Environmentally sustainable production and application of acyl phosphates

Article abstract of DOI:10.1039/d0gc02370a

A versatile and environmentally friendly synthetic method for the formation of acyl phosphates is reported. By employing electrochemical oxidative dehydrogenative coupling, this transformation features a broad range of substrates, as well as metal-free, o

Full text of DOI:10.1039/d0gc02370a

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ISSN 1463-9262
PAPER
Paul T. Anastas et al.
The Green ChemisTREE: 20 years after taking root with the 12
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DOI: 10.1039/D0GC02370A
Journal Name
COMMUNICATION
Environmentally Sustainable Production and Application of
Acyl Phosphates
Shengmei Guo, Sen Li, Wenjie Yan, Zhibin Liang, Zhengjiang Fu, Hu Cai*
Received 00th October 20xx,
Accepted 00th October 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A versatile and environmentally friendly synthetic method for the
Recently, the bloom of electrochemical synthesis in forming
formation of acyl phosphates is reported. By employing diverse molecules has been recognized.8 The benefit of such
electrochemical oxidative dehydrogenative coupling, this method is their environmentally benign character, because in
transformation features broad substrates scope, metal free, these reactions, chemical oxidants, bases are commonly
oxidant free and mild conditions. One-pot synthesis of amides from avoided. Furthermore, the electrons are inherently clean
acyl phosphates with amines was succeed with high efficiency and reactants that can be used to achieve oxidation and reduction
regioselectivity without using any additional catalysts and reactions, which help to preserve the environment.9 We have
additives in 2 hours under room temperature. Mechanism was developed electrochemical strategy to enable selenation of
proposed according control experiments.
phosphonates and phosphine oxides and,10 deborylative
seleno/thiocyanation of arylboronic acids.11 Herein, we report
an electrochemical synthesis of acyl phosphates from carboxylic
acids and phosphites through an anodic dehydrogenative
coupling, which is devoid of metal reagents and chemical
oxidants. Moreover, this method enables a one-pot synthesis of
amides efficiently without additional catalysts and bases under
mild conditions.
Acyl phosphate compounds are of much importance and have
received substantial attention in recent decades because of
their widespread application in organic synthetic chemistry1
and pharmaceutical industry.2 For instance, acylation reactions
of a series of biochemical intermediates are based on acyl
phosphates.3 Therefore, the development of methods for the
preparation of acyl phosphates is essential in organic
chemistry.4 Traditional methods for the synthesis such
compounds utilized below strategies: (1) the substitution
reaction of chlorophosphates and carboxylic acids and,5 (2)
acylation reaction between acyl chlorides and phosphate salts.6
However, the toxicity and unstability of upon chlorides, the
mass waste of the reaction, long reaction time or multistep
process might limit their applications. To overcome these
drawbacks, Li and co-workers employed copper-catalyzed
oxidative dehydrogenative coupling of carboxylic acids and
phosphites to realize the formation of acyl phosphates using
dicumyl peroxide (DCP) as an oxidant.7 The starting materials
are stable, commercially available and low toxic. Although
important progress has been made, this method needs heavy
metal, large amount of peroxide, organic waste was produced
and side products. In term of green synthesis and sustainable
development, novel and green synthetic methods are desirable.
Li's work
O
O
OR
O
COOH
P
Cu2O
O
H
P
OR
+
OR
DCP, 70 o
C
OR
Our work
O
O
OR
O
P
COOH
P
O
H
OR
+
OR
rt
OR
Scheme 1 Formation of acyl phosphates
Inspired by our previous works on the phosphonation
reactions,12 we chose the reaction of 4-methylbenzoic acid (1a,
0.8 mmol) with diethyl phosphonate (2a, 0.4 mmol) as the
model for the optimization of reaction conditions (Table 1).
