 5).
strategies,
also
intramolecular
cyclization
of
2-
nitrophenyl(phenyl)carbodiimide can give access to 2H-2-
(aryl)benzo[d]-1,2,3-triazole with a loss of CO2.[34]
Unfortunately, one or more drawbacks hamper the synthetic
utility of these strategies (Scheme 2). For example,
regioselectivity between N1,3–N2 sites is a common and severe
challenge in the arylations of the N-unsubstituted benzo[d]-1,2,3-
triazoles. Although there have been recently significant advances
to address this shortcoming, most of these methods also suffer
from a poor atom economy. The regioselectivity issue is averted
when 2-substituted azobenzenes or azoxybenzenes are cyclized
oxidatively or reductively, but again, these approaches are
hindered either by the use of transition metal catalysts, highly
sophisticated organocatalytic systems or enzymes, the necessity
to install N-substituents, or by the required (over)stoichiometric
amounts of oxidants or reductants. The published electrochemical
procedures elegantly eliminate the need of redox reagents and
also address other important issues, but they as well have
limitations. In the first communication, only a single isolated yield
was reported, whereas the latter report focuses mainly onto the
synthesis of 2H-2-(2-hydroxyphenyl)benzo[d]-1,2,3-triazoles.[32,33]
The yields obtained with them were excellent, but a significant
drop was realized when the 2-hydroxyphenyl moiety was
substituted (40-63%). Furthermore, the use of LiClO4 as the
supporting electrolyte, the three-electrode arrangement, and the
platinum electrodes combined with the high amount of required
charge present safety, scalability, and economic issues,
respectively. This makes these methods very limited useful for
synthetic applications and not exploitable for technical processes.
Synthetic organic electrochemistry has in recent years
experienced a renaissance and it can be regarded as a pinnacle
of sustainability in many terms. For instance, electrons or holes
can replace chemical redox reagents that reduces both the waste
and the cost of the synthesis. The green aspects of
electrochemistry are evident in numerous different types of
reactions,[35] particularly in arylations.[36] Encouraged by our
recent success in the electroreductive functionalization of
nitroarenes[37] and the discovery of leaded bronzes as safe and
efficient alternatives for lead,[38] we set to develop new general
electrochemical conditions for the synthesis of 2H-2-
(aryl)benzo[d]-1,2,3-triazoles. The most efficient way to identify
suitable electrolytic operational conditions is achieved by
electrosynthetic screening.[39] We report that an operationally very
simple galvanostatic electrolysis of 2-nitroazobenzene derivatives
in an undivided cell can be used to access both 2H-2-
(aryl)benzo[d]-1,2,3-triazoles as well as their N-oxides in high
yields from single starting materials. The control of product
distribution is achieved easily by controlling only the current
density and the amount of applied charge.
Table 1. Screening of the reaction conditions
Entry
Cathode
Additive[a,b]
Current
density
(mA cm-2
Applied
charge
(F / mol)
Yield[c] of
2a / 3a,
(2a+3a)
)
1
2
BDD
AcONa /
AcOH
12
12
12
12
12
12
12
12
12
12
12
4.1
2.4
5
5
18 / 27
45
Pb
AcONa /
AcOH
27 / 15
42
3
GC
AcONa /
AcOH
5
16 / 6
22
4
CuSn10Pb10
CuSn5Pb20
CuSn5Pb20
CuSn5Pb20
CuSn5Pb20
CuSn5Pb20
CuSn5Pb20
CuSn5Pb20
CuSn7Pb15
CuSn7Pb15
AcONa /
AcOH
5
36 / 16
52
5
AcONa /
AcOH
5
40 / 21
61
6
HCOONa /
HCOOH
5
40 / 21
61
7
Na2CO3
5
43 / 16
59
8
-
5
40 / 19
59
9
NaOH
KOH
5
62 / 24
86
10
11
12[d]
13[d]
5
48 / 19
67
LiOH*2H2O
NaOH
NaOH
5
37 / 37
74
8.4
8.4
87 / 9
96
0 / 94
94
[a] In entries 1-11, NBu4BF4 was used 0.01M and in entries 12-13, 0.014M [b]
added as 0.1 M solution in H2O (0.025 M) (entries 1-6) or as a solid (entries 9-
