Electrochemical selenation of phosphonates and phosphine oxides

Article abstract of DOI:10.1016/j.tetlet.2019.151566

An environmentally friendly electrocatalytic strategy for the synthesis of organoselenophospho-rus between phosphonates /phosphine oxides and selenols/diselenides is reported. The reaction was performed with metal-, base- and oxidant-free at room temperat

Full text of DOI:10.1016/j.tetlet.2019.151566

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Electrochemical Selenation of Phosphonates and Phosphine Oxides
Shengmei Guo, Sen Li, Zhebin Zhang, Wenjie Yan, Hu Cai
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S0040-4039(19)31365-6
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https://doi.org/10.1016/j.tetlet.2019.151566
TETL 151566
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Tetrahedron Letters
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Please cite this article as: Guo, S., Li, S., Zhang, Z., Yan, W., Cai, H., Electrochemical Selenation of Phosphonates
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1
Tetrahedron Letters
journal homepage: www.elsevier.com
Electrochemical Selenation of Phosphonates and Phosphine Oxides
Shengmei Guo, Sen Li, Zhebin Zhang, Wenjie Yan, and Hu Cai*
College of Chemistry, Nanchang University, Nanchang 330031, P. R. China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An environmentally friendly electrocatalytic strategy for the synthesis of organoselenophospho-
rus between phosphonates /phosphine oxides and selenols/diselenides is reported. The reaction
was performed with metal-, base- and oxidant-free at room temperature, and affords the
selenophosphorus products in good to excellent yields. The good tolerance of substituents
enables the reaction more attractive in the preparation of organoselenophosphorus compounds.
Available online
2017 Elsevier Ltd. All rights reserved.
Keywords:
Electrochemical
Phosphonate
Selenation
Metal-free
Introduction
might cause environmental issue, the substrate limitation or other.
Thus,
a convenient, practical, efficient, green and atom
Organoselenophosphorus are crucial structural motifs in
various organic molecules1 and natural products2, which possess
special chemical and biological properties, thus are used in many
areas such as organic synthesis, biochemistry3 and pesticide
chemistry.4 Therefore, the synthesis of such compounds has
attracted continuous interest.5 Traditionally, the approach for the
formation selenophosphorus compounds often relies on the
substitution reaction of R2P(O)X or RSeX (X = RSe, Cl, Br, I,
etc.).6 While, these methodologies needed the use of toxic and
unstable reagents,7 and required harsh reaction measures, leading
to inconvenient procedures, low functional groups tolerance and
unsatisfactory yield.8 To overcome upon drawbacks, recently,
efforts have been made to realize the conversion. Zhao’s group
and other groups9 separately developed a metal-catalyzed
phosphonation of phenyldiselenide. While Qiu10 reported the
synthesis of selenophosphorus promoted by phase transfer
catalyst (PTC) under KOH. Huang11 and Chen12 independently
disclosed ways for the Se-P bond formation of phenyl diselenide
and organophosphorus by using AIBN or NaN3/PhI(OAc)2 as
radical initiators. Yan13 used equivalent amounts of iodine to
enable the construction P-Se bonds. Xu14 and Han15 found that
alkaline could efficiently allow for the construction of P-Se
bonds. Besides these, Saha16 developed a strategy for the
formation of organoselenophosphates through appealing the pre-
functionalized RSeH compounds. In addition, Jana17 revealed a
method for the synthesis of such compounds under O2 conditions.
Despite significant progress, those reactions still needed
transition-metal, PTC, strong base, AIBN or oxidant, which
economical synthesis methodology under mild conditions is
desirable.
Regarding
the
global
environment
problem,
environmentally friendly strategies for the synthesis organic
molecules in organic chemistry community are highly
urgent. Electrochemical organic synthesis has been
considered as one of the convenient, environmentally
friendly and mild organic synthesis methods, which do not
need oxidants, transition-metal catalysts comparing to
traditional oxidative reactions and has attracted continuous
attention.18 Here, we reported a metal-, oxidant- and base-
free strategy for the synthesis of organoselenophosphorus
under electrochemical conditions (Scheme 1).
R2SeH
or
O
Pt
Pt
O
R1
R1
R1
R1
R2Se P
+
+
H2
H P
R2SeSeR2
undivided cell
constant current
R1= aryl, alkyoxy
R2= aryl, alkyl, benzyl
metal free
oxidant free
base free
atom economy
21 examples
up to 96% isolated yield
Scheme 1. Selenation of phosphonates

