Electrochemical phosphorylation of arenols and anilines leading to organophosphates and phosphoramidates

Article abstract of DOI:10.1039/d1ob00779c

A practical phosphorylation for generating organophosphates and phosphoramidatesviaelectrochemical dehydrogenative cross-coupling of P(O)H compounds with arenols and anilines is disclosed. This method involves using inorganic iodide salts as both redox catalysts and electrolytes in an undivided cell without the addition of oxidants or bases. A preliminary mechanistic study suggests that radicals are not involved in this process. This method is green and eco-friendly and has good functional group tolerance, high yields and broad substrate scope, with the potential for practical synthesis.

Full text of DOI:10.1039/d1ob00779c

Organic &
Biomolecular Chemistry
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PAPER
Electrochemical phosphorylation of arenols
and anilines leading to organophosphates
and phosphoramidates†
Cite this: Org. Biomol. Chem., 2021,
19, 5342
Zijian Zhong,‡ Pan Xu‡ and Aihua Zhou
*
A practical phosphorylation for generating organophosphates and phosphoramidates via electrochemical
dehydrogenative cross-coupling of P(O)H compounds with arenols and anilines is disclosed. This method
involves using inorganic iodide salts as both redox catalysts and electrolytes in an undivided cell without
the addition of oxidants or bases. A preliminary mechanistic study suggests that radicals are not involved
in this process. This method is green and eco-friendly and has good functional group tolerance, high
yields and broad substrate scope, with the potential for practical synthesis.
Received 22nd April 2021,
Accepted 21st May 2021
DOI: 10.1039/d1ob00779c
rsc.li/obc
compounds sometimes demonstrate different reactivities. In
Scheme 1, the previous studies on using diphenylphosphine
Introduction
Organophosphates and phosphoramidates widely exist in oxide R2P(O)H and dialkyl phosphites (RO)2P(O)H to obtain
nature,1 and they play important roles in both living and non- organophosphates and phosphoramidates are summarized.
living worlds. Among these compounds, the common feature There are reports on using dialkyl phosphites (RO)2P(O)H as a
is that a majority of them contain P(O)–O or P(O)–N bonds in phosphorus source to construct the P(O)–O bond. Prabhu et al.
their main structures. Organophosphates and phosphorami- reported metal-free cross-hetero-dehydrogenative coupling
dates have found wide-ranging applications in the fields of phosphorylation of alcohols with I2/H2O2 as the catalytic
flame retardants,2 pesticides,3 pharmaceutical chemistry,4 system,9 while the Reddy group developed a method to
materials science, semiconductors5 and so on. Furthermore, promote the phosphorylation of arenols instead of alcohols
they have also been employed as efficient chiral phosphorus utilizing the LiI/TBHP catalytic system10 (Scheme 1a).
ligands in many asymmetric organometallic reactions.6 Thus, Diphenylphosphine oxide R2P(O)H is another often used phos-
the development of efficient and eco-friendly synthetic phorus source to construct the P(O)–O bond. The Chen and
methods for the construction of P(O)–O or P(O)–N bonds to Han group reported the phosphorylation of alcohols using Fe
obtain phosphates and phosphoramidates has become a com- (acac)2/1,10-phen as the catalytic system.11 While Chen’s group
pelling interest and attracted significant attention.
developed a metal-free method for the generation of phos-
The traditional method for the generation of phosphates
and phosphoramidates is via direct esterification/amidation of
arenols or anilines7 using toxic and moisture-sensitive phos-
phoryl halides in the presence of a base under an inert atmo-
sphere.8 Over the past years, several other methods for con-
structing P(O)–O or P(O)–N bonds to obtain phosphates and
phosphoramidates have been reported. In these methods,
two hydrophosphoryl compounds were commonly selected
as phosphorus sources; they are diphenylphosphine oxide
R2P(O)H and dialkyl phosphites (RO)2P(O)H. Despite some
similarities in their structures, these two hydrophosphoryl
School of Pharmacy, Jiangsu University, Xuefu Road 301, Zhenjiang, Jiangsu,
212013, China. E-mail: ahz@ujs.edn.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00779c
Scheme 1 Previously reported synthetic methods for obtaining orga-
‡These authors contributed equally.
nophosphates and phosphoramidates.
