Electrochemical synthesis of phosphorus esters from white phosphorus in the presence of copper complexes and ethanol

Article abstract of DOI:10.1023/A:1024408627802

In the presence of white phosphorus the redox potentials of the copper ions change and the potential of the reduction wave of Cu^I/Cu ⁰ shifts noticeably toward more positive values. The Cu ^I-P₄ complex is characterized by a lower value of the electrochemical gap, that is, higher polarizability and reactivity compared to those of the free Cu^I cation. Phosphorus esters can be synthesized from P₄ and ethanol. The latter is in the composition of the copper(II) complexes, which act as a catalyst-charge mediator.

Full text of DOI:10.1023/A:1024408627802

Russian Chemical Bulletin, International Edition, Vol. 52, No. 4, pp. 929—938, April, 2003
929
Electrochemical synthesis of phosphorus esters from white phosphorus
in the presence of copper complexes and ethanol
Yu. H. Budnikova,aꢀ A. G. Kafiyatullina,a O. G. Sinyashin,a and R. R. Abdreimovab
aA. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 75 2253. Еꢀmail: yulia@iopc.knc.ru
bInstitute of Organic Catalysis and Electrochemistry,
142 ul. Kunaeva, 480100 AlmaꢀAta, Kazakhstan.
Eꢀmail: abdreimioce@nursat.kz
In the presence of white phosphorus the redox potentials of the copper ions change and the
potential of the reduction wave of CuI/Cu0 shifts noticeably toward more positive values. The
CuI—P4 complex is characterized by a lower value of the electrochemical gap, that is, higher
polarizability and reactivity compared to those of the free CuI cation. Phosphorus esters can be
synthesized from Р4 and ethanol. The latter is in the composition of the copper(II) complexes,
which act as a catalystꢀcharge mediator.
Key words: catalysis, electrochemistry, copper complexes, white phosphorus, phosphorus
esters.
Since copper(II) is characterized by high complexꢀ
forming ability and its salts and complexes are highly
soluble in organic solvents and can be regenerated by
oxygen, copper(II) complexes are widely used in catalyꢀ
sis,1—8 including the synthesis of organophosphorus comꢀ
pounds from white phosphorus.2—7 At the same time,
only few electrochemical syntheses involving copper cataꢀ
lysts have been studied.8
It is known2—4 that the reaction of white phosphorus
with an ethanol solution of the copper(II) acido comꢀ
plexes under oxygen affords trialkyl phosphates and dialkyl
phosphites. The phosphorusꢀcontaining compounds are
formed in the step of copper(II) reduction to copper(I) by
white phosphorus, and oxygen acts as an oxidizing agent
to recover copper(II). Under aerobic conditions this reacꢀ
tion requires additional precautions. Water, which is proꢀ
duced by the oxidation of CuI with dioxygen, provokes a
side reaction, the oxidative hydroxylation of Р4 to phosꢀ
phoric acid, decreasing the yield of the organophosphorus
products.2—4
of phosphorus esters from white phosphorus, and the
mechanism of these reactions.
Results and Discussion
Voltammetric studies. The published data on the elecꢀ
trochemical behavior of the copper salts and complexes
concern with different conditions and methods and are
incomplete.9—15 In this work, using cyclic voltammetry
(CV) on a stationary glassꢀcarbon (GC) electrode, we
studied chemical transformations of copper(II) chloride
in the presence and absence of white phosphorus in aprotic
(MeCN, DMF), protonꢀdonor (EtOH), and mixed meꢀ
dia (aprotic solvents with EtOH additives) in an argon
flow. The obtained data are presented in Figs. 1—6. The
reduction of CuCl2 in the chosen solvents proceeds in
two steps consuming one electron per the formula unit of
CuCl2 at each peak potential and can be described by the
following scheme:
We suggested that the oxidation of CuI to CuII in this
process can be carried out electrochemically. An imporꢀ
tant advantage of the electrochemical regeneration of the
catalyst could be an enhancement of the yield of the
target products due to elimination of side reactions.
This work is aimed at studying by cyclic voltammetry
and preparative electrolysis the electrochemical behavior
of the copper(II) salts and complexes in the absence and
presence of P4, their role in the electrocatalytic synthesis
C
CuCl2 + e
CuCl + Cl–
Cu0 + Cl–
(Ep ),
1
C
2
CuCl + e
(Ep ).
The first reversible peak has the anodic component
(А1) of CuI oxidation (see Figs. 1, 2, and 4—6) related to
the cathodic process (C1), and the second peak is irreꢀ
versible in most cases (except for a MeCN—EtOH mixꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 882—891, April, 2003.
1066ꢀ5285/03/5204ꢀ929 $25.00 © 2003 Plenum Publishing Corporation

930
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
Budnikova et al.
