Electrochemical deoxygenation of aromatic amides and sulfoxides

Article abstract of DOI:10.1002/ejoc.201402714

The electrochemical deoxygenation of a broad range of aromatic amides was achieved under mild conditions on lead cathodes. Under the optimized reaction conditions, acetal, thienyl, and ether moieties are tolerated. Furthermore, the reduction protocol can be applied to aromatic and aliphatic sulfoxides to obtain the corresponding sulfides. For both aromatic amides and sulfoxides, the deoxygenation reaction ensues without the use of expensive catalysts or hazardous reducing agents. Owing to the high selectivity of the process, simple extraction is sufficient to isolate the product from the substrate. The straightforward purification protocol, the coformation of water, and the use of electric current instead of reducing agents render our method environmentally friendly and economically beneficial. Copyright

Full text of DOI:10.1002/ejoc.201402714

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402714
Electrochemical Deoxygenation of Aromatic Amides and Sulfoxides
[a]
[a]
Carolin Edinger and Siegfried R. Waldvogel*
Keywords: Electrochemistry / Reduction / Amides / Amines / Lead
The electrochemical deoxygenation of a broad range of aro- sues without the use of expensive catalysts or hazardous re-
matic amides was achieved under mild conditions on lead
cathodes. Under the optimized reaction conditions, acetal,
thienyl, and ether moieties are tolerated. Furthermore, the
reduction protocol can be applied to aromatic and aliphatic
ducing agents. Owing to the high selectivity of the process,
simple extraction is sufficient to isolate the product from the
substrate. The straightforward purification protocol, the co-
formation of water, and the use of electric current instead of
sulfoxides to obtain the corresponding sulfides. For both aro- reducing agents render our method environmentally friendly
matic amides and sulfoxides, the deoxygenation reaction en-
and economically beneficial.
Introduction
duction strategy is highly desirable. In this context, electro-
chemical deoxygenation represents an attractive alternative
to conventional methods.
Amines are an important class of compounds with a pro-
duction volume in the range of 100 kta . As structural
[13]
–
1 [1]
Owing to the high reduction potentials of carboxamides,
either these species have to be activated by strong acids in
high concentration or the oxygen species has to be trapped
[
2]
[3]
units, they can be found in agrochemicals, polymers,
[
4]
[5]
[6]
drugs, textiles, and many other products. Further-
more, amines serve as high-value intermediates in various
[14]
[15]
by sacrificial anodes
or silylation agents
to achieve
[
1,7]
organic syntheses.
In this regard, the deoxygenation of
conversion. Some studies report on the electrochemical re-
duction of amides to amines with the coformation of water.
However, the reported protocols require highly concen-
amides is a highly attractive approach for the synthesis of
amines, as it provides access to a broad variety of unsym-
metrically substituted amines with high selectivity.[
8]
[16]
trated sulfuric acid. These drastic conditions along with
In classic organic synthesis, complex hydrides, mostly
the poor selectivity of the reaction represent major draw-
[
9]
lithium aluminum hydride, serve as reducing agents. How-
ever, reductions under these conditions require an inert at-
mosphere and scaleup is challenging owing to the hazard-
ous properties of the reducing agent. A further option is
hydrogenation under high pressure and strongly elevated
temperatures by using molecular hydrogen in combination
with heterogeneous catalysts.[ Conversion at bimetallic
Pd/Re on graphite allows typically “mild” conditions of
[16]
backs. Under similar conditions and with the additional
use of tetraalkylammonium salts, C–N bond cleavage is se-
[17]
lectively viable. To avoid the water byproduct, an indirect
route via thioamides was elaborated by Kindler et al. This
protocol can be electrochemically conducted in alcoholic
hydrochloric acid; however, it leads to a complex reaction
10]
[18]
mixture if primary and secondary amides are converted.
Recently, we presented a comprehensive study of the in-
fluence of ammonium salt additives on the selectivity of
the electroreductive conversion of aromatic amides on lead
160 °C and 0.5 MPa. However, these heterogeneous systems
[
11]
are not compatible with aromatic and olefinic moieties.
More recently, metal-catalyzed hydrosilylations of amides
have attracted considerable attention and various catalyst
systems implementing transition metals such as Fe, Cu, Pd,
Pt, Ir, and Os have been developed.[ The disadvantages
of all these methods are the need for expensive and/or toxic
catalysts and drastic reaction conditions, along with the
generation of large amounts of reagent waste. Hence, a
more inexpensive, mild, and environmentally benign re-
[
19]
cathodes.
We also demonstrated the benign effect of
these additives on the stability of the cathode material
throughout the electrolysis. Prior to this study, we screened
several electrode materials for this type of conversion.
Given that amides are reduced at very negative poten-
12]
[
20]
tials,
hydrogen evolution such as boron-doped diamond,
a material with a very high overpotential for
[21]
glassy carbon, mercury, or lead has to be employed. Among
these cathode materials, lead rendered the best results and
was therefore used throughout our investigations.
[
a] Institut für Organische Chemie, Johannes Gutenberg
Universität,
Duesbergweg 10–14, 55128 Mainz, Germany
E-mail: waldvogel@uni-mainz.de
N-Benzoyl-2-ethylhexylamine (1) served as the model
substrate to establish a mild electrochemical method for the
reduction of amides to the corresponding amines. 2%
H SO in methanol and 0.017 m of N,N,N,NЈ,NЈ,NЈ-hexa-
www.blogs.uni-mainz.de/fb09akwaldvogel/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402714.
2
4
5144
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Eur. J. Org. Chem. 2014, 5144–5148