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Table 1 Optimization of reaction conditionsa,b
phoryl compounds using SelectFluor,12 Han and co-workers
reported an electrochemical method for the synthesis of orga-
nophosphinates via the CDC reaction between alcohols and
diphenylphosphine oxides.13 Recently, the Lu group reported
an electrophilic phosphination of arenols in the presence of
the Tf2O/H2O2 system14 (Scheme 1b). Besides, the groups
of Lee and Zheng also independently reported similar
works.15,16 For phosphoramidate synthesis, dialkyl phosphites
(RO)2P(O)H can again be employed to react with anilines in
the presence of CCl4 17 or TCCA18 (trichloroisocyanuric acid) or
ArSeSeAr19 as the activating agent. The same reaction can also
be catalysed by CuI with air as the oxidant20 (Scheme 1c).
Considering the existing problems reported in these previous
works and encouraged by the recent developments in the field
of electrochemical synthesis, we decided to develop a new,
effective and environmentally-friendly electrochemical method
to obtain organophosphates and phosphoramidates.
Entry Electrode Solvent
Electrolyte J (mA cm−2
)
Yieldb (%)
1
2
3
4
5
6
7
8
Pt–Pt
Pt–Pt
Pt–Pt
Pt–Pt
Pt–Pt
Pt–Pt
Pt–Pt
Pt–Pt
Pt–Pt
C–Pt
CH3CN
CH3CN
CH3CN
THF
DMSO
DMF
NaI
KI
NH4I
NaI
KI
20
20
20
20
20
20
10
90
84
79
Trace
54
69
<5
Trace
Trace
69c
78d
72
TBAI
CH3OH NH4I
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
n-Bu4NPF6 20
n-Bu4NBF4 20
9
10
11
12
13
14
15
NaI
NaI
NaI
NaI
NaI
NaI
20
20
10
30
20
20
Pt–C
Pt–Pt
Pt–Pt
Pt–Pt
Pt–Pt
Electrochemical synthesis has resurged and attracted exten-
sive attention from organic chemists in recent years because it
is a clean, highly efficient, eco-friendly, oxidant-free and step-
80
74e
64f
saving process which can avoid the pre-functionalization step a Reaction conditions: Pt plate as the anode (1 cm × 1 cm × 1 mm), Pt
plate as the cathode (1 cm × 1 cm × 1 mm), undivided cell, constant
for starting materials and the excessive use of toxic oxidants. It
current = 20 mA, 1 (0.5 mmol), 2a (0.6 mmol), catalyst (20 mol%),
has become one of the most attractive tools to obtain special
solvent (5 mL), in an undivided cell at room temperature under air for
molecules which traditional synthetic methods fail to produce. 2 h. b Isolated yields. c Carbon rod as the anode (8 cm × Φ = 6 mm), Pt
plate as the cathode (1 cm × 1 cm × 1 mm). d Pt plate as the cathode
Herein we report an efficient and eco-friendly synthesis of
(1 cm × 1 cm × 1 mm), carbon rod as the anode (8 cm × Φ = 6 mm).
e Electrolyte (10 mol%). f Electrolyte (40 mol%).
organophosphates and phosphoramidates via an electro-
chemical oxidative cross-coupling protocol using diethyl phos-
phite (RO)2P(O)H and arenols/anilines in the presence of an
iodide salt as the electrolyte and catalyst in acetonitrile, a wider electrochemical window and is able to better solubil-
affording the expected product in good yields under mild reac- ize the reactants. Performing the reaction with n-Bu4NPF6 and
tion conditions (Scheme 1).
n-Bu4NBF4 as the electrolyte without an iodide salt under air
Initially, we started our investigation by using diethyl phos- in CH3CN did not generate any product (entries 8 and 9); this
phite 1 and phenol 2a as model compounds in order to find demonstrated that the iodide salt is important for the pro-
the optimal reaction conditions. Platinum plates were used as gress of the reaction, as it is involved in the reaction as a cata-
both the anode and cathode with different electric currents of lyst. A graphite electrode was used as the anode, since it is
10, 20 and 30 mA. Several solvents and electrolytes were often suitable for arenol oxidation, and a platinum plate was
screened at room temperature under air for 2 h. The results used as the cathode; the reaction gave 69% yield of the
are presented in Table 1. The phosphorylation of phenol pro- product (entry 10). A reversal of the anode and cathode in
ceeded smoothly with NaI as the catalyst and electrolyte in entry 10 without any change in the other reaction conditions
CH3CN, and diethyl phenylphosphate 3a was obtained in 90% led to 78% yield of the product (entry 11). Varying the current
yield (Table 1, entry 1). Using KI instead of NaI as the catalyst from 20 mA to 10 mA afforded a lower yield (entry 12), while
and electrolyte under the same reaction conditions provided increasing the current from 20 mA to 30 mA resulted in a
84% yield of product 3a (entry 2). When ammonium iodide decreased yield (entry 13) due to the occurrence of side reac-
was employed as the catalyst and electrolyte instead of KI, 3a tions. Decreasing the electrolyte amount from 20 mol% to
was obtained in a decreased yield (entry 3). Upon changing the 10 mol% led to a decrease in the yield (entry 14), while
solvent from CH3CN to THF and utilizing NaI as both the cata- increasing the electrolyte amount from 20 mol% to 40 mol%
lyst and electrolyte, the reaction only provided a trace amount did not result in a higher yield, and only 64% yield of the
of 3a. Using KI as the electrolyte and catalyst, and DMSO as product was obtained (entry 15).
the solvent, product 3a was obtained in a low yield (entry 5).
