Electrochemical Synthesis of Carbodiimides via Metal/Oxidant-Free Oxidative Cross-Coupling of Amines and Isocyanides

Article abstract of DOI:10.1021/acs.orglett.0c00510

This work discloses an electrochemical oxidative cross-coupling of amines with aryl and aliphatic isocyanides. In an undivided cell, the reaction proceeds without involving any transition-metal catalyst, oxidant, or toxic reagents providing carbodiimides in good yields, thereby circumventing stoichiometric chemical oxidants, with H₂ as the only byproduct. Moreover, carbodiimides were in situ converted into unsymmetrical ureas in moderate to good yields using an electricity ON-OFF strategy.

Full text of DOI:10.1021/acs.orglett.0c00510

pubs.acs.org/OrgLett
Letter
Electrochemical Synthesis of Carbodiimides via Metal/Oxidant-Free
Oxidative Cross-Coupling of Amines and Isocyanides
Bhanwar Kumar Malviya, Pradeep K. Jaiswal, Ved Prakash Verma, Satpal Singh Badsara,
and Siddharth Sharma*
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ABSTRACT: This work discloses an electrochemical oxidative cross-coupling of amines with aryl and aliphatic isocyanides. In an
undivided cell, the reaction proceeds without involving any transition-metal catalyst, oxidant, or toxic reagents providing
carbodiimides in good yields, thereby circumventing stoichiometric chemical oxidants, with H2 as the only byproduct. Moreover,
carbodiimides were in situ converted into unsymmetrical ureas in moderate to good yields using an electricity ON−OFF strategy.
arbodiimides are important structural motifs in organic
Cchemistry and have been used in all areas, from academia
to industry.1 These compounds have been extensively
exploited for many years as the excellent coupling reagent
for the peptide synthesis2 as well as intermediates or precursors
for heterocycles.3,4 Among them, N,N′-diisopropylcarbodii-
mide (DIC) and N,N′-dicyclohexylcarbodiimide (DCC) are
the reagents most commonly used. Additionally, it has drawn
attention because of its ubiquitous applications in agricultur-
al,5a medicinal,5b and polymer chemistry,5c where new desired
heterocycles can be synthesized through judicious choice of
the reaction conditions and coupling partners. These intrinsic
qualities of carbodiimides, along with the avalanche of research
on their use in the synthesis of druglike molecules, make them
a privileged group for synthetic chemists. Thus, extensive
research efforts have been devoted to the development of
methods that provide access to functionalized carbodiimides.
The most common method of preparing carbodiimides are
dehydration of ureas,6 and dehydrosulfurization of thioureas
using mercuric oxide7a,b methanesulfonyl chloride,7c sulfur
dioxide, thionyl chloride,7d and phosgene.7e However, either
these methods involve toxic and hazards reagents or produce
large amount of waste. Alternatively, isocyanide emerged as a
diversified coupling partner of amines or azides for the
carbodiimide synthesis with relatively high atom economy.8
More recently, the group of Wang and Ji developed an elegant
approach for the synthesis of carbodiimides catalyzed by I2
using a direct cross-coupling reaction of isocyanides with
amines under metal-free conditions using cumene hydro-
peroxide as the oxidant.9f Despite major progress in the field,
the use of transition-metal catalysts (e.g., Pd, Au, etc.)9a−d or
oxidants9e is unavoidable, which ultimately restricts their
practical use. Therefore, a general and straightforward
carbodiimide synthesis under mild conditions is still highly
desirable.
In recent years, electrosynthesis has been emerged as a
welcome development in chemistry because it has been
considered as an environmentally benign alternative for various
organic transformations.10 In noncatalytic electro-organic
synthesis, reactive intermediates generated by anodic oxidation
and/or cathodic reduction, is a promising green approach to
produce final products by involving sequence of chemical
transformations.11−13 Consequently, transformation of amines
into other useful motifs is of great significance because of its
widespread availability. Simultaneously isocyanides have been
exploited extensively for the several important heterocycle
syntheses.14 However, isocyanide−amide coupling by electro-
chemical dehydrogenative reaction is not well established.