2
Tetrahedron
Results and discussion
Fortunately, dialkyl phosphonates reacted with selenols to deliver
the corresponding products successfully with good yields
respectively (3a-3e), and the electron effect of the substrates on
the reaction was small. Aryl phosphonates such as diphenyl
phosphonate furnished the desired product in 96% yield (3f), and
dibenzyl phosphonate led the selenation product in 89% yield
With our ongoing interest in the phosphonation reactions19, we
initiated our investigation with the reaction of dibutyl-
phosphonate (1a, 0.5 mmol) and benzeneselenol (2a, 0.6 mmol)
as model to identify the optimal reaction conditions (Table 1).
n
When the reaction was performed in Bu4NBr/CH3CN at room
O
O
P
R
Pt(+) Pt(-)
temperature in air atmosphere by conducting a platinum anode
and a platinum cathode with constant current electrolysis at 12
mA, the expected product was obtained in a high yield (86%,
H
P
R
Se
R
+
+
HSe
2a
H2
nBu4NBr, CH3CN, rt
6 h, under air, 12 mA
undivided cell
R
1
3
n
O
O
P
O
O
entry 1). We found that when the Bu4NBr was replaced by
Se
P
OBu
Se
Se
OEt
Se
P
OMe
Se
P
OiPr
OiPr
nBu4NI, the reaction gave the corresponding product in 46% yield
OBu
OEt
OMe
3a
3b
3c
3g
3d
, 84%
, 86%
, 90%
, 86%
(entry 2). However, no product was formed when other
n
n
O
P
OiBu
O
O
P
O
O
O
electrolytes such as Bu4NF and LiClO4 displacing the Bu4NBr
(entries 3-4). The reaction was detected that no expected product
without electrolytes or constant current (entries 5-6). The
screening of current density found that electric current of 10 mA
or 14 mA could enable the desired product of 3a in 75% and 68%
respectively (entries 7-8). Other solvents such as CH3OH, DCM
(CH2Cl2), DCE(ClCH2CH2Cl), DMF failed to promote the
reactions (entries 9-12). Reducing or increasing the reaction time,
the reaction yields were slightly decreased (entries 13-14). When
we used the graphite rod as anode, the reaction resulted in the
product 3a in 60% yield (entry 15). Besides, operating the
reaction under a nitrogen atmosphere, only a lowered yield was
detected (entry 16).
Se
OiBu
Se
P
OBn
OPh
Se P
OBn
OPh
3e
3f
3h
, 96%
, 86%
, 96%
, 89%
O
P
O
P
O
P
Se
Se
Se
3ic
3jc
3kc
, 92%
, 89%
, 91%
O
Se P
P
O
Se
O
3lc
3m
, 91%
, 87%
a Reaction conditions: constant current = 12.0 mA, (0.5 mmol),
(0.5 mmol), CH3CN (8 mL), air atmosphere, room temperature, 6 h; b Isolated yield; c DCM
(0.6 mmol), nBu4NBr
1
2a
(8 mL), 10 mA, 4 h.
Table 1 Optimization of reaction conditions a
Scheme 2. The scope of the reactions.
O
O
P
OnBu
OnBu
Pt(+) Pt(-)
OnBu
OnBu
+
H
P
HSe
Se
nBu4NBr, CH3CN, rt
6 h, under air, 12 mA
undivided cell
(3g). When 5,5-dimethyl-1,3,2-dioxaphosphinane 2-oxide was
severed as a reaction partner, the selenation product was obtained
in 96% yield (3h). Interestingly, the reaction of diarylphosphine
oxides with selenols gave the desired products in moderate yield.
The yields were increased to excellent, when they were
performed in DCM (8 mL) with electric current of 10 mA for 4
hours (3i-3l). Finally, the dibenzo [c, e] [1,2]oxaphosphinine 6-
oxide also delivered the desired product in 91% yield (3m).
1a
2a
3a
Entry
1
Deviation from standard conditions
none
Yieldb (%)
86
2
nBu4NI instead of nBu4NBr
nBu4NF instead of nBu4NBr
LiClO4 instead of nBu4NBr
no nBu4NBr
46
3
ND
ND
ND
ND
75
4
5
6
no electricity
7
10 mA instead of 12 mA
14 mA instead of 12 mA
CH3OH instead of CH3CN
DCM instead of CH3CN
DCE instead of CH3CN
DMF instead of CH3CN
4 h of reaction time
8 h of reaction time
graphite rod as anode
under N2
8
68
9
29
10
11
12
13
14
15
16
75
51
31
70
78
60
69
aReaction conditions: 1a (0.50 mmol), 2a (0.60 mmol), electroyte (1.0 equiv), solvent (8
mL), air, Pt anode in an undivided cell at room temperature for 6 h; bIsolated yields.
After the optimized reaction conditions were established, we
first explored the substrates scope of this electrochemical
reaction between phosphates/phosphine oxides and selenols
(Scheme 2). Various phosphates were first investigated.