–I
C1
C1
a
1
C2
5 µA
C2´
2
1, 2
1
A1
0
0.4
0.8
1.2
1.6
–E/V
A1
Aa
A´
–I
C2
C3´
b
C2´
10 µA
2
C1
C1´
1
A3´
1, 2
A1
Aa´´´
–0.4
0
0.4
0.8
1.2
–E/V
Aa´´
Aa´
Aa
Fig. 1. CV curves for solutions of CuCl2 (5•10–3 mol L–1) in DMF (a) and in MeCN (b) in the absence (1) and presence (2) of white
phosphorus (1.5•10–2 mol L–1).
ture, where a series of close reversible cathodic peaks of
reduction, most likely, of different copper(I) ethoxides
are observed) (see Fig. 2, b). A great difference between
the potentials of the first and second cathodic processes is
probably due to the high thermodynamic stability of CuI
in these systems, particularly in the aprotic medium (see
Fig. 1). The reverse scan beginning from the more negaꢀ
to both complex formation and other chemical reactions.
In aprotic media and at a low content of EtOH, the poꢀ
tential of CuCl2 reduction somewhat shifts in the anodic
direction, but the CuII/CuI peak remains reversible. Inꢀ
significant changes in the CV curves in these solutions
upon the addition of Р4 can be explained by either the low
rate of formation of the corresponding complexes and
their low stability or a comparable stability of the comꢀ
plexes formed by both components of the redox pair.
Probably, the CuI complex is stable in a solution under
C
2
tive potential than Ep results in the oxidation peaks of
electrodeposited copper. A symmetrical shape of the peaks
without a diffusional tail indicates their adsorption nature.
The oxidation peak of halide ions in solutions of CuCl2
corresponds to oxidation and lies in the anodic potential
region (approximately +1.0 V in DMF).
I
these conditions. Therefore, the peak of its reduction (C2 )
C
2
is observed at a more negative potential Ep , which can
be explained by binding of CuI to form the complex with
phosphorus CuI(P4)n. It is known16 that the electrochemiꢀ
cal redox potentials reflect, to some approximation, the
energies of HOMO and LUMO of compounds even when
The gradual addition of white phosphorus (as a satuꢀ
rated solution in benzene) to a solution of CuCl2 changes
the shape of the CV curves, which is related, most likely,

Electrosynthesis of phosphorus esters
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
931
–I
C2
C3´
a
C2´
C1
2
5 µA
1
Aa´
A1
A1´
–0.3
0
0.3
0.7
1.1
1.5
1.9
–E/V
Aa
A1´
–I
C3´
b
C4
C5
C3
C2´
C2
10 µA
C1´
1
2
A4
A5
C1
A3
A2
A1
C2´
A1´
C1´
2
Aa´´
Aa´´
A1´
Aa
Aa´
0.4
0
0.8
1.2
1.6
2.0
–E/V
Fig. 2. CV curves for solutions of CuCl2 (5•10–3 mol L–1) in DMF (a) and in MeCN (b) with EtOH additives (2•10–2 mol L–1) in the
absence (1) and presence (2) of white phosphorus (1.5•10–2 mol L–1).
the processes are thermodynamically irreversible. Comꢀ
plex formation changes these energies and shifts Ep.
πꢀAcceptor ligands (2,2´ꢀbipyridine, phosphine, and phosꢀ
phites) shift Ep of the reduction of the complexes toward
less negative potentials thus facilitating reduction.17 Anꢀ
odic scan results in a large peak of oxidation of copper
phosphides (EpA ≈ –0.1 V in DMF and –0.3 V in EtOH).
The heights of the Ip and Ip peaks remain unchanged,
indicating the absence of a noticeable chemical reaction
between CuII and Р4 under these conditions. Only in the
presence of great excess EtOH (when it is the only solꢀ
vent), a more complicated, changing in time pattern is
observed (see Fig. 6). Several minutes after phosphorus
was added to a solution of CuCl2 in EtOH (against LiClO4
or Bu4NBF4), the voltammogram exhibits a slight shift
C
C
2
1
of the Ep and Ep potentials with the retention of
reversibility of the first peak. However, after 30 min, in
the same solution (under argon) the heights of reduction
II
peaks (C1 and C2II) decrease simultaneously, i.e., the
concentrations of CuII and CuI in a solution decrease (the
current is proportional to the concentration of the speꢀ
cies). Therefore, it is improbable that CuII oxidizes Р4
and transforms into CuI. It is most likely that copper
polyphosphides, which are electrochemically inactive in
the accessible cathodic region of potentials, are formed in
the solution. The solution becomes brown with time, and
a brown precipitate is gradually formed. After some time,
C
C
2
1
II
II
the C1 and C2 peaks disappear, and both the initial

932
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
Budnikova et al.