When the reaction was conducted at room temperature in an
undivided cell employing platinum as anode and cathode as well
as nBu4NBr/CH3CN as electrolyte and solvent, the desired
product 3aa was afforded in 84% yield along with a small
amount of anhydride in 6 h under 10 mA constant current
(Table 1, entry 1). Meanwhile, the hydrogen released was
detected by GC-TCD (see Supporting Information (SI) for
† Department of Chemistry, Nanchang University, No. 999, Xuefu Rd. Nanchang,
330031, P. R. China. E-mail: Caihu@ncu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2019, 00, 1-4 | 1
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n
n
n
details). Other electrolytes, such as Bu4NCl, Bu4NI, Bu4BF4, phosphonate were also compatible with the reaction, giving the
View Article Online
DOI: 10.1039/D0GC02370A
n
LiClO4, were screened, and it was found that Bu4NBr was the expected products in 80%, 74% and 66% yields, respectively
optimal electrolyte for this electrochemical reaction (Table 1, (3ac, 3ad, 3ae). Interestingly, cyclic phosphine oxides such as
entries 2-5). However, no dehydrogenative coupling product 5,5-dimethyl-1,3,2-dioxaphosphinane 2-oxide (2f) and
was detected without the electrolyte or electric current (Table dibenzo[c,e][1,2]oxaphosphinine 6-oxide (2g) also participated
1, entries 6 and 7). Next, the electric current was examined. We the reaction well, delivering the formation of 3af and 3ag in 73%
found that either decreasing or increasing the electric current,
and 70% yields. Unfortunately, diphenyl phosphonate and
diphenylphosphine oxide failed to carry out the transformation.
Next, the compatibility of carboxylic acids was evaluated
(Scheme 3). Benzoic acid afforded the expected product in 80%
Table 1 Optimization of Conditions a
O
P
O
O
P
OEt
OEt
COOH
OEt
Pt(+) Pt(-)
H
O
O
+
O
H2
+
OEt nBu4NBr, CH3CN, rt
6 h, under air, 10 mA
undivided cell
OR
O
P
COOH
Pt(+) Pt(-)
P
OR
OR
O
H
H2
+
OR
+
nBu4NBr, CH3CN, rt
6 h, under air, 10 mA
undivided cell
1a
Entry
2a
3aa
Yieldb (%)
84
1
2
3
Deviation from standard conditions
none
O
O
P
O
1
O
O
O
OMe
OMe
OBu
OBu
OiPr
OiPr
P
P
nBu4NCl instead of nBu4NBr
nBu4NI instead of nBu4NBr
18
62
2
O
O
O
3
3ab
3ac
3ad
, 74%
, 71%
O
, 80%
nBu4NBF4 instead of nBu4NBr
LiClO4 instead of nBu4NBr
no nBu4NBr
ND
ND
ND
4
5
6
O
O
O
O
O
OiBu
OiBu
P
P
O
O
O
O
O
P
O
ND
67
72
7
8
9
no electric current
3ag
3ae
, 70%
, 66%
3af
73%
8 mA instead of 10 mA
12 mA instead of 10 mA
O
O
O
O
Ph
Ph
OPh
OPh
P
P
O
O
trace
30
10
11
12
CH3OH instead of CH3CN
DCM instead of CH3CN
DMF instead of CH3CN
3ai
0%
3ah
, trace
34
a
1
2
Reaction conditions: (0.80 mmol), (0.40 mmol), electroyte (0.40 mmol),
CH3CN (8 mL) in an undivided cell with two Pt foils (each 10 x 10 x 0.3 mm3)
at rt for 6 h, air, 10 mA; b Isolated yields.