Electrochemical Deoxygenation of Aromatic Amides and Sulfoxides
Scheme 1. Electrolysis of model substrate 1 in the presence and in the absence of additive 2.
methyl-1,3-propanediammonium methylsulfate (2) proved Results and Discussion
to be an efficient electrolyte system for the conversion, as
the product yield could be significantly increased
The current density was systematically optimized,
–2
(
Scheme 1) owing to compact cationic coating of the lead whereby 44 mAcm turned out to be optimal with respect
[
19,22]
surface (Figure 1).
to the product yield. Among several tested electrolyte sys-
tems, H SO in MeOH rendered the best results. Systematic
2
4
variation of the acid content revealed that a concentration
of 2 vol.-% was ideal, as it provided sufficient ionic conduc-
tivity for the relatively high current density and mild
enough conditions to avoid C–N bond cleavage. Additive 2
proved to be superior to monoammonium salts and poly-
ammonium salts. Various diammonium salts were tested
throughout our study to find the optimum alkylene spacer
length between the quaternary nitrogen atoms.[
19]
De-
pending on the subjected amide, the best yield of the amine
was achieved with an excess amount of charge (typically
–1
8
Fmol ). Noteworthy, galvanostatic electrolysis under our
elaborated conditions was feasible. This is of significance
for synthetic applications, as potentiostatic electrolyses with
a three-electrode arrangement are difficult to scaleup.[
A practical advantage of this method is the straightfor-
ward workup and isolation of the products. Best quality
23]
Figure 1. Left: Concept of enhanced conversion of the amide owing
to the decoration of the lead cathode with alkylammonium ions. was achieved by the following procedure: After electrolysis,
Right: Hydrogen evolution and lead corrosion as the dominating addition of 10% NaOH and extraction by using tert-butyl
reactions in the absence of any alkylammonium species.[
19]
methyl ether provided unconverted starting materiel next to
the amine. A simple acidic extraction of the organic layer
Upon the addition of 2, the amine could be isolated in separated analytically pure product from unconverted
55% yield with a current efficiency of 28%. In accordance amide. Mass spectrometric studies (ESI+) indicated that the
with the enhanced product yield and current efficiency, cy- mass balance of 100% could be attributed to the formation
clic voltammetry revealed a cathodic shift of the hydrogen of oligomers that remain in the aqueous phase after extrac-
evolution reaction (HER) by ΔE = –157 mV. Furthermore, tion. This oligomer formation and the evolution of small
we demonstrated that cathodic corrosion is entirely sup- amounts of hydrogen explain the fact that the Faradaic
pressed upon the addition of alkylammonium additives yields are in almost every case lower than the product yields
[
19,22]
such as 2.
The advantages of our method are the very (see Table 1). No cleavage of the C–N bond was observed,
mild reaction conditions, extremely simple isolation of the which rendered oligomeric compounds as the only byprod-
product, and the selective conversion in the absence of any ucts besides water. Both the product and the recovered
expensive or toxic reagents/catalysts.
starting material were obtained essentially lead free, which
Herein, we investigate the influence of the reaction tem- was demonstrated by using inductively coupled plasma
perature, the current density, the applied charge, and the mass spectrometry (ICP-MS, Pb content Ͻ2.5 ppm for the
[
19]
acid concentration. Furthermore, we explore the scope of crude product).
this electroreductive approach, including the tolerance of The results of our temperature studies are depicted in
different aromatic moieties and functional groups. In ad- Figure 2. Whereas the mass balance was not significantly
dition, the deoxygenation reaction of sulfoxides to the cor- affected by the temperature, the product yield steadily grew
responding sulfides is explored.
between 25 and 40 °C and increased sharply between 40
Eur. J. Org. Chem. 2014, 5144–5148
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5145