–I
Unlike the known catalytic oxidative alkoxylation of
Р4 in the presence of oxygen,2—4 in this work we used the
electrochemical regeneration of CuII. In this method, CuI
should be oxidized on the anode either through a mediaꢀ
tor (for example, Cl2 formed by the oxidation of the Cl–
ions) or immediately on the anode through a heterogeꢀ
neous reaction (in the absence of the Cl– ions in a soꢀ
lution).
5 µA
The potential of the beginning of oxidation of the
components of the CuCl2—P4—EtOH system is close to
the oxidation potential of Cl–. When the synthesis is carꢀ
ried out in an undivided electrochemical cell, EtOH can
be reduced on the cathode, and either Cl– or copper
polyphosphides can be oxidized on the anode (at low
concentrations of EtOH or in its absence, the CV curves
contain a pronounced diffuse anodic peak at low anodic
potentials (+0.2—+0.3 V)). This peak (AI) appears due to
the superposition of several peaks of oxidation of the copꢀ
per phosphides with close oxidation potentials. Although
the main potential of the broad peak is close to the oxidaꢀ
tion potential of CuI (AI), its shape is more complicated
(great currents and peak widths).
2.0
2.2
2.4 –E/V
Fig. 3. CV curves for solution of white phosphorus
(1•10–2 mol L–1) in MeCN with a benzene additive (10 vol.%)
against 0.1 М Et4NBF4.
CuII ions and CuI ions are absent from the solution (see
Fig. 4), whereas the peak of excess phosphorus is obꢀ
served at its standard potential of –2.35 V (see Fig. 3).
The observed influence of the EtOH concentration on
the reaction of CuII with phosphorus (Table 1) suggests
that EtOH is the third reactant in this reaction, and the
phosphorusꢀcontaining intermediate products contain the
EtO groups.
We studied the electrochemical behavior of the
copper(II) complexes with 2,2´ꢀbipyridine (bpy) in the
absence and presence of Р4 to stabilize the copper ions
–I
C1
5 µA
1
C2´
C2
2
C1´
C1
C2´´
1
A2
C1´´
3
4
A2´´
A2
Aa´´
A2´
Aa
A1
–0.8
–0.4
0
0.4
0.8
1.2
–E/V
A1´
Fig. 4. CV curves for solutions of CuCl2 (5•10–3 mol L–1) in EtOH against 0.1 М LiClO4 in the absence (1) and presence (2—4) of
white phosphorus: 2, 5•10–3 mol L–1, immediately after addition; 3, the same after 0.5 h; and 4, 1 h after addition of Р4
(1•10–2 mol L–1).

Electrosynthesis of phosphorus esters
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
933
–I
C4´
10 µA
EtOH
C2
C3
1
C2
C1
1´
A1
2
A1
C2´
C1´
A0
0.4
0.8
1.2
1.6
1.8
2.0 –E/V
A1´
Fig. 5. CV curves for solutions of CuCl2 (5•10–3 mol L–1) + bpy (1.5•10–2 mol L–1) in DMF (1) and a DMF—EtOH mixture
(2•10–2 mol L–1) (1´) in the absence (1, 1´) and presence (2) of white phosphorus (1•10–2 mol L–1).
–I
C4´
10 µA
C3´
C2´
3
2
C2
C1
C1´
2
1
A1
Aa
A´
1
0.4
0.8
1.2
1.6
2.0
–E/V
Fig. 6. CV curves for solutions of CuCl2 (5•10–3 mol L–1) + bpy (1.5•10–2 mol L–1) in MeCN (1) in the absence (1) and presence of
phosphorus in concentrations of 5•10–3 (2) and 1.5•10–2 mol L–1 (3).

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Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
Budnikova et al.
Table 2. Change in the reduction potential of CuI/Cu0 (∆E C
)
2
Table 1. Yields of the products of electrolysis of solutions of P4
in EtOH, DMF—EtOH, and MeCN—EtOH in the presence of
CuCl2bpy in the undivided cell
in various solvents with white phosphorus additives (P4 : CuII = 3)
in the presence of the CuCl2 and CuCl2bpy3
∆E C /V
2
Product
Yield (%)
Catalyst
EtOH DMF—EtOH MeCN—EtOH
DMF MeCN DMF—EtOH MeCN—EtOH
(EtO)2P(O)Et*
(EtO)2PHO*
(EtO)3PO*
17.5
16.0
28.5
62
5.4
15.3
79.3
90
15.5
29.5
55.0
81
CuCl2
CuCl2bpy3
0.41
0.61
0.55
0.36
0.73
0.80
0.51
0.36
Sum of products**
value. It is likely that the electrochemical gaps, viz., difꢀ
ferences between the potentials of reduction and oxidaꢀ
tion of a species G = EA – E C (V), can most correctly be
used instead of particular reduction potentials for the estiꢀ
mation of the reactivity of species.18 The electrochemical
gap characterizes the polarizability and donor ability of a
complex. The electrochemical gap values for CuI (CuCl
or CuClbpy) in the absence and presence of white phosꢀ
phorus are presented in Table 3. In most cases, with some
exception, G(CuIP4) < G(CuI). Thus, CuI—P4 is a more
easily polarized ("softer") and, hence, more reactive comꢀ
plex than CuI (CuCl or CuClbpy) in the absence of Р4.
