Anodic oxidation of bisamides from diaminoalkanes by constant current electrolysis

Article abstract of DOI:10.3762/bjoc.14.72

In general, bisamides derived from diamines and involving 3 and 4 methylene groups as spacers between the two amide functionalities behave similar to monoamides upon anodic oxidation in methanol/LiClO₄ because both types undergo majorly mono- and dimethoxylations at the α-position to the N atom. However, in cases where the spacer contains two methylene groups only the anodic process leads mostly to CH₂-CH₂ bond cleavage to afford products of type RCONHCH₂OCH₃. Moreover, upon replacing LiClO₄ with Et₄NBF₄ an additional fragmentation type of product was generated from the latter amides, namely RCONHCHO. Also, the anodic process was found to be more efficient with C felt as the anode, and in a mixture of 1:1 methanol/acetonitrile co-solvents.

Full text of DOI:10.3762/bjoc.14.72

Anodic oxidation of bisamides from diaminoalkanes
by constant current electrolysis
Tatiana Golub and James Y. Becker*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 861–868.
Department of Chemistry, Ben-Gurion University of the Negev, Beer
Sheva 84105, Israel
doi:10.3762/bjoc.14.72
Received: 20 December 2017
Accepted: 27 March 2018
Published: 16 April 2018
Email:
James Y. Becker* - becker@bgu.ac.il
* Corresponding author
This article is part of the Thematic Series "Electrosynthesis II".
Guest Editor: S. R. Waldvogel
Keywords:
anodic oxidation; bisamides; constant current electrolysis;
methoxylation
© 2018 Golub and Becker; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
In general, bisamides derived from diamines and involving 3 and 4 methylene groups as spacers between the two amide functionali-
ties behave similar to monoamides upon anodic oxidation in methanol/LiClO4 because both types undergo majorly mono- and
dimethoxylations at the α-position to the N atom. However, in cases where the spacer contains two methylene groups only the
anodic process leads mostly to CH2–CH2 bond cleavage to afford products of type RCONHCH2OCH3. Moreover, upon replacing
LiClO4 with Et4NBF4 an additional fragmentation type of product was generated from the latter amides, namely RCONHCHO.
Also, the anodic process was found to be more efficient with C felt as the anode, and in a mixture of 1:1 methanol/acetonitrile
co-solvents.
Introduction
It is well known that the anodic oxidation of amides involving a anodic route could also be important from a synthetic point of
hydrogen atom at the α-position to the N atom could undergo view, affording ring-expansion [8-10] and annulation of rings
alkoxylation, carboxylation and hydroxylation at this position [1,11,12] (Scheme 3).
[1-5] (Scheme 1). It has been found that anodic methoxylation
of amides (and carbamates) can be utilized to form new Interestingly, in the case of anodic oxidation of aromatic amides
carbon–carbon bonds [6,7] (Scheme 2). Furthermore, this of type Ph2CHCONHAr, where no hydrogen atom is present at
861

Beilstein J. Org. Chem. 2018, 14, 861–868.
Scheme 4: Anodic bond cleavages in amides of type Ph2CHCONHAr.
Scheme 1: Anodic oxidation of amides.
Scheme 5: Type of products obtained (n = 0, 1, 2).
Scheme 2: Anodic oxidation of an amide in the presence of alkene.
the α-position to the N atom, they undergo three types of
bond cleavages (instead of the common substitution) [13]
(Scheme 4).
Previously we investigated [14] the effect of the ring size (5-,
6-, and 7-membered) and the nature of the supporting elec-
trolyte on the anodic methoxylation of N-acylazacycloalkanes at
the α-position to the nitrogen. The outcome revealed the forma-
tion of four types of products of which two involve saturated
and two unsaturated cyclic amides (Scheme 5).
Scheme 6: Synthesized cyclic N-acyl and N-sulfonyl piperidines for
electrolysis.