C. Edinger, S. R. Waldvogel
SHORT COMMUNICATION
Table 1. Electrochemical deoxygenation of amides at a decorated
[
a]
lead cathode.
Figure 2. Influence of the temperature on the electrochemical de-
2 4
oxygenation of 1. Conditions: catholyte: 2% H SO in MeOH,
–
2
–1
–1
j = 44 mAcm , additive 2: 0.017 molL , Q = 8 Fmol , cathode
material: lead, anode material: platinum.
yield decreased to 40% for 5 with a current efficiency of
20% (Table 1, entry 3); however, an unexpected high
amount of starting material was recovered (45%). Cathodic
treatment of the corresponding tertiary amide yielded in-
ferior amounts of amine 6 (Table 1, entry 4). Cyclic amine
7
was formed in a better yield of 32% under the same con-
ditions. With N-benzoylmorpholines as substrates (Table 1,
entries 6–9), better results were obtained. This can poten-
tially be attributed to the indirect participation of the
morpholine oxygen atom in the reduction process through
[
24]
chelation of the activating proton. Amine 8 was therefore
generated most effectively with a current efficiency of 52%
(
Table 1, entry 6). The optimum current density turned out
–2
to be 22 mAcm to avoid formation of oligomers with
simultaneously high conversion of the substrate.
[
a] Conditions: unless otherwise noted, catholyte 2% H
2
SO
4
in
MeOH, T = 45 °C, j = 44 mAcm , additive 2 0.017 molL , an-
ode: platinum. [b] Conditions: catholyte 2% H SO in MeOH, T =
5 °C, j = 22 mAcm , additive 2 0.017 molL , anode: platinum.
c] Conditions: catholyte 2% H SO in MeOH, T = 45 °C, j =
–2
–1
2
–1
4
–2
4
[
2
4
–2
–1
3
7 mAcm , additive 2 0.017 molL , anode: platinum. [d] brsm =
based on recovered starting material.
Scheme 2. Electrochemical deoxygenation of aromatic amides on a
decorated lead cathode.
and 45 °C (Figure 2). A further increase in the temperature
did not lead to improved results. Therefore, all electrolyses
were performed at a temperature of 45 °C.
Alterations in the aromatic system also led to significant
changes in the product yield (Table 1, entries 7–11). A tert-
The results of our studies on the scope of the electro- butyl group in the 4-position did not seem to affect the
chemical deoxygenation according to Scheme 2 are pre- results significantly; amine 9 was obtained in 54% yield. A
sented in Table 1. The electrochemical deoxygenation reac- significantly more electron-rich substrate based on veratric
tion to amine 4 provided the best efficiency and led to isola- acid provided corresponding amine 10 in a slightly lower
tion of the products in yields up to 62% (Table 1, entry 1). yield (Table 1, entry 8). It is known that veratryl cations un-
Variation of the N-substituent led to significant changes in dergo alkylation reactions to form oligomers such as cyclo-
[
25]
the product yield. With 55% yield of secondary amine 3, veratrylenes.
The reduction of naphthoic amide gave
the reduction of substrate 1 delivered comparably appealing naphthylmethylmorpholine (11) in 45% yield without af-
results. With decreasing length of the N-alkyl chains, the fecting the aromatic moiety (Table 1, entry 9). Finally, the
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Eur. J. Org. Chem. 2014, 5144–5148