An increase in the EtOH concentration in a solution
of CuCl2bpy3 (unlike CuCl2) does not affect the process,
and white phosphorus does not react for several days.
According to the 31Р NMR and CV data, no changes
occur in the system.
Thus, the redox properties of the copper complexes
change in the presence of white phosphorus, and the reꢀ
duction potential of CuI/Cu0 shifts strongly toward more
positive values. The CuI—P4 complex is characterized by
a lower electrochemical gap, i.e., it is more readily polarꢀ
izable and reactive than the initial CuI. The reactivity of
the copper ions changes in the presence of the bpy ligand.
Preparative electrolysis. Preparative electrosynthesis
of phosphorus esters from white phosphorus and EtOH in
the presence of the copper complexes was carried out in
an undivided electrochemical cell. The supporting elecꢀ
trolyte was Et4NBF4 (0.02 М solution in EtOH or in a
mixture of EtOH with an aprotic solvent). Electricity (5 F
per P atom) was passed through the electrolyte at 50 °C.
The catalyst was CuCl2 or the CuCl2—bpy complex. After
* The yield calculated per P is presented.
** The total yield is presented.
and intermediates by complex formation. The CuCl2bpy3
complex is characterized by two cathodic peaks in a soluꢀ
tion of MeCN and by three cathodic peaks in DMF. The
addition of bpy to a solution of CuCl2 in DMF or MeCN
remarkably shifts the peaks of the CuII/CuI redox sysꢀ
tem to the cathodic direction with retention of their
reversibility (see Figs. 5 and 6). A great difference in
C
C
2
1
the Ep and Ep potentials gives evidence for a high
thermodynamic stability of CuI in a DMF solution
and a lower stability in MeCN. Minor EtOH additives
(with respect to the main solvent) to the electrolyte
(CuCl2 : EtOH = 1 : 4) have almost no effect on the shape
of the CV curves. The changes concern the C2 and C3
peaks in DMF (they join), and a new irreversible oxidaꢀ
tion peak (approximately +0.35 V), which does not coinꢀ
cide with the peak of Cl– oxidation, appears in all cases.
The transformation of CuIIL3 into CuIL2 is reversible,
and the transformation of CuIL2 into Cu0L2 is irreversꢀ
ible. The addition of phosphorus only insignificantly shifts
the C1 peak to the anodic region in MeCN and exerts no
effect on its position in DMF. In the presence of phosꢀ
phorus, the first C1 wave is reversible in MeCN and irreꢀ
versible in DMF. It is most likely that the fast chemical
reaction of CuI with P4 occurs in the latter case. The
addition of Р4 to the copper complex does not substantially
change the heights of the C1 and C2 peaks (or C2 + C3).
This indicates the absence of a fast chemical reaction of
CuCl2bpy3 with white phosphorus, likely, due to the abꢀ
sence of free coordination sites for P4 in this complex. It
is noteworthy that the peak of reduction of Р4 itself, which
is observed at high cathodic potentials (Ep = –2.35 V), is
much higher than the diffusional peak, i.e., this is rather
catalytic peak. The EtOH additives to the CuCl2bpy3 comꢀ
plex do not remarkably change the shape of the curves in
both the absence and presence of white phosphorus.
The shifts of the reduction potential of CuI upon phosꢀ
phorus addition depend on the solvent nature and the
presence of the bpy ligand (Table 2). For example, for
CuCl2bpy3 the greatest shift is observed in DMF, and for
CuCl2, in MeCN. In a solvent—EtOH mixture, the presꢀ
Table 3. Electrochemical gap for CuI (G = EA – E C) in various
solvents in the absence (figures in the numerator) and presence
of P4 (figures in the denominator)
Catalyst
G/V
DMF MeCN DMF—EtOH MeCN—EtOH
CuCl2
1.80
1.33
1.43
0.84
1.23
0.64
1.05
0.99
2.01
1.19
1.92
1.27
1.11
0.50
0.93
1.00
CuCl2bpy3
ence of the ligand has no noticeable effect on the ∆E C
2

Electrosynthesis of phosphorus esters
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
935
electrolysis, the color of the solution changed from light
green to brown. A minor amount of a brown phosphorusꢀ
containing precipitate is formed in a solution of EtOH or
in an EtOH—MeCN mixture, whereas no precipitate is
formed in an EtOH—DMF mixture. In all the cases, triꢀ
ethyl phosphate, diethyl ethylphosphonate, and diethyl
phosphite were the reaction products (composition was
determined from the integral intensity in the 31Р NMR
spectrum, see Table 1).
