Urethanes synthesis from oxamic acids under electrochemical conditions

Article abstract of DOI:10.1039/d0cc05069e

Urethane synthesis via oxidative decarboxylation of oxamic acids under mild electrochemical conditions is reported. This simple phosgene-free route to urethanes involves an in situ generation of isocyanates by anodic oxidation of oxamic acids in an alcoholic medium. The reaction is applicable to a wide range of oxamic acids, including chiral ones, and alcohols furnishing the desired urethanes in a one-pot process without the use of a chemical oxidant.

Full text of DOI:10.1039/d0cc05069e

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DOI: 10.1039/D0CC05069E
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Urethanes Synthesis from oxamic acids under Electrochemical
Conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Ikechukwu Martin Ogbu,a Jonathan Lusseau,a Gülbin Kurtay,a Frédéric Robert,a and Yannick
Landais*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Urethanes synthesis via oxidative decarboxylation of oxamic acids isocyanates (Figure 1a). However, the reaction performed in a
under mild electrochemical conditions is reported. This simple two-phase medium (H2O-CH2Cl2) did not allow for a direct
phosgene-free route to urethanes involves an in-situ generation of access to urethane without isolation of the carcinogenic
isocyanates by anodic oxidation of oxamic acids in an alcoholic isocyanate and led to the latter with moderate yields.
medium. The reaction is applicable to a wide range of oxamic Leveraging this, we recently showed that urethanes can be
acids, including chiral ones, and alcohols furnishing the desired accessed through a one-pot metal-free visible light-mediated
urethanes in a one-pot process without the use of a chemical oxamic acids decarboxylation, thereby generating isocyanates
oxidant.
in-situ, which in the presence of an alcohols furnished
urethanes in good to excellent yields (Figure 1b).8a This
strategy involved the use of an organophotocatalyst (4CzIPN)
and hypervalent iodine (III) (BIOAc) as an oxidant to trigger the
free-radical conversion of oxamic acids into isocyanates.
The urethane (or carbamates) functional group is present in a
large number of substrates of pharmaceutical and
agrochemical interest.1 Urethanes exhibit excellent proteolytic
stabilities2 and are consequently used frequently as peptide
bond surrogates.3 Carbamates have been popularized in
organic synthesis as amine protecting groups,4 showing
convenient orthogonality and stability towards acids, bases or
reducing conditions depending on the nature of the
substituent. The addition of alcohols to isocyanates is likely
one of the most reliable method to access urethanes, but also
polyurethanes (Bayer reaction).5 However, the known
isocyanates carcinogenicity and the synthesis of the latter
from toxic phosgene constitute a major limit to the use of this
reaction. Isocyanates may also be conveniently accessed from
parent oxamic acids. The latter have emerged as versatile
synthons for the synthesis of amide-containing organic
compounds.6 Oxamic acids are stable7 and easily accessible
through the coupling between amines and oxalic acid
derivatives.8 Minisci and co-workers originally showed that
oxamic acids are useful precursors of isocyanates.9 This
approach involved the use of ammonium persulfate as an
oxidant in the presence of Ag(I) and Cu(II) salts as catalysts to
achieve oxidative decarboxylation of oxamic acids into
(a) Minisci synthesis of isocyanates from oxamic acids (Ref. 9)
O
(NH4)2S2O8
N
R
C
RHN
CO2H
O
Cu(OAc)2, AgNO3
H2O-CH2Cl2
(b) Urethanes from oxamic acids using visible light/hypervalent iodine oxidation (Ref. 8a)
4CzIPN (1 mol%)
BIOAc (1.5 eq.)
O
O
R'OH
+
CO2H
RHN
OR'
RHN
DCE, 24-48 h, RT
Blue LED
(c) Urethanes through electrochemical decarboxylation of oxamic acids (this work)
C(+)
C(-)
O
O
undivided cell
R'OH
RHN
OR'
RHN
CO2H
Figure 1 Oxamic acids as green precursors of urethanes
Encouraged by these results, we sought to simplify further this
useful process, designing a metal-, photocatalyst-, light- and
oxidant-free route relying on electrochemistry to mediate
single electron transfers during the oxamic acid
decarboxylation. There is an increasing interest in the use of
simple electrochemical set-up in organic synthesis due to its
a. University of Bordeaux, Institute of Molecular sciences (ISM), UMR-CNRS 5255,
351, Cours de la Libération, 33405 Talence Cedex, France.