After obtaining the optimal reaction conditions, we then
Similarly, replacing the solvent and electrolyte/catalyst with investigated the substrate scope and generality of this coupling
DMF and TBAI (tetra-n-butylammonium iodide) led to 69% reaction; a variety of substituted phenols with both electron-
yield of the product (entry 6). When CH3OH was used as the withdrawing and electron-donating substituents on their
solvent with ammonium iodide as the electrolyte and catalyst, benzene ring were selected and reacted with diethyl phosphite
the reaction furnished less than 5% of product 3a. These 1 at room temperature under air with NaI the catalyst and elec-
results showed that the solvent used had a great effect on this trolyte, and the results are shown in Table 2. It can be seen
reaction, and that CH3CN is better than CH3OH because it has that the electronic properties of the substituents had some
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Table 2 Electrochemical phosphorylation of diethyl phosphite with
arenolsa,b
reaction of sterically hindered neighbouring 2-phenyl phenol
with diethyl phosphite 1 demonstrated that steric hindrance
did have some effect on the reaction yield, as a decreased 75%
yield was achieved (3k). Notably, phenol with the strong elec-
tron-donating substrate –OMe (3m) also worked well in this
system, affording the desired product in 84% yield, but the
yield was lower than that obtained when using phenol with
the methyl group. This might be due to its low oxidation
voltage, leading to the formation of unknown byproducts.
Naphthol was selected to react with diethyl phosphite 1, and
the reaction exhibited good reactivity and gave phosphate 3o
in 85% yield. Alcohols and heteroarenols like benzyl alcohol
and 3-hydroxypyridine were also reacted with diethyl phosphite
1, and both reactions gave a slightly decreased yield of 3p
(62%) and 3q (68%). We also expanded the scope of phos-
phites by using dimethyl phosphite and dibutyl phosphite. To
our delight, phenol reacted well with dimethyl phosphite,
affording 95% yield of 3r. However, when dibutyl phosphite
was reacted with phenol instead of diethyl phosphite, the reac-
tion only gave a trace amount of the expected product 3s.
The success of electrochemical phosphorylation of arenols
with diethyl phosphite 1 led us further to explore the cross-
coupling reactions of anilines under the same reaction con-
ditions. After extensively searching for the optimal reaction
conditions, we found that using KI as both the catalyst and
electrolyte at 50 °C in CH3CN gave the best yield; therefore, we
continued the electrochemical phosphorylation of diethyl
phosphite 1 with different aniline analogs.
A variety of substituted anilines and amines with both elec-
tron-withdrawing and electron-donating substituents on their
benzene ring were reacted with diethyl phosphite 1 at 50 °C
under air in CH3CN with KI as the catalyst and electrolyte. The
results shown in Table 3 indicate that the electronic properties
of the substituent had almost no noticeable effect on the reac-
tion efficiency. All anilines with electron-donating and elec-
tron-withdrawing substituents afforded good yields of phos-
phoramidates (5a–5i). Anilines with a methyl or ethyl group on
the N atom gave decreased yields of 84% or 80% (5j and 5k),
respectively. Interestingly, using aniline analogs with an elec-
tron-donating group (5m) afforded a lower yield compared
with using aniline itself. Amines were also employed instead
of aniline analogs, and all three reactions resulted in good
yields of 79% (5l), 81% (5m) and 82% (5o), respectively. But
compared with the yields obtained using aniline analogs as
reactants, using amines to prepare phosphoramidates led to
slightly lower yields. When dimethyl phosphite was employed
to react with p-methylaniline instead of diethyl phosphite, the
reaction afforded 95% yield of 5p. To expand the generality of
the reaction, both 2-aminopyridine and indole were selected to
react with diethyl phosphite and dibutyl phosphite; surpris-
ingly, both reactions provided trace amounts of the products
(5q and 5r). When dibutyl phosphite was reacted with aniline,
the reaction also did not give the expected product 5s.