Thus, the most optimal conditions for the synthesis of
phosphorusꢀcontaining products from Р4 are those with
the use of the CuCl2bpy catalyst in a DMF—EtOH mixꢀ
ture. In this case, phosphorus is completely converted to
organic derivatives, among which triethyl phosphate preꢀ
dominates. An increase in the amount of the passed elecꢀ
tricity to ∼6 F makes it possible to transform all the diethyl
phosphite into triethyl phosphite. Diethyl ethylphosꢀ
phonate is the product of rearrangement of triethyl
phosphite.
For comparison, we carried out electrooxidation in a
divided cell. A saturated solution of white phosphorus in
toluene was added to the anolyte containing CuCl2 in a
solution of EtOH. This solution was electrolyzed in the
galvanostatic regime at 50 °C. At the moment of addition
of white phosphorus (0.5 moles of Р4 per mole of CuCl2)
the potential of the anode did not change. After the reꢀ
peated addition of phosphorus (in the first minutes of
electrolysis) to the ratio Р4 : CuCl2 = 3 : 1, the color of the
solution sharply became brown, a brown precipitate
formed, and the anodic potential began to shift gradually
to the anodic region, to the oxidation potential of Cl–.
These observations agree with the results obtained by the
CV method: some time after the beginning of electrolysis,
the solution contains no copper ions and the cell voltage
increases dramatically. The use of, e.g., Et4NCl as supꢀ
porting electrolyte maintains the conductivity of the meꢀ
dium. In this case, the anodic oxidation of Cl– can be
used for regeneration of free copper ions in that or anꢀ
other oxidation state, although the phosphorusꢀcontainꢀ
ing precipitate remains in the solution even after passing
6 F electricity per P atom. The electrolysis products are
ethyl phosphorus esters, viz., (EtO)3PO, (EtO)2PHO,
(EtO)2P(O)OH, and others. The electrolyte is acidified.
Thus, undivided electrolysis provides the lower yield and
selectivity.
As mentioned in the literature,2—4 the CuII : P4 ratio
exerts a substantial effect on the rate and selectivity of the
oxidative alkoxylation of white phosphorus in the presꢀ
ence of the copper catalysts, and in excess copper salt
(molar ratio CuII : P4 = 15—20) Р4 can rapidly be transꢀ
formed into triethyl phosphate without intermediate forꢀ
mation of insoluble copper polyphosphides or Cu0. We
carried out the chemical reduction of CuCl2 by white
phosphorus (CuCl2 : P4 = 20) in EtOH in an argon atmoꢀ
sphere at 55—60 °C. The primary emerald color of the
solution became light green within 1 h but no precipitate
was formed.
According to the 31Р NMR spectroscopic data, the
resulting solution contains triethyl phosphate bound to
the copper ions (6 ppm). The CV data showed that the
–I
2
100 µA
3
1
0.4
0.8
1.2
1.6
–E/V
Fig. 7. CV curves in EtOH against 0.1 М Et4NBF4 after the reaction (1 h) of CuCl2 (0.2 mol L–1) with P4 dissolved in toluene:
1, CuCl2 : P4 = 20 immediately after the reaction at 60 °C (the color of the solution changed from emerald to light green); 2, under the
same conditions after cleaning of the electrode; and 3, CuCl2 : P4 = 10 (a brown solution).

936
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Budnikova et al.
CuII concentration decreased and CuI appeared (the secꢀ
ond peak increased over the first peak), although CuI
exists in a solution, most likely, in different forms, beꢀ
cause it is characterized by two rather one peaks (Fig. 7).
However, since under these conditions all copper comꢀ
pounds exist in a solution, CuI can electrochemically be
oxidized to CuII. Then Р4 can be added again, CuII can be
reduced to CuI, and the electrochemical oxidation of CuI
can be repeated to accumulate (RO)3PO in the solution.
However, when the process is performed in an undivided
cell, much smaller excess copper chloride and a higher
concentration of phosphorus can be used to obtain higher
yields of the product based on the solution volume at a
much higher rate of the process.