Herein, we report the first electrochemical synthesis of
carbodiimides in an undivided cell under remarkably mild
reaction conditions (Scheme 1b). These reactions avoid metal
catalyst and potentially minimize the amounts of requisite
reagents and chemical waste.
At the outset of our studies, we explored the coupling of
amine (1a) and isocyanide (2a) under different reaction
parameters. After considerable experimentation, an optimal
Received: February 8, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00510
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
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Letter
Extending the reaction time did not alter the reaction yield
much but reducing the reaction time to 8h reduced the yield of
3a (Table 1, entries 11 and 12). Further investigation showed
that, reaction proceeded with low yields when 1 mmol nBu4NI
was used, this is possibly due to the dual role of nBu4NI, where
it was used as both electrolyte and reagent in the reaction
(Table 1, entry 13). No desired product was isolated when I2
was used at 50 °C as stochiometric reagent for the synthesis of
3a (Table 1, entry 11).9f
Scheme 1. Some Previous Conventional Reports and
Present Electrochemical Approach for the Synthesis of
Carbodiimides
With the optimized conditions in hand (Table 1, entry 1),
many efforts have been utilized to explore the applicability of
this transformation (Table 2). Initially, we investigated the
a
Table 2. Substrate Scope
yield of product 3a was obtained in 82% yield under constant
current (12 mA) for 12 h in the presence of nBu4NI and
CH3CN as the solvent (Table 1, entry 1), while platinum
Table 1. Optimization study: Effects of Reaction
a
Parameters.
a
b
entry
variation from standard conditions
none
DMA (10 mL) instead of CH3CN
NH4I instead of nBu4NI
yield (%)
1
2
3
4
82
62
17
8
nBu4NBF4 instead of nBu4NI
5
6
7
8
RVC(+)|Pt(−) instead of Pt(+)|Pt(−)
RVC(+)|RVC(−) instead of Pt(+)|Pt(−)
20 mA instead of 12 mA, 12 h
6 mA instead of 12 mA, 12 h
rt instead of 50 °C
no electricity, N2
8 h instead of 12 h
20 h instead of 12 h
nBu4NI (1 equiv)
76
37
26
43
18
NR
61
77
46
0
9
10
11
12
13
a
Isolated yield.
c
14
I2
developed protocol using various substituted amines with tert-
butyl isocyanide for the synthesis of carbodiimides 3a−j
(Table 2). Anilines bearing electron-donating groups (EDG)
as well as electron-withdrawing groups (EWG) were well
tolerated in this transformation (3a−j) and furnished the
desired carbodiimides in good yields (up to 86%). Electron-
donating groups such as methoxy and methyl afforded the
corresponding products in 68 to 84% yields (3a, 3c, 3f, 3g, and
3i). Halide substituents such as Cl afford the desired products
in good yields (3d and 3h). It is to be noted that EWGs such
as p-NO2 bearing aniline furnished the carbodiimide 3e in
excellent yield (yield = 86%), whereas 2,4-dimethyl-substituted
aniline furnished the desired carbodiimide 3j in 64% yield.
Further, the scope of isocyanides was investigated (Table 2,
3k−f′), although no specific electronic effect on the reaction
yield was observed when aliphatic isocyanides were subjected
to the reaction. Secondary aliphatic isocyanides, i.e., cyclohexyl
isocyanide gave the desired products 3k−p in good yields (up
to 80%) with meta and para EWG as well as EDG-substituted
anilines. It is worth noting that tertiary aliphatic isocyanides
a
Reaction conditions: All reactions were performed with platinum
anode, platinum cathode, constant current = 12 mA, 1a (0.25 mmol),
2a (0.50 mmol), nBu4NI (1 mmol), CH3CN (6 mL), 50 °C, 12 h,
undivided cell. Isolated yield. I2 (1 equiv) was used as reagent at 50
b
c
°C. NR = no result, DMA = dimethyl acetamide.
anode and platinum cathode electrodes were found to be
beneficial. On the contrary, the yield was slightly diminished by
using DMA as the solvent choice (entry 2). Among a variety of
supporting electrolytes, NH4I and nBu4NBF4 demonstrated
poor efficiency compared to nBu4NI (17, 8% yield, entries 3
and 4). In order to test the electrode effect, platinum plates/
RVC and RVC/RVC were applied for and furnished
compound 3a in 76 and 37% yields respectively (entries 5
and 6). Moreover, changing the operating current (20 or 6
mA) was proved to be less efficient as well (26 and 43%,
entries 7 and 8). When the reaction was carried out at room
temperature, 3a was obtained in 18% yield only (entry 9). No
desired product was obtained without electricity (entry 10).