77
69
13
14
4 h of reaction time
8 h of reaction time
Scheme 2 The scope of the phosphites
15
16
graphite felt as anode
72
66
in N2
yield. The acids with electron-donating groups on the para
position of phenyl ring showed good tolerance with this
transformation. Which led to the product rang from70% to 82%
yields (3ca-3fa). The effect of groups at the ortho position of
benzene ring on the reaction was tested. We found that OMe-,
Me-, Ph-, Cl- were suit for the reaction with good yields (3ga-
3ka). Di-substituted carboxylic acids such as 1k and 1l were also
led to the desired products in 78% and 58% yields, respectively
(3ka, 3la). While, 2,4,6-trimethylbenzoic acid (1m) did not
produce the dehydrogenative coupling product, probably due
to the steric hindrance. In addition, the reactions with a series
of medicinally relevant heterocyclic acids such as thiophene-2-
carboxylic acid, furan-2-carboxylic acid, furan-3-carboxylic acid
and 2,5-dimethylfuran-3-carboxylic acid as partners were
successfully carried out, which afforded the electrochemical
coupling product in moderate to fair yields (3na-3qa). 2-
Naphthoic acid was also reacted with 2a to give 3ra in
satisfactory yield. Beside, 2,3-dihydrobenzo[b][1,4] dioxine-6-
carboxylic acid afforded the corresponding product in 80% yield
(3sa). Unfortunately, pentafluorobenzoic acid failed to the
reaction because of strong acidity (3ta). To highlight this
strategy, gram-scaled synthesis of the 3aa could be realized
with a slight drop of the yield.
a
1a
2a
(0.40 mmol), electroyte (0.40 mmol), solvent
Reaction conditions:
(0.80 mmol),
(8 mL) in an undivided cell with two Pt foils (each 10 x 10 x 0.3 mm3) at room temperature
for 6 h, air, 10 mA; b Isolated yields.
the reaction led to low yields of the desired products (Table 1,
entries 8 and 9). For the solvents, polar solvents such as CH3OH,
dichloromethane (DCM), and N, N-dimethylformamide (DMF)
were used in this transformation, the yields were decreased
rapidly, perhaps due to a good solubility of substrate in CH3CN
(Table 1, entries 10-12). When the reaction times were
shortened or prolonged, the yields of the product were
inefficient (Table 1, entries 13 and 14). The Faradic efficiency of
the model reaction is 41% in 4 h, while, 30% is found in 6 h. This
be because the electric resistance was rising with the reaction.
We found the beginning voltage was 3.40 V, after 4h, which was
rising to 3.80 V, while the voltage dramatically increased to 5.70
V after 6h. Poor efficiency for the acylated reaction was found
with graphite felt as an anode (Table 1, entry 15). It should be
noted that when the reaction atmosphere was switched to
nitrogen, the conversion did not enhance (Table 1, entry 16).
Under the optimal reaction conditions, the scope of
phosphites in this coupling was investigated (Scheme 2). When
dimethyl phosphonate was employed to the reaction, the
desired product was afforded in 71% yield (3ab). Dibutyl
phosphonate, diisopropyl phosphonate and diisobutyl
2 | J. Name., 2019, 00, 1-4
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The common avenue for the synthesis of amides is relied on aniline and adamantan-1-amine were able to give satisfactory
View Article Online
DOI: 10.1039/D0GC02370A
the transformation of carboxylic acids or their derivatives (acyl results (4f, 4g, 4h). The amides from thiophene-2-carboxylic
chlorides) with amines. While, bases and promotor such as acid and 2,5-dimethylfuran-3-carboxylic acid were obtained in
DMAP is normally needed. We found that our synthetic method 86% and 78% yields (4i, 4j). The product of acylation reaction
could be applied in one-pot synthesis of amides efficiently from 5-bromothiophene-2-carboxylic acid with diethyl
without any additional additives under room temperature phophites is hard to isolated because its polarity is similar to the
(Scheme 4). After the electrochemical dehydrogenative corresponding acid, while the amidation product was easily
coupling was completed, morpholine was added into the isolated with 88% yield (4l). It is noteworthy that the acyl
reaction mixture, and was stirred 2 hours without current. The phosphates showed high selectivity in the amidation reaction.
amidation product 4a was obtained in 93% yield altogether, for instance, when piperidin-4-amine was applied as an amine
which is beyond the quantitative transformation (84% yield of source, the regioselective product was obtained (4l).
O
3aa) due to the small amount produced anhydride. Pyrrolidine
O
COOH
1) standar. cond.