3
O
P
O
P
R
Pt(+) I Pt(-)
R1Se
R
H
R
+
1RSeSeR1
nBu4NBr, CH3CN, rt
6 h, under air, 12 mA
undivided cell
R
1
4
3
e
e
H
B
A
1RSe
dimerization
1RSeSeR1
O
P
O
O
Bn
Bn
Se
OBu
Se P OBu
OBu
Se P OPh
OPh
P
1RSeH
OBu
O
Se
1RSeSeR1
O
3a
3m
3n
3o
, 84%
, 78%
, 85%
, 80%
O
Br
cathode
R1Se P R
R
C
O
O
Se P Ph
Ph
P
1RSe
D
P
Br P R
O
Se
O
Se
1/2 Br2
O
O
R
E
anode
O
3pb
, 78%
3q
3r
, 75%
, 84%
H P R
R
Scheme 5. The plausible mechanism.
P
P
P
Se
Se
O
Se
O
O
O
On the basis of the above results and the previous reports17,20, a
proposed reaction pathway of selenation phosphonates and
phosphine oxides is depicted in Scheme 5. Firstly, at the anode,
selenol was oxidized to generate selenium radical A, followed by
a dimerization to form diselenide B. This intermediator was then
reduced at the cathode to generate C, which was composed to
lead A and selenium anion D. On the other hand, Br- was
oxidized to form Br2, in the presence of phosphonate, the
phosphorobromidate E was obtained. Finally, E was attacked by
D to afford the desired selenation product.
3s
3tb
,57%
3ub
, 65%
, 78%
a
Reaction conditions: (0.5 mmol), (0.3 mmol), nBu4NBr (0.5 mmol), CH3CN
(8 mL), room temperature, 6 h. isolated yield; b DCM (8 mL), 10 mA, 4h.
1
4
Scheme 3. The scope of the reactions.
The determination of reaction solution by GC-MS showed that
there exists diselenides. So, we wondered that the diselenides
might be the reaction intermediates. Then the scope of
diselenides was subsequently studied with phosphonates
/phosphine oxides (Scheme 3). 1,2-Diphenyldiselane was treated
with dialkyl phosphates, and the reaction proceeded well, leading
to 3a and 3m in 78% and 85% yields, respectively. The dibenzyl
diselenide performed well in the reaction systems, resulting the
corresponding products of 3n, 3o, 3p and 3q in 80%, 84%, 78%
and 84% yield. Moreover, the reaction of dialkyldiselenides with
dibenzo [c, e] [1,2] oxaphosphinine 6-oxide also proceeded
smoothly, yielding the desired products in 75–78% (3r-3s).
Finally, when the diphenylphosphine oxide was subjected to
dialkyldiselenides, the product was isolated in 57% and 65%
yield (3t-3u).
Conclusions
In summary, a concise, convenient, environmentally friendly
and efficient electrocatalytic-mediated selenation of phosphates/
phosphine oxides was developed. The substrates scope was
broad, and products were obtained in good to excellent yields
under mild conditions. The use of electrocatalytic strategy avoids
the use of metal catalyst, oxidant, base, thus reduced formation of
organic wastes. These features enable the method more useful in
organic synthesis.
Two control experiments were conducted to clarify the
reaction mechanism (Scheme 4). To explore whether the reaction
occurs through a radical procedure, we used TEMPO (2.0 equiv.)
as radical scavenger, and the desired product 3a was decreased
64% yield, along with the radical product of TEMPO with
Acknowledgements
benzeneselenol
(detected
by
GC-MS).
Next,
1,1-
diphenylethylene (2.0 equiv.) instead of TEMPO was used, and
the desired product 3a was isolated in 68% yield. It means that
the reaction is might undergo a radical process.
We thank the financial support from the National Science
Foundation of China (21861024, 21571094, 21761021).
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OnBu
O
standard conditions
OnBu
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GC-MS
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Scheme 4. Control experiments.

4
Tetrahedron
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R2SeH
O
Pt
Pt
O
R1
R1
R1
R1
or
R2Se P
+
+
H2
H P
R2SeSeR2
undivided cell
constant current
R1= aryl, alkyoxy
R2= aryl, alkyl, benzyl
metal free
oxidant free
base free
21 examples,up to 96% yield
Our work has below advantages:
(1) Metal-, oxidant-, and base free in this transformation
(2) The Scope of substrate are broad
(3) The reaction is atom economy with high yield