E-mail : yannick.landais@u-bordeaux.fr
simplicity,
promising
environmental
benefits
and
sustainability, limiting the recourse to expensive chemicals and
minimizing waste.10 We thus report herein a practical urethane
synthesis via a mild anodic oxidation of oxamic acids.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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The process was first optimized, as summarized in Table 1,
using oxamic acid 1a as a model compound, and MeOH 2a,
both as a solvent and a nucleophile, to trap the isocyanate
generated in situ. Unless indicated, graphite electrodes were
used both as cathode and anode. The reaction was first
conducted under basic conditions using triethylamine (TEA),
and a constant current of 60 mA was applied.11 Gratifyingly,
the corresponding methyl carbamate 3a was obtained in 72%
yield along with 20% of 1,1-diethyl-3-phenethylurea as a result
of the amine oxidation12 (see 8, vide infra) (Table 1, entry 1).
Decreasing the amount of the base to 0.1 eq resulted in a
slight increase in yield (entry 2) and the absence of urea.
Replacement of TEA with other bases including ammonia, or
guanidines such as TBD or MTBD did not help improving the
yield of 3a (entries 3-5). The reaction time was equally
extended to 8h, but no significant effect was observed on the
yield (entry 6). Furthermore, when the current was lowered to
5 mA and reaction time increased up to 12h, conversion into
3a reached 80% (entry 7). Interestingly, the reaction worked
also well in the absence of a base, 3a being isolated in 77%
(84% conversion) without any additives (entry 8). Under these
conditions, the reaction time was further extended to 18h,
without yield improvement (entry 9). The role of a supporting
Table
1
View Article Online
DOI: 10.1039/D0CC05069E
Electroc
hemical
conversi
on
oxamic
2a
MeOH
O
H
H
(+)Cgr / (-)Cgr
N
N
OMe
OH
Current, Time
Additive
O
O
of
1a
3a
acid 1a into urethane 3a.
entrya
1
2
3
4
5
6
7
8
Current (mA)
Additive (eq.)
TEA (1.0)
TEA (0.1)
NH3 (0.1)
TBD (0.1)
MTBD (0.1)
TEA (0.1)
TEA (0.1)
Time (h)
5
5
5
5
5
8
12
12
18
12
12
12
12
12
12
Yield (%)b
72
78 (72)
76
70
68
79
80
84 (77)
84
83
60
60
60
60
60
60
5
5
5
5
5
-
-
9
10
11
12c
13d
14e
15f
n-Bu4NClO4 (0.1)
n-Bu4NI (0.1)
26
40
ND
83
5
5
5
5
-
-
-
-
84
a Unless otherwise mentioned, all reactions were performed with 1a (0.5 mmol),
2a (3 mL), graphite anode and cathode, under argon atmosphere unless
indicated, in an ElectraSyn vial, undivided cell at room temperature. b Yields of 3a
determined by 1H NMR with 1,3,5- trimethylbenzene as an external standard
(Isolated yields of 3a under brackets). c Platinum anode. d Stainless steel anode.
Nickel anode. f Reaction in air.
electrolyte
was
also
studied,
showing
that
tetrabutylammonium perchlorate (n-Bu4NClO4)11b did not bring
any improvement (entry 10), while n-Bu4NI led to a drastic
decrease in yield, possibly as a result of its competitive
decomposition at the anode (entry 11).13 Finally, the nature of
electrodes was investigated. The use of a platinum anode
significantly decreased the yield (entry 12), while stainless
steel did not provide the product at all (entry 13). Nickel foam
exhibited a similar performance as graphite anode (entry 14),
in good agreement with literature precedent, showing that
graphite and nickel anodes favour Hofer-Moest type reactions,
as multi-electron transfer is improved due to a better
absorption on foam as compared to metallic surface.11b,14 The
reaction was finally performed in air with success, indicating
that the process could proceed appreciably without much
precautions to exclude air (entry 15).
e
O
O
C(+)
C(-)
RHN
OH
RHN
OMe
2a
MeOH , 5 mA
O
RT, 6-18 h
3a-m
1a-m
O
O
O
N
H
OMe
N
OMe
N
H
OMe
H
F
MeO
3a
3b
3c
, 72%
, 78%
O
, 76%
O
O
N
OMe
N
H
OMe
N
H
OMe
H
Cl
Cl
3d
3e
3f
, 53%
O
, 71%
O
, 68%
O
Under these optimal conditions (entry 8), substrate scope was
extended by varying the nature of oxamic acids in the
presence of MeOH 2a as a nucleophile and solvent (Scheme 1).
These conditions thus allowed the conversion of a wide range
of oxamic acids 1a-m, furnishing methyl carbamates 3a-m in
moderate to high yield. These mild conditions were found
compatible with arenes bearing electron-withdrawing (3b, 3e-
f) or electron-donating (3c) substituents. Interestingly, while
oxamic acids having benzylic substituents (1b-c, 1e-h) were
shown to be sensitive to oxidation, using our previous
photocatalytic conditions,8a the corresponding urethanes were
obtained in good yields under electrochemical conditions. Bis-
oxamic acids 1k-m were also shown to undergo the reaction,
resulting in the corresponding bis(methyl carbamates) 3k-m in
satisfying yields. In this case, supporting electrolyte was used
in small quantities as it was shown to improve conductivity
and conversion.