13) (0.1 M) [c] Conversion based yield was determined with 1H or 19F NMR-
spectroscopy using 1,3,5-trimethoxybenzene or NBu4BF4 as an internal
standard. [d] MeOH was used as a solvent instead of MeCN:H2O (3:1)
We initiated the study by choosing the cyclisation of 4’-fluoro-2-
nitroazobenzene (1a) to 2H-(4-fluorophenyl)-2-benzo[d]-1,2,3-
triazole (2a) and 2H-(4-fluorophenyl)-2-benzo[d]-1,2,3-triazole 1-
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We then continued by screening several different buffers and
additives. Formate buffer (pH 3.7, 0.1 M), Na2CO3 (2.67 equiv.), and
electrolysis in the absence of any additive gave comparable results
to AcONa:AcOH buffer (Entries 6-8), whereas the use NaOH-
additive increased the yield notably (Entry 9). We then explored the
use of 2.67 equiv. of KOH, and LiOH*2H2O but these modifications
did not improve the obtained yields (Entries 10-11). Changing the
anode to stainless steel (SS, 1.4301) undergoes severe corrosion,
whereas another SS (VA 1.4571) and nickel furnished slightly lower
yields than GC. Finally, after lowering the current density from 12 to
4.1 mA cm-2, exchanging the cathode from CuSn5Pb20 to
CuSn7Pb15, and replacing the solvent to MeOH due to the poor
solubility of some of the starting materials and the slight immiscibility
of MeCN and H2O caused by the added NaOH, we were able to
obtain 2a in 87% NMR yield (Entry 12). In general, the reaction
mixtures were clearer in MeOH than in MeCN / H2O (3:1).
Interestingly, 3a could be accumulated as the main product when
the current density was even further attenuated to 2.4 mA cm-2
(Entry 13)
To our delight, also electron-rich 2-(4-MePh) (2d) and 2-(4-OMePh)
(2e) substituted compounds were obtained in excellent 92% and
88% yields, respectively. The latter example is in particular
important, as the Sb-1 inhibitor (Scheme 1) can be easily accessed
from 2e via demethylation by a literature-known protocol in 82%
yield.[40] We then explored the halogen-series as they are excellent
synthetic handles for further functionalization and they have easy
accessible LUMOs that can accept electrons instead of NO2 or its
subsequently reduced reaction intermediates. This tendency could
lead to side-reactions such as dehalogenations in the employed
protic media. Interestingly, 2-(4-ClPh) (2c) was obtained in an 83%
yield whereas 2-(4-BrPh) (2g) was isolated in a 62% yield, together
with 6% of the dehalogenated compound 2b (Scheme 3). Also, 5-
Br-2-Ph (2f) substituted benzo[d]-1,2,3-triazole could be obtained in
a moderate 57% yield. In this case, dehalogenated side product
was also observed in the reaction mixture. Unfortunately, we were
unable to isolate the 2-(4-IPh) derivative 2k as the major component
(23:77 I/H) with our reaction conditions. Furthermore, we obtained
2f and 2g more selectively, when the current density was lowered
from 4.1 to 2.4 mA cm-2. Moreover, 2-(3,5-bis-(CF3)Ph) derivative
1i converted readily to 2i with an excellent yield of 89%. The 2-(4-
CNPh) was first obtained in a 29% yield but when we changed the
additive from NaOH to Et3N (12 equiv.), 45% yield was received.
Surprisingly, 2-(2-COOMePh) (2h) derivative was obtained in a
12% yield. For 2c, 2f-2h, 2j-k we used 3:2 THF:MeOH mixture as
a reaction solvent, as MeOH alone did not sufficiently solubilize
their starting materials.
[a] 3:2 THF:MeOH instead of MeOH [b] Current density of 2.4 mA cm-2 was
used [c] 12 equiv. of Et3N instead of NaOH
Scheme 3. Scope to 2H-2-(aryl)-benzo[d]-1,2,3-triazoles with isolated yields
With this set of reaction conditions, we then started to explore the
scope of the reaction. We first attempted the synthesis of different
benzo[d]-1,2,3-triazoles with a 4.1 mA cm-2 current density. In
addition to 2a, that could be isolated in an 86% yield (Scheme 3),
also unsubstituted substrate 2b was received in a high 89% yield.
