Electrochemically driven P-H oxidation and functionalization: Synthesis of carbamoylphosphonates from phosphoramides and alcohols

Article abstract of DOI:10.1039/c8nj05739g

An electrochemical method to achieve carbamoylphosphonates from phosphoramides and alcohols via P-H oxidation and functionalization by using n-Bu₄NI as a catalyst is reported. A series of carbamoylphosphonates were obtained with good to excellent yields under mild reaction conditions. The electrochemical reaction is carried out under constant current electrolysis, with the alcohol being used as a solvent and a substrate, and is an attractive green synthesis method.

Full text of DOI:10.1039/c8nj05739g

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Electrochemically driven P–H oxidation
and functionalization: synthesis of
carbamoylphosphonates from phosphoramides
and alcohols†
Cite this: New J. Chem., 2019,
43, 1531
Qiu-Li Wu, Xing-Guo Chen, Cong-De Huo,
Zheng-Jun Quan
Xi-Cun Wang
and
*
Received 12th November 2018,
Accepted 10th December 2018
An electrochemical method to achieve carbamoylphosphonates from phosphoramides and alcohols via
P–H oxidation and functionalization by using n-Bu NI as a catalyst is reported. A series of carbamoyl-
4
phosphonates were obtained with good to excellent yields under mild reaction conditions. The electrochemical
reaction is carried out under constant current electrolysis, with the alcohol being used as a solvent and
a substrate, and is an attractive green synthesis method.
DOI: 10.1039/c8nj05739g
rsc.li/njc
such as cyclopentylamide phosphate and cis-2-aminocyclohexyl-
carbamoylphosphonic acid (cis-ACCP) are potential candidates for
Introduction
8
,9
In recent years, electrosynthesis has been regarded as an matrix metalloproteinase (MMPs) inhibitors.
In addition,
increasingly important method for molecular synthesis, and some carbamoylphosphonic acids have anti-tumor necrosis
1
0
there have been considerable advances in the activation of C–H factor alpha (TNFa) and carbonic anhydrase (CA) inhibitory
1
11
functionalizations. The Stahl and Zeng groups have developed activities. Some typical carbamoylphosphonate derivatives are
indirect electrochemical procedures for the halogenation of shown in Fig. 1.
2
C–H, N–H and O–H bonds. In these procedures, they used a
Generally, carbamoylphosphonates are prepared by the
halide such as TBAX (X = Cl, Br, I) as the redox mediator and/or substitution reaction of phosphonoformates or phosphono-
11a,12
electrolyte through halide generation, allowing reoxidation of thiolformates with amines (Scheme 1a),
and by the addition
the halide ions to halogens. Diverse nucleophilic reaction reaction of dialkyl phosphonates into isocyanates and isothio-
1
3
products under the halogenation electrolysis conditions were cyanates (Scheme 1b). Alternatively, they can be synthesized
À
then obtained. More generally, I has been used for anodiza- via a multistep transformation from an amide derivative and
3
14,15
tion and as a catalytic substrate to obtain target products.
phosphorus halides (Scheme 1c).
However, several limita-
Generation of a halide in the nucleophilic substitution step has tions remain in these strategies, including the requirement of
the benefit of allowing reoxidation of the halide to a halogen at excess toxic regents, low yields, substrate limitations and/or
the anode and permitting these reactions to be performed with harsh reaction conditions. Recently, we reported the synthesis
4
catalytic quantities of halide.
of carbamoylphosphonates by iodine-promoted P–H phosphoryla-
Organophosphorus compounds and their derivatives have tion and oxygenation of phosphinecarboxamides with alcohols
1
6
important application value in many fields, such as drug (Scheme 1d). We proposed that this transformation was
pharmacophores, agrochemicals, flame retardants and transition completed by forming a P–I intermediate as the key step.
5,6
metal catalyst ligands. Bisacylphosphonates have influences on
tissue calcification, bone resorption, hydroxyapatite (HAP) formation
and dissolution activity. The bisacylphosphonate molecules can be
7
fully ionized at physiological pH. Carbamoylphosphonates (CPOs)
Gansu International Scientific and Technological Cooperation Base of Water-Retention
Chemical Functional Materials and College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, Gansu 730070, People’s Republic of China.
E-mail: quanzhengjun@hotmail.com
†
Electronic supplementary information (ESI) available: Experimental procedures,
1
13
31
characterization data, and copies of H, C and P NMR spectra. See DOI: 10.1039/
c8nj05739g
Fig. 1 Pharmaceutically-relevant organophosphorus compounds.