B
https://dx.doi.org/10.1021/acs.orglett.0c00510
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Letter
also led to the corresponding products 3q−x in good yields
(up to 82%) except with disubstituted anilines, which
furnished the moderate yields of desired carbodiimides 3w
(55%) and 3x (57%). It is to be also noted that the primary
aliphatic isocyanides gave the desired carbodiimides 3y−d′ in
61−75% yields with substituted anilines. It is noteworthy that
aromatic isocyanide 2,6-dimethylphenylisocyanide furnished
isocyanides furnished the unsymmetrical urea 4a−f in
moderate to good yields (54−73%) utilizing the intermediate
carbodiimide to final unsymmetrical urea 4a−f in an electricity
OFF condition. Secondary isocyanide, i.e., cyclohexyl iso-
cyanide with 4-chloroanilines furnished the good yield (73%)
of unsymmetrical urea 4f, whereas 4-methoxyaniline furnished
the moderate yield (54%) of 4e. Aromatic isocyanides also
afforded the unsymmetrical aromatic urea 4g−i in moderate
yields (48−56%) in one pot following the same ON−OFF
electrochemical reaction conditions. Additionally, 2-amino
quinazolinone derivative 6 was also synthesized by the reaction
of anthranilamide 5 and cyclohexyl isocyanide using a modified
procedure, where 10 mol % Yb(OTf)3 was used as an
additional catalyst in addition to the standard reaction
condition (Scheme 2). The desired compound 6 was isolated
in 63% isolated yield.
̀
the desired carbodiimides 3e′,f in 57−62% yields. In addition,
we were also interested in the coupling of aliphatic amine and
isocyanide to enhance the scope of the reaction for the
synthesis of coupling agent DCC (3g′). However, the yield of
compound 3g′ was found to be relatively poor and isolated in
26% yield.
To further demonstrate the utility of our above reactions,
and in order to evaluate the versatility of this novel
electrochemical system, we applied the carbodiimides to
unsymmetrical urea transformation in a one-pot reaction
employing an electricity ON−OFF approach (Table 3). After
Scheme 2. One-Pot Synthesis of Substituted
Aminoquinazolinone 6
Table 3. One-Pot Unsymmetrical Synthesis of Urea 4a−i via
a,b
In Situ Generated Carbodiimides
In addition, cyclic voltammetry (CV) experiments were
performed to investigate the redox potential of the substrates
(Figure 1) and were in accordance with previous reports.15
Figure 1. Cyclic voltammograms of reactants and the mixture in 0.05
M nBu4NI/CH3CN using a glassy carbon-disk working electrode Pt
disk as counter electrode; Ag/AgCl as reference electrode, at 0.1 V/s
scan rate: (a) only CH3CN (without nBu4NI), (b) nBu4NI (0.05M),
(c) 2a (0.01 M) + nBu4NI (0.05M), (d) 1a (0.01 M) + 2a (0.01 M)
+ nBu4NI (0.05M).
a
Reaction conditions: All reactions were performed with Pt anode (25
mm × 15 mm × 3 mm), Pt cathode (25 mm × 15 mm × 3 mm),
constant current = 12 mA, 1a−c (0.5 mmol), 2a−c (1.0 mmol),
nBu4NI (2 mmol), CH3CN (6 mL), 50 °C, 12 h, undivided cell
Curve a (in the absence of nBu4NI) showed no oxidation peak
(0.0−1.6 V vs Ag/AgCl). The CV of nBu4NI had two
oxidation peaks at 0.60 and 1.21 V (curve b), which
b
followed by addition of 5 mL of 0.1 M HCl at 80 °C for 6 h. Isolated
yield.