The addition of triethyl phosphate to a solution of
CuCl2 in EtOH shifts the potentials of the CuII/CuI and
CuI/Cu0 peaks to the cathodic region (to ∼0.16 V). The
formation of the CuII complex with (EtO)3PO that formed
explains the aboveꢀmentioned cathodic shift of the
equilibrium potential of the CuII/CuI pair approxiꢀ
mately by the same value after the reaction ceased in the
CuCl2—EtOH—P4—arene—O2 system.
Based on analysis of the published data2—4 and our
experimental results, we can assume that under optimal
conditions the oxidative alkoxylation of Р4 in the presꢀ
ence of the CuII salts and complexes (CuX2) includes two
main reactions: homogeneous reduction of CuХ2 with
white phosphorus, which affords phosphorus esters and
electrochemical oxidation of CuХ on the anode.
Trialkyl phosphite undergoes dealkylation to dialkyl
phosphite and isomerization to alkyl phosphonate.
Trialkyl phosphate is formed due to the homogeneous
oxidative alkoxylation of dialkyl phosphite in the presꢀ
ence of CuX2.19
It is of interest to reveal routes of intermediate transꢀ
formation of the copper complexes.
(RO)2PHO + 2 CuX2 + ROH
(RO)3PO + 2 CuX + H2.
CuII
[Cu—Pn]n+
CuI or Cu0
CuII.
It has previously been asserted2—4 that at CuCl2 : P4 =
1—12 the CuII atoms are reduced by tetraphosphorus to
Cu0 avoiding formation of copperꢀphosphorus intermeꢀ
diates, and at greater excess CuCl2 (CuII : P4 ≥ 20), they
are reduced to CuI. The CuII → CuI reaction is confirmed
by the results of our experiments on the synthesis of trialkyl
phosphate from Р4 and the CV data for the oxidized and
reduced copper species (see above). However, our experiꢀ
ments at CuII : P4 < 10—12 gave insoluble copper polysulꢀ
fides instead of Cu0. It can be assumed that Cu0 forms a
complex with the P4 ligand. However, similar M0L comꢀ
plexes (M = Ni, Co, Cu; L = bpy, PPh3, and others) are
reduced in the accessible potential region ahead of the
ligand.17 Our CV study did not detect the Cu0P4 complex
in a solution likely because of its low stability.
In further experiments, we replaced copper(II) chloꢀ
ride by acetate, which, as can be assumed, can more
easily be reduced to elemental copper with white phosphoꢀ
rus. The CV data showed that the reaction of Cu(AcO)2
with P4 (in a ratio of 10 : 1) in EtOH against Et4NBF4
results in the disappearance of the copper(II) ions. A black
precipitate containing copper and phosphorus is formed
in a solution, which contradicts the assumption2—4 on the
reduction of CuII to Cu0 with white phosphorus. The
reaction products contained (EtO)3PO and (EtO)2PHO.
The height of the peak corresponding to CuI increases in
the CV curves. The elemental composition of the precipiꢀ
tate formed is close to Et4N+Cu2P–•2H2O.
The electrochemical reduction of CuX on the anode
closes the catalytic cycle
CuX + X– – e
CuX2.
In the case of electrosynthesis involving the CuCl2bpy
complex instead of CuCl2, the direct oxidation of white
phosphorus with copper(II) does not occur, and the proꢀ
cess proceeds, likely, through other several steps. In these
systems, the copper complexes with white phosphorus
and EtOH are reduced on the cathode in the initial steps
of electrolysis (EC = –1.8—2.0 V during electrolysis), and
the chloride ions and copperꢀphosphorus complexes are
oxidized on the anode. The main routes of the reaction of
phosphorus are presented in Scheme 1.
Electrosynthesis using CuCl2 as catalyst has
several disadvantages. Considerable excess CuCl2
(CuCl2 : P4 = 20) is needed for the selective transformaꢀ
tion of white phosphorus into liquid products, predomiꢀ
nantly (RO)3PO. In fact, the copper(II) salt acts as an
oxidizing agent rather than catalyst. The regeneration of
CuCl2 by the electrooxidation of CuI is limited by the
slow chemical transformation of CuII into CuI, so that
electrosynthesis should be carried out at a low current
density and with a low efficiency. The reaction occurs
differently, depending on the Cu : P4 ratio. When excess
copper or its high concentration is present in a solution,

Electrosynthesis of phosphorus esters
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
937
Scheme 1
the phosphorus acid derivatives can be prepared by
electrosynthesis from Р4 in the presence of the copper(II)
compounds, which act as the catalyst and charge transfer
agent.