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a
Table 1 Optimization of the reaction conditions
b
Anode/
Entry cathode Electrolyte
Current/
mA
Time/ Yield
Solvent
h
(%)
1
2
3
4
5
6
7
8
9
1
1
1
C/C
C/Pt
KI (1.5 eq.)
KI (1.5 eq.)
KI (1.5 eq.)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
5
10
15
—
10
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
3
3
3
3
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
CN/CH
20
20
20
20
20
20
20
5
2
1
20
1
26
61
75
76
80
77
—
91
96
75
—
—
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
KI/LiClO
4
n
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
NI/LiClO
NBr/LiClO
NBF /LiClO
NI/LiClO
NI/LiClO
NI/LiClO
NI/LiClO
NI/LiClO
4
n
n
n
n
n
n
n
4
4
4
4
4
4
4
4
0
1
2
c
3
OH
a
Conditions: 1a (0.2 mmol), 0.1 eq. of catalyst, 1.5 eq. of electrolyte,
solvent (10 mL), and two platinum electrodes (10 Â 10 Â 0.2 mm), in an
b
undivided cell at room temperature in air. Yield of isolated compound.
c
3 3
CH CN (10 mL)/CH OH (4 eq.).
greatly shortened with increasing current and the yield was also
improved (Table 1, entries 8–11). Increasing the current to
Scheme 1 Phosphonate-forming reactions.
15 mA led to a relatively lower yield (Table 1, entry 10). The
experimental results demonstrated that using 10 mA of current
gave the best results. Of course, the reaction cannot be
carried out in the absence of electricity (Table 1, entry 11).
Afterwards, different solvents were investigated and MeCN
A typical method for the electrochemical functionalization
of N–H by C–H activation amination has been reported more
recently. We have now unraveled the power of electrochemical
P–H activation to enable the first electrochemical domino-reaction
of P–H iodination–phosphonation–oxygenation using a catalytic
17
(4 eq.) as the substrate was found to be detrimental to the
process (Table 1, entry 12).
amount of n-Bu NI as a mediator in an undivided cell under
4
After deriving the optimal conditions, we further investigated the
range and limitation of the conversion of N-arylphosphonamide to
N-arylaminoformylphosphonate through this electrochemical route
constant current conditions (Scheme 1e). Key features of our
approach include: (i) electrochemical P–H iodination/phosphona-
tion/oxygenation, (ii) no toxic sacrificial halide reagents, (iii) a
(Scheme 2). Various phosphonamides could be smoothly trans-
4
catalytic amount of n-Bu NI (10 mol%) as an iodine source, and
formed into the corresponding carbamoylphosphonates in good to
(iv) metal-free P–H functionalization under ambient conditions.
Results and discussion
Initially, methoxyphosphoramide 1a in methanol 2a was used
as the starting material and electrolyzed by using 1.5 equivalents of
KI (1.5 eq.) as electrolyte and CH OH as a solvent at a constant
3
À2
current of 0.5 mA cm in an undivided cell (Table 1, entry 1).
Fortunately, the target product 3aa was obtained in 26% yield.
Then, different electrodes were investigated, and the best choice
was found to be platinum electrodes (Table 1, entries 1–3).
Notably, when we used 0.1 eq. of KI and 1.5 eq. of LiClO
4
, the
target product 3aa was obtained in 76% yield (Table 1, entry 4). KI
n
4
and Bu NBr underwent the reaction smoothly, and TBAI gave a
better result (Table 1, entries 5 and 6); 1.5 eq. of electrolyte loading
could maintain good conductivity for the electrolysis, and 3aa
n
could be achieved in excellent yield. However, Bu NBF was not
4
4
compatible with this transformation (Table 1, entry 7). These
results showed that the role of the halogen salts was not only as
the electrolyte, but also as a redox mediator. The reaction time was Scheme 2 Substrate scope of the phosphinecarboxamide and alcohol.
1
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Scheme 3 The reaction of a phosphinecarboxamide and two different
alcohols.
Scheme 5 Synthesis of aliphatic phosphate carbamoylphosphonates.
excellent yields. First, the functional group tolerance was studied by
using methanol as a solvent and phosphoramides with different
substituted functional groups (3ba–3ga). Phosphoramides contain-
ing electron donating groups (3ca and 3da) or electron withdrawing
groups (3ea–3ga) were well tolerated. The reaction also exhibited
good compatibility with a series of phosphonamide reactants, which
have groups in the para or o-substituent positions (3ca, 3da). In
particular, dimethoxy-substituted phosphonamides also exhibited
excellent reactivity (3cb, 3cd). Therefore, steric hindrance has no
significant effect on the reaction. Ethanol and n-propanol alcohols
were also compatible with the reaction with 1 and provided the
product in good yield (3ab–3eb, 3ac–3dc). It was found that a high
yield of diisopropyl phosphonate (3ad–3ed) was also obtained by
using isopropanol as the solvent. However, n-butanol, tert-butanol
and propane-1,3-diol (3ad–3af) were not viable under the optimized
conditions. Herein, the relatively large resistance hampered the
completion of the reaction.