2) amines , rt, 2 h
OR
OR
O
O
N
O
P
H
P
H2
+
OEt
OEt
+
COOH
R
Pt(+) Pt(-)
OEt
OEt
P
H
+
O
O
R
H2
+
nBu4NBr, CH3CN, rt
6 h, under air, 10 mA
undivided cell
R
1
2
4
1
2
O
O
O
3
N
N
N
O
P
O
O
O
O
OEt
OEt
OEt
O
OEt
P
P
O
O
O
OEt
OEt
4a, 93%
4b, 83%
4c, 62%
O
Et
O
O
Ph
nBu
3aa, 84%
3ba, 80%
3ca, 76%
N
N
H
N
H
Et
nBu
O
O
O
O
O
O
OEt
OEt
OEt
OEt
P
P
P
4d, 71%
4e, 77%
4f, 89%
O
O
O
OEt
OEt
O
O
O
iPrO
PhO
MeO
S
N
N
N
H
3ea, 71%
3fa, 70%
3da, 82%
H
O
O
O
O
O
O
O
OEt
OEt
OEt
OEt
OEt
OEt
4i, 86%
4h, 66%
4g, 73%
P
P
P
O
O
O
O
O
O
NH
S
Br
OMe
3ga, 75%
Ph
N
N
N
H
3ha, 76%
3ia, 81%
O
O
O
O
OMe
O
O
4j
4k, 88%
4l, 69%
, 78%
O
OEt
OEt
O
O
OEt
OEt
OEt
OEt
P
a
P
P
O
1
2
Reaction conditions: (1) (0.8 mmol), (0.4 mmol), electroyte (0.4 mmol),
O
O
CH3CN (8 mL) in an undivided cell with two Pt foils (each 10 x 10 x 0.3 mm3)
at rt for 6 h, air, 10 mA. (2) amines (0.4 mmol), rt, air, 2h; bIsolated yields.
Cl
3la, 58%
3ka, 78%
3ja, 74%
Scheme 4 The scope of the amidation
O
P
O
O
O
O
O
OEt
OEt
OEt
OEt
OEt
O
S
P
P
To investigate the mechanism of this transformation, several
control experiments were conducted (Scheme 5). When the
model reaction was added ethene-1,1-diyldibenzene (DPE), the
yields of product were dramatically dropped, and (2-
bromoethene-1,1-diyl)dibenzene 5 was detected by GC-MS
O
O
O
OEt
3na, 70%
3oa, 74%
3ma, 0%
O
O
O
O
OEt
OEt
O
O
OEt
OEt
OEt
P
P
P
O
O
O
OEt
O
O
O
OEt
OEt
O
P
H
+
P
stand. cond.
OEt
OEt
O
O
O
O
Ph
Ph
H
3pa, 83%
3qa, 60%
3ra, 62%
DPE (2.0 equiv)
O
P
F
F
O
O
P
O
Br
5 GC-MS
O
P
O
1a
2a
3aa, trace
OEt
OEt
OEt
OEt
OEt
OEt
F
F
O
O
O
O
O
Ph
O
O
O
Ph
Br
F
OEt
O
H
+
P
3aa, 78%b, 0.849 g
stand. condi.
DPE (2.0 equiv))
O
6
OEt
OEt
3ta, 0%
O
O
3sa, 80%
O
O
OEt
H
P
a
O
O
O
1
2
Reaction conditions: (0.80 mmol), (0.40 mmol), electroyte (0.40
mmol),CH3CN (8 mL) in an undivided cell with two Pt foils (each 10 x 10
b 1a
2a
23%
1o
2a
3oa, trace
x 0.3 mm3 ) at room temperature for 6 h, air, 10 mA.
(4 mmol), electroyte (4 mmol), 30 h. c Isolated yields.
(8 mmol),
O
P
O
OEt
OEt
O
H
+
stand. condi.