[a] 3:2 THF:MeOH instead of MeOH
Scheme 4. Scope to 2H-2-(aryl)-benzo[d]-1,2,3-triazole N-oxides with isolated
yields
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The reductive cyclization of 2-nitroazobenzenes to 2H-2-
(aryl)benzo[d]-1,2,3-triazoles is a 4e-/4H+ process. Therefore, 2H-
A plausible mechanistic rationale from preliminary studies is
proposed in Scheme 6. At the beginning, 1a is reduced to nitroso
intermediate (Int-2) at the cathode either by addition of 2e-/2H+,
or alternatively, by 4e-/4H+ that generates first the hydroxylamine
intermediate (Int-1) that is further reversibly oxidized at the anode
with a loss of 2e-/2H+ to Int-2.[42] Cyclic voltammetry (CV) of 1a
shows two broad reductive waves at -0.71 V and -1.17 V (vs.
Ag/Ag+, see SI for further details). Furthermore, from CV of 2a, we
deduce that the latter reductive wave of 1a is associated to 2a.
Therefore, 2a can accumulate already during the reductive phase
of the CV of 1a and the oxidation of Int-1 to Int-2 is not a
prerequisite for the generation of 2a. Next, 5-atom 6-
electrocyclization converts Int-2 to 2a. Calculations at DLPNO-
CCSD(T)/def2-QZVPP/SMD(MeOH) // PBE0-D3/def2-TZVP/
COSMO(MeOH) level of theory reveal that the electrocyclization
of Int-2(b) to 2b has a ΔG‡ of 8.2 kcal mol-1 and the cyclization is
overall exothermic by 17.1 kcal mol-1 (SI). After electrocyclization,
2e- / 2H+ reduction of N-oxide furnishes the final product. At the
anode, methanol oxidation to sodium formate supplies electrons
for the reduction.
2-(aryl)benzo[d]-1,2,3-triazoles-N-oxides
should
become
analogously accessible if the starting materials are only reduced
by 2e-/2H+. Based on this assumption, we then attempted
synthesis of benzo[d]-1,2,3-triazole-N-oxides. In all cases, we
lowered the current density to 2.4 mA cm-2 to make reactions
more selective (Scheme 4). Again, the electron-rich 2-(4-MePh)
(3d) and 2-(4-OMePh) (3e) were obtained in excellent yields of
91% and 82%, respectively. Also, unsubstituted 2-(Ph) (3b), 2-(4-
FPh) (3a), and 2-(4-ClPh) (3c) derivatives were obtained in high
83%, 97% and 81% yields, respectively. In addition, 2-(3,5-bis-
(CF3)Ph)-derivative 1i furnished 3i in an 88% yield and 1h yielded
31% of 2-(2-COOMePh) derivative 3h. Moreover, we were able to
isolate 5-Br-2-Ph (3f) and 2-(4-BrPh) (3g) with good yields of 83%
and 86% and, surprisingly, also 2-(4-IPh) (3k) could be obtained
in a 74% yield. Even though we were unable to obtain 2H-2-(4-
iodophenyl)benzo[d]-1,2,3-triazole (2k) directly, the relatively high
yield of its N-oxide 3k indicates that the utilization of iodine-
derivatives is also possible in a multi-step synthesis. It is
conceivable that after stopping the reductive cyclization at the N-
oxide state, the iodo substituent can be utilized as a synthetic
handle for further functionalization that is followed by
electrochemical deoxygenation to functionalized benzo[d]-1,2,3-
triazoles. As with the synthesis of 2H-2-(aryl)benzo[d]-1,2,3-
triazoles, 3c, 3f-3h, 3k were obtained in 3:2 THF:MeOH mixture.