Then, the products with two different alcohols were tested.
The reaction of 1a with methanol and isopropanol (1 : 1) gave
a mixture of products, 3aa, 3ad, and 6aad, in about 30% yield
of each product (Scheme 3). It was shown that the relevant
products were not selective.
With the success of the carbamoylphosphonate synthesis,
we next attempted to construct methyl phosphinates by apply-
ing the electrochemical method (Scheme 4). Gratifyingly,
methanol could react with substrate 1 efficiently under the
optimal conditions. Similar to what was observed before in the
carbamoylphosphonate synthesis, the methyl phosphinate was
also largely unaffected by steric or electronic properties. A high
yield could be achieved to obtain the methyl methyl phosphinates
that could be used as substrates demonstrated the broad
functional-group tolerance of the reaction.
Finally, N-cyclopentylphosphanecarboxamide and N-cyclohexyl-
phosphanecarboxamide were tested in this transformation and
they were well-tolerated in the reaction affording the desired
products (8ab and 8bb). Bis(azanediyl)bis(carbonyl)bis(phos-
1
1c
phonate) (8cb) was also achieved. By using the reported method,
these compounds can be further converted into carbamoyl-
phosphonic acids, which are MMP inhibitors (Scheme 5).
Finally, to further elucidate the role of TBAI, the effect of
TBAI on the reaction was investigated by cyclic voltammetry
(
(
see Fig. 2). Obviously, a redox wave was observed for TBAI
black curve). The increase in peak (ipa) current is associated
1
8
with the regeneration of the iodine compound. However,
substrate N-(4-methoxyphenyl)phosphanecarboxamide 1b did not
display a significant peak (red curve). When we treated the mediator
TBAI with substrate 1b (green curve), the anodization peak (ipa)
disappeared significantly, indicating that TBAI participated in the
reaction. These results can be clearly interpreted as a single electron
transfer mechanism, which suggests that TBAI serves as a mediator
19
and catalyst during the reaction (green curve vs. black curve).
According to the results described above and the literature reports,
we proposed the following possible mechanisms in Scheme 6.
(5aa–5dc). As a result, the diverse phosphoramide substituents
Fig. 2 Cyclic voltammograms: platinum rod (CHI 102) as the working
electrode, platinum wire as the counter electrode (CHI 115) and Ag/AgCl as
the reference electrode in MeOH (10 mL) with 0.03 M LiClO
4
as the
À1
supporting electrolyte. (black line) TBAI (0.002 mol L ), (red line) 1b
À1
Scheme 4 Substrate scope of the 1-methyl-N-phenylphosphanecarbox-
amides and alcohols.
(0.2 mmol), and (green line) TBAI (0.002 mol L ) and 1b (0.2 mmol). Scan
À1
rate: 100 mV s
.
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Notes and references
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Scheme 6 Proposed reaction mechanism.
2
At the anode surface, iodine ions are oxidized to I , which reacts
with substrate 1 to generate intermediate A. Then, intermediate A
20
undergoes reaction with an ethoxy anion generated in the cathode
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Conclusions
In summary, we disclose an efficient electrochemical route for
the synthesis of phosphonates via phosphonamides and alcohols
under mild conditions by using TBAI as a redox catalyst in an
undivided electrochemical cell. This transformation offers a new
method for phosphonate formation by oxygenation of phosphon-
amides and alcohol coupling. During the reaction, the in situ
2
electrogenerated I plays a vital role. Only a catalytic amount of
TBAI is required, and iodine and its salts have low toxicity and are
safe as compared to bromine oxidation catalysts. On the other
hand, the alcohol is both a reactant and a solvent. It not only
provides a medium for the reaction, but also reacts with the
cathode as a reactant, which reduces environmental pollution.
This process is simple, and the reaction conditions are mild, green
and environmentally friendly.
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We are thankful for the financial support from the National
Nature Science Foundation of China (No. 21562036), and the
Scientific and Technological Innovation Engineering program
of Northwest Normal University (NWNU-LKQN-15-1).
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