a Reaction conditions: Pt plate as the anode (1 cm × 1 cm × 1 mm), Pt
plate as the cathode (1 cm × 1 cm × 1 mm), undivided cell, constant
current = 20 mA, 1 (0.5 mmol), 2a (0.6 mmol), catalyst (20 mol%),
solvent (5 mL), under atmospheric air at room temperature for 2 h (3 F
mol−1). b Isolated yields.
effect on the reaction efficiency and the reaction could tolerate
a wide range of substituents with electron-donating and elec-
tron-withdrawing groups at different positions on the phenyl
ring. All the reactions proceeded smoothly to give the corres-
ponding organophosphates 3a–o in good to excellent yields
ranging from 75 to 92%. Generally, using arenols with alkyl
groups on their benzene rings afforded slightly higher product
yields (3a, 3b, 3c, and 3l) compared to using arenols with
halide elements on their benzene rings (3e, 3f, 3g, 3h, and 3i).
When arenols with strong electron-withdrawing –CN or –NO2
substituents (3j and 3n) were used, the reaction also provided
high yields of 80% and 87%, respectively. The results of the
To gain insights into the mechanism of the two reactions,
several control experiments were carried out under the optimal
conditions (Scheme 2). When the radical scavenger TEMPO
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Table 3 Electrochemical phosphorylation of diethyl phosphite with
anilinesa,b
Scheme 2 Control experiments.
under air, after 2 h, the reaction afforded only less than 5% of
product 3a, while replacing phenol with aniline improved the
reaction yield to 51%. This fact demonstrated that the pres-
ence of a base or the removal of HI is very important for reac-
tion progress.
From Fig. 1, it can be seen that diethyl phosphite 1 did
not show any oxidation peak. The oxidation peak of phenol
2a was observed at about 1.80 V, while aniline 4a showed two
oxidation peaks at 1.20 V and 2.10 V. The CV of KI presented
obvious anodic currents at 1.13 V and 1.51 V, respectively,
−
which obviously corresponded to the halide system of (I−–I3
–
I2). The CV of the mixture of 1, 2a and KI presented oxidation
peaks at 1.25 V and 1.82 V, which corresponded to the shift
of the oxidation peaks of the iodide ion and phenol, while
the mixture of 1, 4a and KI presented an oxidation peak at
1.20 V.
Based on the control experiment results and previous
research in the literature, we proposed a plausible mechanism
as shown in Scheme 3. Initially, the anodic oxidation of the
a Reaction conditions: Pt plate as the anode (1 cm × 1 cm × 1 mm), Pt
plate as the cathode (1 cm × 1 cm × 1 mm), undivided cell, constant
current = 10 mA, 1 (0.6 mmol), 4a (0.5 mmol), catalyst (20 mol%),
solvent (5 mL), under atmospheric air at 50 °C temperature for 2 h (1.5
F mol−1). b Isolated yields.
(2,2,6,6-tetramethyl-1-piperidinyloxy) or BQ (1,4-benzoquine) iodide anion generates iodine, which subsequently reacts with
was added to the reaction, the reaction still proceeded well, diethyl phosphite 1 to give the phosphoryl iodide intermedi-
resulting in organophosphate 3a and phosphoramidate 5a in ate. Then nucleophilic phenol 2a or aniline 4a react with the
79% and 83% yields, respectively (Scheme 2a and b), indicat- resultant phosphoryl iodide to give the cross-coupling product
ing that both reactions took place via a non-radical pathway in 3 or 5, releasing HI. Once HI is released, it immediately under-
this electrochemical phosphorylation process. Under the stan- goes cathodic reduction of its protons to give hydrogen gas at
dard reaction conditions without phenol or aniline, diethyl the cathode, and at the same time produces the iodide anion;
phosphite 1 reacted with I2, which was generated from the oxi- this greatly promotes the speed of the reaction of phenol/
dation of the iodide anion on the anode, to give phosphoryl aniline with diethyl phosphite iodide, moving toward the
iodide in 45% yield. When diethyl phosphite 1 and phenol direction of product generation. The resulting iodine anion
were placed together in the presence of I2 at room temperature then participates in the next cycle.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This investigation was generously supported by funds provided
by Jiangsu University (1281290006).
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Conclusions
In conclusion, we have developed an efficient and eco-friendly
synthetic protocol to obtain organophosphates and phosphora-
midates via an electrochemical oxidative cross-coupling protocol
using dialkyl phosphite and arenols/anilines. This protocol
involves using iodide salts both as redox catalysts and electro-
lytes in acetonitrile, generating good yields of the expected pro-
ducts. The mechanistic studies revealed that the reaction pro-
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and P–N bonds. The main features of this protocol are the short
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