S
N
H
OMe
N
H
OMe
N
H
OMe
3g
3h
3i
, 73%
, 41%
, 70%
H
O
O
MeO
N
N
OMe
N
H
OMe
H
O
a
3j
3k
, 50%
, 70%
O
O
N
H
OMe
N
OMe
H
H
N
H
MeO
N
MeO
O
O
a
a
3l
3m
, 64%
, 54%
Scheme 1 Electrochemical decarboxylation of oxamic acids in the presence of
MeOH 2a. Oxamic acid scope. a (n-Bu4)NClO4 (0.1 eq.) was added.
Other alcohols 2b-l were then submitted to the optimal
reaction conditions above. Although alcohols with higher
2 | J. Name., 2012, 00, 1-3
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molecular weight than MeOH 2a display lower electrical conditions, as already observed using the photocatalyzed
View Article Online
conductivity,15 they were shown to be competent solvents and process,8a due to the competitive decarbonylation of the
DOI: 10.1039/D0CC05069E
nucleophiles in the presence of a supporting electrolyte, putative carbamoyl radical intermediate (vide infra).9b
affording the desired urethanes in moderate to good yields Degradation was also observed using benzyl alcohol
(Scheme 2).
(oxidation) or 2-bromoethanol, which led instead to the ester
of the oxamic acid (ESI). It is worth noticing that although the
alcohol is used as a solvent, vacuum distillation at the end of
the process was successful, allowing about 90% alcohol
recovery with satisfying purity (1H NMR).
Finally, chiral oxamic acids 5a-c derived from the
corresponding (L)-amino acids,8b were also submitted to the
above electrochemical conditions (Scheme 3), leading to the
expected urethanes 6a-i in satisfying yields without
racemization, as shown by chiral HPLC on racemic and
homochiral 6f (ESI).
O
O
C(+)
C(-)
RHN
+
R'OH
OH
RHN
4a-w
OR'
n-Bu4NClO4, 5 mA
O
50°C, 6-12h
1a-m
2b-l
O
O
O
CF3
N
OEt
N
H
O
N
O
H
H
4a
4b
4c
, 70%
62%a
, 78%
, 50%
30%a
gram-scale 61%
O
O
O
O
N
H
OEt
N
O
N
H
H
4d
4e
4f, 71%
, 63%
, 21%
Cl
O
CF3
O
O
O
O
O
C(+)
C(-)
H
H
N
N
H
O
N
N
H
O
R'O
N
O
+
R'OH
OMe
HO
OMe
H
Bu4NClO4, 5 mA
50°C, 6-12h
O
R
O
R
4g
4h
4i
, 45%
, 53%
O
, 54%
O
5a
2
6a-i
, R = CH2CO2Me
O
5b, R = Me
5c
, R = CH2Ph
H
N
O
N
O
N
H
O
O
H
H
O
H
O
F
H
N
EtO
N
CO2Me
O
EtO
N
4j
4k
4l, 23%a
, 45%
, 52%
OMe
OMe
OEt
O
O
Me
O
Me
CO2Me
O
O
Cl
6a
6b
6c
, 61%
, 51%
, 50%
N
H
OEt
N
O
N
H
O
H
O
O
Cl
H
H
N
H
N
OMe
O
N
CO2Me
CO2Me
4m
4n
4o
, 41%
, 50%
O
, 50%
O
MeO
O
MeO
O
O
O
O
O
OMe
O
OEt
HN
HN
6d
6e
6f
, 65%
, 41%
, 72%
N
H
OEt
H
H
H
MeO
N
CO2Me
N
O
CF3
N
O
4p
4q
4r
, 62%
, 71%
32%a
, 80%
OMe
MeO
MeO
O
O
O
CO2Me
, 78%
OMe
O
O
O
6g
6h
6i
, 50%
, 40%
OEt
O
O
N
H
N
O
H
Scheme 3 Electrochemical decarboxylation of amino-acids derived oxamic acids
in the presence of alcohols 2.