Electrochemical Deoxygenation of Aromatic Amides and Sulfoxides
tolerance of acetals, which are typically cleaved under acidic Sulfones and aliphatic amides exhibit no significant reactiv-
conditions, was tested. It was found that 12 could be iso- ity under our conditions.
lated in 46% yield. Interestingly, aminomethylthiophene de-
rivative 13 was obtained in good yield despite the use of
lead cathodes (Table 1, entry 11). If no additive was used,
Experimental Section
the electrochemical treatment delivered a significantly in-
ferior yield of 15% and a completely deteriorated lead cath- General Procedure for the Electrochemical Deoxygenation:[19] The
anodic and cathodic compartments of an H-type cell were sepa-
ode, most probably due to cathodic desulfurization pro-
rated by using a Nafion 324 membrane. Each half-cell was filled
with electrolyte (2% H SO in MeOH, 50 mL) and the amide or
2 4
sulfoxide (1.7 mmol); the additive (0.85 mmol) was then added to
cesses and organic lead corrosion. Two facts strongly sup-
port the hypothesis that the conversion of the substrate oc-
curs without direct contact to the electrode surface: First,
the product yield of 13 was dramatically enhanced, and sec-
ond, upon the addition of 2 the cathode surface remained
intact during electrolysis (see Figure 1).
the cathodic chamber. As anodic material, a platinum sheet
2
(
(
5.0 cm ) was used, and lead functioned as the cathodic material
2
6.8 cm ). The electrolysis was performed under galvanostatic con-
ditions at 45 °C. After applying the specific charge quantity (Q =
The scope of our method is limited to aromatic amides. 4–8 F), the cathodic mixture was transferred to a separation funnel
Aliphatic amides were highly stable even with 30% meth- and brought to pH Ն 10 by using aqueous 10% NaOH solution.
–
2
anolic H SO and charge densities up to 110 mAcm .
Extraction with tert-butyl methyl ether (3ϫ 60 mL) rendered un-
converted amide or sulfoxide next to the desired product. For
amides, the organic layer was treated with 0.01 m HCl (3ϫ 50 mL)
to separate the unconverted starting material from the desired
amine. The combined organic extract was dried with Na SO , and
2 4
the solvent was removed under reduced pressure to recover analyti-
cally pure starting material. The aqueous layer was treated with
2
4
The fact that sulfur-containing moieties were tolerated
prompted us to study our deoxygenation strategy on sulfox-
ides and sulfones. We found that sulfides 15 could be readily
prepared by subjecting corresponding sulfoxides 14 to our
elaborated electrolysis protocol (Scheme 3). Whereas di-
phenyl sulfide (15a) could be prepared in a good yield
1
0% NaOH solution for pH Ն 10 and extracted with tert-butyl
methyl ether (3ϫ 60 mL). The combined organic extract was dried
sion of aliphatic sulfoxide 14a required more drastic condi- with Na SO and concentrated to yield the analytically pure amine.
(
73%) under the conditions shown in Table 1, the conver-
2
4
tions; a higher acid concentration (10% H SO in meth- For sulfides, the organic layer was extracted with water (3ϫ 50 mL)
2
4
anol) and an increased current density of 95 mAcm–2 of- to separate the sulfide from the unconverted sulfoxide, and it was
then washed with brine (3ϫ 50 mL). After drying with Na
the solvent was removed to yield the analytically pure sulfide.
2 4
SO ,
fered a yield of 63% for alkyl congener 15b. On the other
hand, sulfones such as diphenyl sulfone and dibutyl sulfone
proved to be resistant towards electrochemical reduction on Supporting Information (see footnote on the first page of this arti-
1
13
lead cathodes under the conditions described above.
cle): H NMR and C NMR spectra, an illustration of the experi-
mental setup, and a detailed description of the deoxygenation reac-
tion.
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