Amides and polyamides have been found as key units in many
biologically active and pharmaceutical compounds. For
The selectivity of the anodic process was found to be highly de- instance, symmetrical and unsymmetrical bisamides derived
pendent on the electrolyte used, and to a lesser extent, on the from diamines are significant components as structural subunits
substrate concentration. Notably the importance of the former for the construction of peptidomimetric frameworks [18] and as
parameter in electrolysis has been well documented [15,16].
lubricants [19]. To the best of our knowledge nothing has been
known so far about electrochemical properties of α,ω-bisamides
More recently the effect of the functional group attached to the derived from α,ω-diaminoalkanes. However, notably that the
N atom in various piperidine derivatives (Scheme 6) on the bisamide 3,5-diaza-2,6-heptanedione was obtained from
anodic oxidation of 'cyclic amides' was explored in methanol N-methylacetamide by electrolysis on a Pt anode in water [20].
under different experimental conditions [17]. The results indi-
cate that this type of amides mostly undergo mono- and The present work describes the electrochemical behavior of
dimethoxylation at the α and α'-positions to the N atom. It was eight synthesized bisamides (from diamines, Scheme 7) and the
also found that the relative ratio among products was strongly outcome from their preparative electrolysis (at constant current)
dependent on the nature of the supporting electrolyte, the anode under different experimental conditions, as a function of the
material and the substituent group attached to the N atom.
length of spacer between the two amide functionalities.
Scheme 3: Intramolecular cyclization via anodic oxidation of an eneamide.
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Beilstein J. Org. Chem. 2018, 14, 861–868.
Scheme 7: Type of bisamides (derived from diamines) studied (n = 2, 3, 4).
Results and Discussion
Table 1: Oxidation potentials of α,ω-bisamides (Scheme 7) measured
by cyclic voltammetrya.
The electrochemical properties of the bisamides described in
Scheme 6 were studied by cyclic voltammetry and all of their
redox potentials were found to be irreversible in acetonitrile.
Their first oxidation potentials (in the range of 2.1–2.35 V vs
Ag/AgCl) are summarized in Table 1. The first and second
columns indicate that the longer the spacer the higher the oxida-
tion potential. Also it is not surprising that the derivative with
EWG (IV) is more difficult to oxidize than that with EDG (III).
All bisamides derivatives exhibit one irreversible cathodic wave
(not shown, at −2.2 to −2.4 V).
entry
Ep(ox) (V)
I
II
III
IV
n = 2
n = 3
n = 4
2.10
2.23
2.27
2.13
2.28
2.35
2.17
2.33
aIn CH3CN/0.1 M LiClO4; potentials are quoted versus Ag/AgCl refer-
ence electrode. Working electrode: glassy carbon disk (1.5 mm in
diameter). Auxiliary electrode: a Pt wire.
Constant current electrolysis (CCE) at a current density of
20 mA/cm2 was carried out for all of the above synthesized
bisamides under various experimental conditions, using differ-
ent supporting electrolytes, anodes, and electricity consumption.
Bisamide II (n = 3, will be designated as II-3 hereafter) was
arbitrarily chosen as a model compound for initial electrochemi-
cal studies. The spectrum of products obtained is described in
Scheme 8. Except for the expected monomethoxylated II-3a
and dimethoxylated II-3b products, fragmentation products (II-
3c, 3d, 3e) were observed too.
Table 2 below summarizes the type of products and their rela-
tive ratios obtained from initial electrochemical oxidation of
II-3 under various experimental conditions. It appears that the
selectivity and efficiency of the anodic process depends on both
the anode material and electricity consumption (F/mol). Thus
the oxidation of II-3 on a C anode (Table 2, entries 1 and 2) is
quite selective providing mostly mono-II-3a and dimethoxy-
II-3b products in addition to ≈10% of methyl benzoate (II-3d)
as a fragment. Notably that 20–30% of unreacted substrate
Scheme 8: Type of products obtained from anodic oxidation of II-3.
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Beilstein J. Org. Chem. 2018, 14, 861–868.
Table 2: The effects of anode material and electricity consumption on the results of anodic oxidation of substrate II-3 in MeOH/LiClO4a.
entry F/mol anode material
products
unreacted substrate
II-3a
II-3b
II-3c methyl benzoate (II-3d) benzoic acid (II-3e)
1
2
5
C
C
40
27
26
20
43
–
18
40
8
–
5
–
5
–
–
–
12
9
–
–
30
19
10
5
3
Pt
21
5
10
20
3
35
4
10
10
10
10
Pt
40
12
–
10
5b
6c
7d
GC
PbO2
DSA
5
36
–
–
90
–
–
–
–
≈100
aYield of products was determined by 1H NMR relative integration. bGC = glassy carbon. cUnidentified products (≈10%). dDSA = dimensionally stable
anode, coated with oxides of Ru and Ir.
remains. An increase in electricity consumption (entries 2 vs 1) 10 F/mol). The results are shown in Table 3. It appears that
promotes the formation of the dimethoxy product II-3b over the except for entries 5 and 6, the amounts of unreacted starting ma-
monomethoxy one II-3a.
terials are considerably high (60–70%), indicating that the reac-
tion is far from being efficient under these conditions, presum-
ably because of favorable oxidation of the solvent methanol.