N
O
H
4u
, 47%
4t
, 60%
4s
, 65%
O
N
O
O
H
H
H
A
tentative
mechanism
for
the
electrochemical
O
N
O
N
N
O
H
decarboxylation of oxamic acids is finally proposed, as
depicted in Figure 2. Oxidation of oxamic acid A at the anode
should generate a ketocarboxyl radical I, which would then
O
O
4v
4w
, 50%
, 63%
34%a
Scheme 2 Electrochemical decarboxylation of oxamic acids in the presence of
alcohols 2b-l. Alcohol scope. a room temperature.
suffer decarboxylation leading to the carbamoyl radical II.17
A
second oxidation event would then convert II into the
carbamoyl cation III (that may also be written as a protonated
isocyanate IV), the reaction of which with alcohol B furnishing
the urethane C along with one proton. Reduction of H+ at the
cathode produces in turn hydrogen. The formation of the
intermediate carbamoyl radical is supported by the formation
of diamide 7 during electrolysis of oxamic acid 1 in the
presence of MeOH 2a in CH2Cl2 (Scheme 4). The presence of
the coupling product 7 also suggests that the oxidation of
radical II into III is a slow process, in good agreement with
Minisci’s observations.9 As mentioned above, when TEA was
used as a base, competing oxidation of the amine occurs,
which is in line with the oxidative potential of NEt3 and oxamic
acids respectively of +0.95 V18 and +1.32 V (vs SCE) (see ESI).
Electrochemical oxidation of Et3N is known to lead, in the
presence of traces of water, to acetaldehyde and
Improved yields were obtained by heating up the
electrochemical cell during electrolysis. Beside a known
enhanced reactivity of alcohols towards isocyanates with
increasing temperature,5a the mild heating increases the
conductivity of the medium,15 and also likely improves the
solubility and mass transport of ions to the electrodes.16 The
reaction conditions are compatible with alcohols bearing a Cl
(4i, 4m), ethers (4r-u) and CF3 groups (4c, 4g) as well as a
double bond (4h, 4n). The reaction was equally shown to work
appreciably well in a one gram-scale experiment, furnishing
the product 4a in 61% yield. Double addition was also
observed with bis-oxamic acids leading to the corresponding
bis-urethanes 4v-w in reasonable yields. Some limitations
were however noticed during this study. For instance, oxamic
acids derived from anilines were not successful under these
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diethylamine,12 the latter then adding to carbamoyl cation IV
to afford 8, thus supporting the formation of IV as an
intermediate.
2
3
4
B. Testa, and J. M. Mayer, Hydrolysis in Drug and Prodrug
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3a
, 10%
+
C(+)
C(-)
O
H
undivided cell
Ph
N
O
Ph
Ph
OH
5 mA
H
N
5
6
S. Ozaki, Chem. Rev., 1972, 72, 457.
O
MeOH (3 eq.), CH2Cl2
N
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1a
H
6-18h
O
7
, 15%
3a
, 72%
C(+)
C(-)
+
O
H
N
undivided cell
O
Ph
OH
20 mA
MeOH, NEt3 (1 eq.)
Ph
N
N
O
H
1a
8
, 21%
2-3h
Scheme 4 Mechanistic studies supporting the formation of carbamoyl radical II
and cation III-IV.
e-
e-
7
8
9
O
O
OH
RHN
- H+
2 H+
+ 2e-
A
O
- e-
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O
RHN
I
O
O
- CO2
O
R
R'
N
O
C
H
R
R
N
H
II
'
- H+
B
ROH
O
- e-
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O
R
N
H
C
N
H
III
IV
Anode
Cathode
Figure 2 Mechanism of the electrochemical decarboxylation of oxamic acids.
In summary, we reported the preparation of urethanes from
oxamic acids and alcohols through a practically simple
electrochemical process. The method allows the formation of
various urethanes in moderate to good yields without isolation
of the carcinogenic isocyanates generated in situ and directly
trapped by alcohols. This metal-, photocatalyst-, light- and
oxidant-free reaction proceeds under mild conditions and
should serve as a useful tool to access these important targets
of pharmaceutical interest.
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IMO thanks the Alex-Ekwueme Federal University Ndufu-Alike
Ikwo (AE-FUNAI) for a PhD grant. We are grateful to the
University of Bordeaux (UBx) and to CNRS for financial
support. We gratefully acknowledge Dr. D. Deffieux (UBx) and
Dr. D. Zigah (Ubx) for helpful discussions.
This paper is dedicated to Prof. Ilhyong Ryu (Osaka Prefecture
University) on the occasion of his 70th birthday.
Conflicts of interest
“There are no conflicts to declare”.
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Notes and references
1
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4 | J. Name., 2012, 00, 1-3
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Graphical Abstract
View Article Online
DOI: 10.1039/D0CC05069E
Urethanes Synthesis from oxamic acids under
Electrochemical Conditions
Electrochemical decarboxylation of oxamic acids in the
presence of alcohols provides urethanes.
O
O
C(+)
C(-)
R
OH
R
N
N
OR'
H
R'OH, 5 mA
H
O
oxamic acid
urethane