BHT (2.0 equiv)
OEt
O
O
H
P
Scheme 3 The scope of the carboxylic acids
OEt
2a
1a
3aa, trace
and azepane gave the desired products in 83% and 62% yields,
respectively (4b, 4c). Primary and secondly alky amines such as
1-phenylpropan-1-amine and dibutylamine led to the expected
products in 71% and 77% yields (4d, 4e). Cyclohexanamine,
Scheme 5 Control experiments
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(Details please see SI). Which showed us that the reaction goes According to the previous work and our experimenVtise,w13A,r1ti4c,le15Otnwlinoe
a radical procedure and generates the bromide radical.14 When path way of this transformation was demonstrated (Scheme 6).
DOI: 10.1039/D0GC02370A
the reaction of 1o and 2a was employed with DPE under Firstly, molecular bromine is generated from Br ion by anodic
standard conditions, the radical addition product of 6 was oxidation, which oxidizes 2 to obtain the intermediate
isolated in 23% yield as well as the product of 5, which phosphoryl bromide. On the other hand, acid anion is produced
confirmed that bromide radical might generate in this by dissociation of acid, which attacks the phosphoryl bromide
transformation.13 to further confirm our hypothesis, BHT then to give the coupling product. In the cathode, hydrogen ion is
was employed into the reaction, and trace amount of the reduced to hydrogen.
acylation product was obtained. To further investigate the
procedure of the reaction, cyclic voltammetry (CV) experiments
were then carried out. (Figure 1). In curve B, 2a showed no
Conclusions
oxidation peak (0.0−3.0 V versus Ag/AgCl). Therefore, 2a could
not be oxidized in this condition. Curve C displayed that one
new oxidative peakCOOH at 2.38 V vs curve A was obtained,
In conclusion, we have developed an efficient strategy for the
formation of acyl phosphates from carboxylic acids and
phosphites. The electrochemical CDC reaction was carried out
to realize this conversion without transition-metals and
chemical oxidants at room temperature. This reaction feathers
high atom economy, metal free and utilization electron as green
oxidant, which can significantly reduce pollution. A range of
acids and phosphites could be compatible with the
transformation. Furthermore, one-pot synthesis of amides from
acyl phosphates and amines was achieved with high
regioselectivity and no additional additives. This step-economic
method tolerates primary, second and aryl amines. Mechanism
studies showed that nucleophilic addition might go through in
the reaction.
1.0
A
B
0.8
C
D
0.6
E
0.4
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Acknowledgements
Potential (V) vs Ag/AgCl
We thank the financial support from the National Natural
Science Foundation of China (21861024, 21861026, 21761021).
Figure 1. Cyclic voltammograms of reactants and the mixtures in 0.1
M LiClO4/CH3CN using a Pt disk working electrode (diameter, 3 mm).
Pt wire as counter; Ag/AgCl as reference electrode, at 100 mV/s scan
Conflicts of interest
There are no conflicts to declare.
n
rate: (A) background; (B) 2a (0.05 M); (C) 1a (0.05 M); (D) Bu4NBr
(0.02 M); (E) 1a (0.05 M), 2a (0.05 M), nBu4NBr (0.02 M).
corresponding to the oxidation of 1a.15 The CV of Bu4NBr had
n
Notes and references
two oxidation peaks at 0.70 and 1.02 V (curve D), which
correspond to the oxidation of Br− to Br3− and Br3− to Br2,
1
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n
respectively.16 The CV of the mixture of 1a, 2a, and Bu4NBr,
showed an apparent oxidation peak at 2.68 V, which reflected
that there have been a chemical interreact between the three
compounds. Besides, the CV of 1a using Pt disk showed that a
clear reduction potential, while using glass carbon-disk was not
appeared, indicated that glass carbon-disk was not a better
electrode (SI, Figure 8).
2
3
4
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e
e
PhCOOP(O)R2 + Br-
H2
Br2
2Br
BrP(O)R2
+ H+
PhCOO
5
6
PhCOOH
2 Br-
HP(O)R2
anode
cathode
Scheme 6 The plausible mechanism
4 | J. Name., 2019, 00, 1-4
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7
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H. Fu, T. Yang, J.-Q. Shang, J.-L. Zhou, M. Sun, Y.-M. Li, Org.
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