Another reason for this observation could stem partially from
the limited solubility of some of the substrates in this solvent
(e.g., II-2, III-2 and IV-2). The yield of total products is in the
range of 30–80%, depending on the nature of the substituent at-
tached to the carbonyl moiety, roughly in the order of:
Ph > CH3 > p-MeOC6H4, p-NO2C6H4. In terms of type of prod-
ucts, they are mostly analogous to those described in Scheme 8
for II-3. However, it is obvious that in the case of anodic oxida-
tion of substrates with n = 2, an additional new fragmented
product (type f) was formed due to CH2–CH2 bond cleavage,
which is exclusive for this type of bisamides (Scheme 9).
In comparison to the above results, oxidation of II-3 on a Pt
anode (Table 2, entries 3 and 4) affords similar products but
with less selectivity because of the formation of an additional
fragmentation product, benzoic acid (II-3e) in 10–20% yield.
Other anodes were tested as well (Table 2, entries 5–7) at an
electricity consumption of 10 F/mol. It appears that a GC anode
favors the formation of the monomethoxy product II-3a where-
as the anodes of PbO2 and DSA are the worst because most of
the substrate remained unreacted. Apparently at these two
anodes the oxidation of the solvent methanol prevails.
Based on the results in Table 2 in which a C anode afforded
better selectivity and efficiency compared to the other anodes
studied, all other substrates outlined in Scheme 7 were oxidized Obviously the fact that considerable amounts of starting materi-
at this anode and under the same conditions (namely, in metha- als were left unreacted (in most cases, except for Table 3,
nol/LiClO4, at 20 mA/cm2, and with electricity consumption of entries 5 and 6) has been dissatisfying and therefore, prompted
Table 3: Results of anodic oxidation of all substrates on a C rod anode in MeOH/LiClO4. (10 F/mol; 20 mA/cm2)a.
entry
substrate
monomethoxy dimethoxy type methoxylated
methyl
benzoate 'd'
RCONHCH2OCH3
unreacted
substrate
type 'a'
'b'
amide type 'c'
'f'
1
2
I-2 (n = 2)
I-3 (n = 3)
I-4 (n = 4)
II-2
10
30
5
–
10
30
–
–
–
–
–
5
–
–
–
–
–
25
–
65
60
65
63
19
30
70
65
3
–
–
4
9
5
23
–
5
II-3
27
26
15
–
40
27
–
9
6
II-4
17
–
–
7
III-2
15
10
8b
IV-2
–
–
aRelative yields of all products were determined by 1H NMR integration. Analogous type of products (a–d) are described in Scheme 7. bPhCONH2
was obtained in 10% yield along with 25% of unidentified products.
864

Beilstein J. Org. Chem. 2018, 14, 861–868.
Scheme 9: Anodic splitting of C–C bond in bisamides in the presence
Scheme 10: Anodic splitting of C–C bond in bisamides in the pres-
of LiClO4 electrolyte.
ence of Et4NBF4 electrolyte.
us to change some parameters. At first, the LiClO4 electrolyte parallel, the C rod anode was replaced with a C felt (that has a
was replaced by Et4NBF4 and the results are shown in Table 4. considerable larger surface area) in attempts to improve both
Clearly the bisamides with n = 2 afforded now, in addition to f, efficiency and selectivity. The results are described in Table 5
new fragmented products, aldehydes of type g (Scheme 10).
and they show a pronounced difference compared to the ones in
Table 4 because product yields are higher now (Table 5, entries
In addition, the relative yield of the fragmented products (f and 1, 4 and 5 show almost completion) even after consuming only
g, both derived exclusively from bisamides with n = 2) in- 5 F/mol. In addition, although the spectrum of major products is
creased considerably in the presence of this electrolyte. Howev- similar in both tables, the weight of monomethoxylated prod-
er, again, the amounts of unreacted starting materials were still ucts (51–56%, entries 2 and 5) and fragmented ones (of type f,
significant in some cases.
85–95%, entries 1 and 4) increased at the expense of consumed
starting material (in all entries except for entry 8).