2 EtOH + 2 e
2 Cl– – 2 e
2 EtO– + H2
Cl2
Experimental
x+
[Cu—P]n – x e
[Cu—P]n
CuII + [P]
CV curves were recorded on a GC electrode 1.5 mm in
diameter. A silver electrode, i.e., a 0.01 M solution of Ag/AgNO3
in MeCN, served as reference electrode. A Pt wire 1 mm in
diameter was used as auxiliary electrode. Measurements were
carried out in a cell with the temperature maintained at 25 °C in
an argon atmosphere. CV curves were detected using a PIꢀ50ꢀ1
potentiostat, a PRꢀ8 programmer, and a twoꢀcoordinate reꢀ
corder at a potential sweep of 50 mV s–1. The supporting salts
were Et4NBF4, Bu4NBF4, and LiClO4.
P4 + CuL2+
[CuP4L]2+
[(RO)3P]Cu2+
[CuPxL]
[(RO)3PO]Cu2+
Preparative electrolysis was carried out using a B5ꢀ49 dc
source in a thermostatted (at 50 °C) threeꢀelectrode 40ꢀmL cell
in EtOH or its mixture (1 : 3) with DMF or MeCN under argon.
The electrolyte was magnetically stirred. In experiments on unꢀ
divided electrolysis, a Pt cylinder with a working surface of
40 cm2 was used as cathode, and a coaxial Pt rod with a working
surface of 15 cm2 served as anode. In experiments with a divided
cell, a butterꢀpaper diaphragm was used, the same Pt cylinder
was used as anode, and a GC plate with a surface of 8 cm2
served as cathode. The catholyte was a saturated solution of
Et4NBF4 in EtOH. White phosphorus was introduced into
the catholyte in the solid state or as a saturated solution of Р4
in C6H6.
L = bpy
A. Homogeneous oxidation. B. Electrochemical reduction on
electrode. C. Electrochemical oxidation on electrode.
The 31Р NMR spectra were recorded on a Bruker СХРꢀ100
spectrometer with a working frequency of 36.5 Hz relatively to
the external standard (85% H3PO4) with positive downfield
chemical shifts.
phosphorus is homogeneously oxidized with copper(II)
via route А, and CuI that formed is oxidized on the anode.
At low concentrations of the copper salt, the reaction of
phosphorus either occurs slowly if at all, and at the elecꢀ
trolysis potentials (–1.8 V, all copperꢀphosphorus comꢀ
plexes and intermediates are reduced on the cathode) the
complex is reduced to transform the tetrahedral phosphoꢀ
rus via route В.
The use of CuCl2bpy as catalyst makes it possible to
perform electrosynthesis more efficiently. The copper
complex is used in catalytic amounts, white phosphorus is
taken in great excess, and now the process rate is not
determined by the rate of CuI formation. The active interꢀ
mediates are generated at more positive (on the anode)
and, correspondingly, more negative potentials (on the
cathode), so that this results in the individual step (genꢀ
eration of the reactant on the electrode and volume chemiꢀ
cal reaction) rateꢀbalanced transformation of white phosꢀ
phorus. In this case, the mechanism of electrosynthesis is
more complicated and requires a special study. However,
this approach seems to be most promising.
Undivided electrolysis at the ratio Cu : P4 = 0.25. An undiꢀ
vided cell was loaded with EtOH (10 mL), DMF or MeCN
(30 mL), CuCl2 (0.1 g, 0.74 mmol), bpy (0.12 g, 0.77 mmol),
white phosphorus (0.372 g, 3 mmol), and Et4NBF4 as supportꢀ
ing salt to a concentration of 1•10–2 mol L–1. In pure EtOH
electrolysis was carried out under the same conditions. The soꢀ
lution was heated under argon to 50 °C. Electricity (1.61 A h,
5 F electricity per P atom) was passed through the electrolyte in
the galvanostatic regime at a current of 200 mA. The potentials
to –1.8 V on the cathode and to +1.7 V on the anode were
maintained during electrolysis. In a DMF solution the reaction
occurs without formation of the heterogeneous phase, and a
minor amount of a dark brown unstable in air phosphorusꢀ
containing precipitate with the phosphine odor appears in MeCN
and EtOH. After electrolysis, excess solvent was distilled off
from the electrolyte, the supporting salt was precipitated with
diethyl ether, and the etheric extract was concentrated by
evaporation and distilled in vacuo. The physical and spectroꢀ
scopic characteristics of the synthesized phosphorus esters, viz.,
(EtO)3PO, (EtO)2P(O)Et, and (EtO)2PHO, correspond to the
published data. The yields of the products are presented in
Table 1.