Regarding the anodic counter reaction, during reaction
monitoring in MeOH, we observed evolution of a 1H NMR
resonance at 8.49 ppm that we confirmed to originate from a
sodium formate by comparing to an authentic sample. Thus, it is
plausible that sodium hydroxide additive has multiple roles in the
reaction. On the one hand, it might contribute to the decrease of
the oxidation potential of the methanol in the anodic counter
oxidation,[41] and on the other hand it is an extra electrolyte that
lowers the terminal voltage. At this point, we became interested
whether NBu4BF4 could be omitted altogether from the reaction
mixture as this would simplify any downstream processes.
Gratifyingly, 2a and 2d were obtained in a 92% and a 91% yield
(NMR) in the absence of NBu4BF4 after passing 11.7 F and 10.6
F of charge, respectively (Scheme 5). Furthermore, reaction of 1d
to 2d was scaled-up from 0.18 mmol to 3.4 mmol (19x) in the latter
experiment, which confirms the excellent robustness and
scalability of the studied reaction.
Scheme 6: Plausible mechanistic rationale (left) and CV Epc potentials of 1a-2a
(right). CV measurements with glassy carbon (GC) working, GC counter, and
Ag/Ag+ reference electrode in sat. LiCl/EtOH using 150 mVs-1 scan rate in 0.1
M NaOH in MeOH.
To conclude, we have realized an efficient synthetic protocol for
accessing both 2H-2-(aryl)-benzo[d]-1,2,3-triazoles and their N-
oxides from the same starting materials. The developed
electrochemical conditions have several elements that make it
highly interesting over its precedents in academic and industrial
settings. The reactions are set-up under operationally very simple
galvanostatic conditions in an undivided cell and the reductive
cyclizations can be optionally performed by using inexpensive
NaOH as both the supporting electrolyte and additive which
simplifies possible downstream processes. Furthermore, leaded
bronze cathodes and glassy carbon anodes are safe and
sustainable materials and the scalable and robust reaction
proceeds in general with high yields and tolerates several
challenging functional groups.
[a] at 3.4 mmol (m = 814 mg)
Scheme 5. Experiments without NBu4BF4 supporting electrolyte
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Experimental Section
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General protocol for the synthesis of 2H-2-(aryl)benzo[d]-1,2,3-
triazoles and 2H-2-(aryl)benzo[d]-1,2,3-triazole N-oxide derivatives:
Starting material (0.18 mmol), NBu4BF4 (0.07 mmol), and NaOH (0.5
mmol) or Et3N (2.16 mmol) were dissolved to either MeOH or 3:2
THF:MeOH (5 mL). The reaction mixture was then stirred for ~30 min at
33-37 °C (RT) and during that time the CuSn7Pb15 cathode was gently
polished and glassy carbon anode was cleaned with H2O and acetone
before constant current electrolysis (either 2.4 or 4.1 mA cm-2) was started.
The reaction was controlled by analyzing the crude reaction mixture
periodically by 1H or 19F NMR. After the reaction was complete, the
reaction mixture was evaporated. Purification of the crude material by flash
column chromatography (SiO2) using mixtures of cyclohexane:ethyl
acetate as eluents afforded the title compounds.
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Acknowledgements
Support by the DFG (Wa1276/17) is highly appreciated. TW
gratefully acknowledges the fellowship by the Oskar Huttunen
Foundation.
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Keywords: electrochemistry • reduction • nitrogen heterocycles
• azo compounds • sustainable chemistry
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Entry for the Table of Contents (Please choose one layout)
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COMMUNICATION
Electrosynthesis takes the lead:
Electrosynthesis enables green and
atom-economic access to 2H-2-
(aryl)-benzo[d]-1,2,3-triazoles or
their N-oxides from 2-
Tom Wirtanen, Eduardo Rodrigo, and
Siegfried R. Waldvogel*
Page No. – Page No.
nitroazobenzenes. This
operationally very simple protocol is
Insert TOC Graphic here))
Selective and Scalable
Electrosynthesis of 2H-2-(Aryl)-
benzo[d]-1,2,3-triazoles and Their
N-oxides Using Leaded Bronze
Cathodes
conducted in an undivided cell by
constant current electrolysis.
Optionally inexpensive NaOH can
be used both as a supporting
electrolyte and additive.
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