The pronounced difference between the results obtained by the
two electrolytes, namely affording 'f' with LiClO4, and 'g' in ad- Previously (Table 3 vs Table 4) we observed a marked differ-
dition to 'f' with Et4NBF4, could stem from the different compo- ence in results upon replacing LiClO4 with Et4NBF4. Whereas
sition of the solution at the electrode surface (caused by differ- in the former case mono- and dimethoxylated products were
ent solvation of the electrolyte anion) than that of the bulk of predominant, fragmentation products of type f and g became
the solution. Such a phenomenon was discussed in the litera- major in the latter case. Based on these observations a further
ture previously [15,16,21]. For instance, Nyberg [16] already attempt to improve the results outlined in Table 5 was con-
demonstrated the effect of ClO4− vs BF4− in the anodic oxida- ducted by employing similar conditions except for using
tion of hexamethylbenzene in aqueous acetonitrile, and pro- Et4NBF4 (instead of LiClO4) this time. Some selected sub-
posed that tetraflouroborate anion preferentially brought water strates from Table 5 that left a considerable amount of unre-
into the anode surface giving high yield of ArCH2OH (com- acted starting material, namely I-4, II-4, III-2 and IV-2, were
pared to ArCH2NHCOMe with perchlorate). Therefore also in chosen to be reoxidized under these modified conditions. The
our case, the electro generated carbocation intermediate formed outcome described in Table 6 indicates that this approach was
(RCONHCH2+) could meet with methanol (to form 'f') or water useful for one substrate only, II-4.
(to form 'g') preferentially at the electrode surface, dictating the
ratio between products 'f' and 'g'.
Mechanism
A mechanism of formation of mono- and dimethoxylated
In order to increase the solubility of the substrates with a amides is well-documented in the published literature
limited one a mixture of MeOH/MeCN (1:1) was used, and in [1-6,14,17,23]. It is generally accepted that the initial electron
Table 4: Results of preparative electrolysis of selected bisamides in MeOH/Et4NBF4 (at C rod anode, 10F, 20 mA/cm2)a.
entry
substrate
monomethoxy type 'a' dimethoxy type 'b' RCONHCH2OCH3 'f' RCONHC(O)H 'g' unreacted substrate
1
2
3
4
5
6
I-2
I-3
10
10
13
–
–
10
–
51
–
26
–
13
80
87
14
77
18
I-4
–
–
II-2
–
69
23
51
17
–
III-2 (p-OMe)
IV-2 (p-NO2)
–
–
–
–
31
aYields were determined by 1H NMR relative integration. Analogous type of products a,b are described in Scheme 7.
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Beilstein J. Org. Chem. 2018, 14, 861–868.
Table 5: Resultsa of anodic oxidation of bisamides on a C felt anode in MeOH/MeCN (1:1)/LiClO4; 20 mA/cm2; 5 F/mol).
entry
substrate
monomethoxy dimethoxy type
ester type
benzoic acid
RCONHCH2OCH3
unreacted
substrate
type 'a'
'b'
'd'
'e'
'f'
1
2
3
4
5
6
7
8
I-2
I-3
5
–
7
–
–
–
–
–
–
5
–
–
5
95
–b
–
–
56
20
–
34
50
–
I-4
30
–
–
II-2
II-3
II-4
III-2
IV-2
15
8
85
(h)c
–
51
19
10
–
24
22
–
5
16
5
43
60
85
25
5
–
5
aYield are determined by 1H NMR relative integration. Analogous type of products (type a, b, d, e) are described in Scheme 7. bAldehyde (3%):
MeCONHCH2CH2CHO (from, I-3) [22]. cUnsaturated bisamide (7%): PhCONHCH2CH=CHNHCOPh (II-3h).
Table 6: Results of anodic oxidation of selected substrates on C felt anode in MeOH/MeCN (1:1)/Et4NBF4; 20 mA/cm2; 5 F/mol.
entry
1
substrate
monomethoxy
dimethoxy type
ester type
fragmented
products
unreacted
substrate
type 'a'
'b'
'd'
I-4
10
–
–
MeCONHCHO
85
(I-4g, 5%)
2
3
4
II-4
12
–
74
–
8
–
6
ArCONHCH2OCH3
III-2 (p-OMe)
IV-2 (p-NO2)
15
31
75
69
(III-2f, 10%)
–
–
–
transfer forms an iminium cation radical followed by deproton- Conclusion
ation and further oxidation to generate an iminium ion/carbocat- In general, bisamides derived from diamines and involving 3
ion that undergoes methoxylation in methanol, as described in and 4 methylene groups as spacers between the two amide func-
Scheme 11.
tionalities behave similary to monoamides upon anodic oxida-
tion in methanol/LiCiO4 because both types undergo majorly
mono- and dimethoxylations at the α-position to the N atom.