correspond to the oxidation of I− to I3 and I3 to I2,
respectively. Interestingly, we found that the CV of 2a (in the
presence of nBu4NI) presented only one oxidation peak at 0.54
V, and of oxidation peak I3− to I2 disappeared, possibly due to
the formation of carbonimidic diiodide intermediate in curve c
(Figure 1). The CV of the mixture of 1a, 2a, and nBu4NI
demonstrated an apparent oxidation peak at 0.58 V (curve d),
due to the possible chemical interaction between the three
compounds. As far as aniline is concern, in trace d, there is no
obvious peak for aniline was observed because aniline showed
−
−
the completion of the amine and isocyanide coupling reaction
in the electricity ON condition, subsequent conversion of this
carbodiimide to the final unsymmetrical ureas 4a−i was
achieved in the electricity OFF condition using aq HCl. As can
be seen in Table 3, most aliphatic as well as aromatic
isocyanides underwent smooth transformation to afford the
corresponding unsymmetrical urea 4a−i in moderate to good
yields (up to 73%). Tertiary as well as secondary aliphatic
C
https://dx.doi.org/10.1021/acs.orglett.0c00510
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pubs.acs.org/OrgLett
Letter
Authors
Bhanwar Kumar Malviya − Department of Chemistry,
oxidation peak only at very high scan rate and high
concentration of aniline.16
On the basis of our mechanistic studies, the proposed
mechanism of electrochemical oxidative coupling of amines
and isocyanides is depicted in Scheme 3. The reaction might
Mohanlal Sukhadia University, Udaipur 313001, India
Pradeep K. Jaiswal − Department of Biochemistry and
Biophysics, Texas A&M University, Texas 77843, United States
Ved Prakash Verma − Department of Chemistry, Banasthali
University, Vanasthali 304022, India; orcid.org/0000-
0003-2142-2459
Scheme 3. Plausible Mechanism
Satpal Singh Badsara − Department of Chemistry, Rajasthan
University, Jaipur 302004, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00510
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a grant from the DST-India in the
form of INSPIRE Faculty (IFA-13, CH-116) to S.S. S.S. is also
thankful to UGC-New-Delhi for a Start-up Research Grant.
S.S. also thanks Dr. Prabhat Kumar Baroliya, Department of
Chemistry, MLSU, for providing access to instrumentation for
CV studies. S.S. is also thankful to Dr. Ajay K. Singh, CSIR-
IICT Hyderabad, for scientific discussions at the time of
revision.
be initiated by the I2 generation by the nBu4NI oxidation at
anode in pathway I. Generated I2 was reacted with isocyanide
for the generation of carbonimidic diiodide intermediate A.9e,17
Subsequently intermediate A was coupled with amines
provided required carbodiimide and HI. Second, nBu4NI
oxidation at anode generated iodide radicals which was
subsequently trapped by isocyanides to generate carbonimidic
diiodide intermediate A in pathway II followed by the coupling
with amines provided carbodiimides. Finally, H2 gas emerged
by the reduction of HI at cathode, and I2 was regenerated at
anode.
In conclusion, we have developed a novel and practical
electrochemical oxidative cross-coupling reaction of amines
with isocyanides using readily accessible amines via C−N bond
formation, affording carbodiimides under metal-free and
oxidant-free conditions. This oxidative cross-coupling using a
general, reliable, and straightforward methodology delivers a
variety of substituted carbodiimides in good to excellent yields
(up to 86%). Furthermore, the utility of this approach was
successfully extended to the synthesis of diverse unsymmetrical
ureas in moderate to good yields (up to 73%) in a one-pot
reaction using an electricity ON−OFF approach. Thus, this
work not only represents a metal-free strategy for cross-
coupling of isocyanides with amines for carbodiimide synthesis
but also provides an efficient approach to unsymmetrical
functionalized urea synthesis under mild conditions. Further
synthetic applications including multigram-scale synthesis are
currently ongoing in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00510.
Experimental details and spectral data of all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Siddharth Sharma − Department of Chemistry, Mohanlal
Sukhadia University, Udaipur 313001, India; orcid.org/
0000-0003-2759-4155; Email: siddharth@mlsu.ac.in,
sidcdri@gmail.com
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