However, in cases where the spacer contains two methylene
groups only the anodic process leads mostly to CH2–CH2 bond
cleavage to afford products of type RCONHCH2OCH3. More-
over, upon replacing LiClO4 with Et4NBF4 an additional frag-
mentation type of product was generated from the latter amides,
namely RCONHCHO. Also, the anodic process was found to be
more efficient with C felt as the anode, and in a mixture of
1:1 methanol/acetonitrile co-solvents.
Scheme 11: A suggested mechanism for anodic methoxylation of
amides.
Experimental
Materials
Plausible mechanisms for the formation of various fragmenta- Reagents, electrolytes and solvents (all analytical grade) were
tion products are described in Scheme 12. supplied by various vendors, as mentioned in [23].
Actually whenever benzoic acid or methyl benzoate were Preparation of bisamides I–III
formed (top of Scheme 12), the corresponding aldehyde from All types of bisamides were prepared according to our own pro-
the other part of the molecule was detected too and fully charac- cedure by reacting the corresponding diamines (commercially
terized.
available) with acetic anhydride, or benzoyl chloride or
866

Beilstein J. Org. Chem. 2018, 14, 861–868.
Scheme 12: Mechanisms of formation of fragmentation products.
4-methoxybenzoyl chloride. In a typical experiment, 30 mmol Constant current electrolysis
of a diamine (ethylenediamide, 1,3-diaminopropane or 1,4- Constant current electrolysis at preparative scale was per-
diaminobutane) were introduced into a 500 mL Erlenmeyer formed at constant currents using a PAR Potentiostat/Galvano-
flask with 50 mL of DCM and 20 mL of saturated aqueous stat Model 273A, and a beaker-type undivided cell equipped
bicarbonate solution. Then 70 mmol of acetic anhydride (or with a C rod, C felt, PbO2, GC, DSA or a Pt foil (immersed area
benzoyl chloride or 4-methoxybenzoyl chloride) were added of ≈5 cm2) as the anode, and a Pt foil as the cathode. In a
dropwise by a separatory funnel to the diamine solution while typical electrolysis α,ω-bisamides (1 mmol) were dissolved in
stirring by a magnetic stirrer. Then the reaction mixture was methanol (or 1:1 methanol/acetonitrile, 25 mL) containing
filtered under vacuum and the solid residue was recrystallized 0.1 M supporting electrolytes. Electrolysis took place at room
from a mixture of ethyl acetate and water (9:1). The resulting temperature with a current density of 20 mA/cm2 and was
white precipitate (except for the yellowish one derived from terminated after the desired consumption of electricity
p-nitrobenzene derivative) was dried, weighed and verified by was passed (Tables 2–5). Then the reaction mixture
NMR spectra. The isolated yields of the bisamides were around was concentrated by rotary evaporator till all the solvents
72–86%.
evaporated. The relative yield of products was determined by
1H NMR integration. Notably this procedure of analyzing a
mixture of products simultaneously and successfully is
General methods
Instruments used in this study for 1H NMR and 13C NMR mea- based on prior separation and characterization of the
surements, mass and IR spectra, high-resolution mass analyses, individual products in the mixture. Their previous separation
and cyclic voltammetry were described in [23].
was carried out either by silica gel column chromatography or
preparative coated glass plates, using different mixtures
Thin-layer chromatography (TLC) was carried out on alumi- of ethyl acetate (20–50%)/hexane or acetone/ethyl
num sheets coated with aluminum oxide 60 F254 and silica gel acetate, as eluent. Also since some of the products undergo
60 F254. Retention time was evaluated by UV (for amides with facile hydrolysis/decomposition it is suggested that the
benzene ring) or by using a general purpose stain of cerium analysis will be done immediately after terminating the electro-
molybdate [containing a mixture of Ce(NH4)2(NO3)6 - lysis.
(NH4)6Mo7·4H2O in H2SO4] for amides of type I. Preparative
TLC was carried out by using 20 × 20 cm of glass plates coated Characterization of products
with silica gel 60 F254. Evaporation of solvents was performed See Supporting Information File 1 for spectral data and copies
at reduced pressure using a rotary evaporator.
of 1H and 13C NMR spectra.
867

Beilstein J. Org. Chem. 2018, 14, 861–868.
22.Kalisiak, J.; Trauger, S. A.; Kalisiak, E.; Morita, H.; Fokin, V. V.;
Adams, M. W. W.; Sharpless, K. B.; Siuzdak, G. J. Am. Chem. Soc.
2009, 131, 378–386. doi:10.1021/ja808172n
Supporting Information
Supporting Information File 1
23.Golub, T.; Becker, J. Y. Electrochim. Acta 2016, 205, 207–214.
doi:10.1016/j.electacta.2016.04.074
Spectral data and copies of 1H and 13C NMR spectra.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-72-S1.pdf]
License and Terms
Acknowledgments
This is an Open Access article under the terms of the
Creative Commons Attribution License
The authors are thankful to Dr. M. Karpasas for HRMS mea-
surements and to Mrs E. Solomon for technical assistance.
(http://creativecommons.org/licenses/by/4.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
References
1. Steckhan, E. Anodic oxidation of nitrogen-containing compounds. In
Organic Electrochemistry, 4th ed.; Lund, H.; Hemmerich, O., Eds.; Ch.
15; Marcel Dekker, Inc.: NY, 2001; pp 570–602.
2. Grimshaw, J. Electrochemical reactions and mechanisms in organic
electrochemistry; Ch. 8; Elsevier, 2000; pp 282–290.
3. Torii, S. Electroorganic synthesis, Part 1: Oxidations; Ch. 5.2;
Kodansha VCH, 1985; pp 171–180.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(https://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
4. Shono, T.; Matsumura, Y.; Tsubata, K.; Sugihara, Y.; Yamane, S.;
Kanazawa, T.; Aoki, T. J. Am. Chem. Soc. 1982, 104, 6697–6703.
doi:10.1021/ja00388a037
doi:10.3762/bjoc.14.72
5. Shono, T.; Matsumura, Y.; Onomura, O.; Yamamoto, Y.
Tetrahedron Lett. 1987, 28, 4073–4074.
doi:10.1016/S0040-4039(01)83864-8
6. Genies, M. Bull. Soc. Chim. Fr. 1975, 389–392.
7. Shono, T.; Hamaguchi, H.; Matsumura, Y. J. Am. Chem. Soc. 1975,
97, 4264–4268. doi:10.1021/ja00848a020
8. Wong, P. L.; Moeller, K. D. J. Am. Chem. Soc. 1993, 115,
11434–11445. doi:10.1021/ja00077a048
9. Moeller, K. D.; Rutledge, L. D. J. Org. Chem. 1992, 57, 6360–6363.
doi:10.1021/jo00049a060
10.Moeller, K. D.; Rothfus, S. L.; Wong, P. L. Tetrahedron 1991, 47,
583–592. doi:10.1016/S0040-4020(01)87048-4
11.Okita, M.; Wakamatsu, T.; Mori, M.; Ban, Y. Heterocycles 1980, 14,
1089–1092. doi:10.3987/R-1980-08-1089
12.Okita, M.; Wakamatsu, T.; Ban, Y. Heterocycles 1983, 20, 401–404.
doi:10.3987/R-1983-03-0401
13.Golub, T.; Becker, J. Y. Org. Biomol. Chem. 2012, 10, 3906–3912.
doi:10.1039/c2ob06878h
14.Golub, T.; Becker, J. Y. J. Electrochem. Soc. 2013, 160,
G3123–G3127. doi:10.1149/2.017307jes
15.Eberson, L.; Olofsson, B. Acta Chem. Scand. 1969, 23, 2355–2366.
doi:10.3891/acta.chem.scand.23-2355
16.Nyberg, K. J. Chem. Soc. D 1969, 774–775.
doi:10.1039/c29690000774
17.Golub, T.; Becker, J. Y. Electrochim. Acta 2015, 173, 408–415.
doi:10.1016/j.electacta.2015.04.133
18.Harichandran, G.; Amalraj, S. D.; Shanmugan, P. Indian J. Chem.
2011, 50B, 77–82.
19.Williams, J. B.; Geick, K. S.; Falter, J. A. J. Vinyl Addit. Technol. 1997,
3, 216–219. doi:10.1002/vnl.10194
20.Couch, D. E. Electrochim. Acta 1964, 9, 327–336.
doi:10.1016/0013-4686(64)80040-2
21.Mayeda, E. A.; Miller, L. L. Tetrahedron 1972, 28, 3375–3380.
doi:10.1016/0040-4020(72)88098-0
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