Divided electrolysis at the ratio Cu : P4 = 1. The anodic area
of a divided cell was loaded with EtOH (30 mL), CuCl2 (0.0537 g,
0.4 mmol), and a saturated solution of white phosphorus in
benzene (1.6 mL, 0.4 mmol). A saturated solution of Et4NBF4
Thus, in our experiments, we found the dependence
of the redox properties of the copper catalysts in the abꢀ
sence or presence of white phosphorus and showed that

938
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 4, April, 2003
Budnikova et al.
in EtOH was in the cathodic area. The solution was heated
under argon to 50 °C. After passing 200 mA h electricity (<1/3 of
the amount obtained based on 5 F per P atom) at the anodic
potential from +1.0 to +1.26 V, the cell voltage increases draꢀ
matically (from 16 to 75 V), because the copper ions are sepaꢀ
rated from the solution within a brown precipitate and the soluꢀ
tion becomes nonconducting. The absence of the copper ions in
the solution is confirmed by the CV data. The addition of the
supporting electrolyte does not change the situation. Thus, it is
impossible to continue electrolysis under these conditions beꢀ
cause the solution contains no anodic depolarizer.
Divided electrolysis at the ratio Cu : P4 = 20. The anodic
area of the divided cell was loaded with EtOH (30 mL), CuCl2
(0.268 g, 2 mmol), and a saturated solution of white phosphorus
in benzene (4 mL, 0.1 mmol). A saturated solution of Et4NBF4
in EtOH was in the cathodic area. The solution was stirred
under argon at 50 °C for ∼1 h. The color of the solution changed
from saturated green to brown but the solution remained homoꢀ
geneous. The CV curves demonstrated a decrease in the height
of the CuII/CuI peak and an increase in the height of the more
cathodic peak (presumably, CuI). According to the 31Р NMR
spectroscopic data, (EtO)3PO is selectively formed under these
conditions. The anodic oxidation of CuI was carried out at a
current of 200 mA and a potential of +0.2 V (5 F per initial
phosphorus atom or 1 F per initial CuII salt). Then the proceꢀ
dure on loading and consumption of phosphorus was repeated
thus accumulating triethyl phosphate in the solution.
Reaction of copper(II) acetate with white phosphorus in an
ethanol medium at the ratios Cu : P4 = 20 and 10. A saturated
solution of white phosphorus (0.2 mL, 0.05 mmol) in benzene
was added to a solution of Cu(AcO)2•H2O (0.2 g, 1 mmol) in
EtOH (10 mL), so that the initial molar ratio CuII : P4 was 20. In
order to record the CV curves, Et4NBF4 (5•10–2 mol L–1) was
added. The solution was stirred for 1 h at ∼20 °C, due to which
its color changed from blue to brown. After the repeated addiꢀ
tion of a saturated solution of white phosphorus in benzene
(0.2 mL, 0.05 mmol), the Cu : P4 molar ratio in the solution
became equal to 10. The solution was stirred with the added
second portion of Р4 for 2 h, and a black precipitate was formed.
Heating of the solution to 50 °C accelerates the process, so that
the reaction ceases within 20 min. The precipitate was filtered
off and washed with diethyl ether. The elemental composition of
the precipitate corresponds to the formula C8H24Cu2NPO or
Et4N+Cu2P–•2H2O. Found (%): C, 33.3; H, 6.9; Cu, 44.1;
N, 4.9; P, 10.8. Calculated (%): C, 29.63; H, 7.40; Cu, 39.20;
N, 4.32; P, 9.57. The products of phosphorus transformation are
(EtO)3PO and (EtO)2PHO with the total yield to 30%.
References
1. V. Dambrin, M. Villieras, J. Lebreton, L. Toupet, H. Amri,
and J. Villieras, Tetrahedron Lett., 1999, 40, 871.
2. Ya. A. Dorfman, M. M. Aleshkova, G. S. Polimbetova, L. V.
Levina, T. V. Petrova, R. R. Abdreimova, and D. M.
Doroshkevich, Usp. Khim., 1993, 62, 928 [Russ. Chem. Rev.,
1993, 62 (Engl. Transl.)].
3. Ya. A. Dorfman and R. R. Abdreimova, Zh. Obshch. Khim.,
1993, 63, 289 [Russ. J. Gen. Chem., 1993, 63 (Engl. Transl.)].
4. Ya. A. Dorfman, R. R. Abdreimova, and D. N. Akbaeva,
Kinet. Katal., 1995, 36, 103 [Kinet. Catal., 1995, 36 (Engl.
Transl.)].
5. Ya. A. Dorfman, R. R. Abdreimova, and D. M.
Doroshkevich, Teor. Eksp. Khim., 1991, 659 [Theor. Exp.
Chem., 1991 (Engl. Transl.)].
6. Ya. A. Dorfman and R. R. Abdreimova, Koord. Khim., 1996,
22, 764 [Russ. J. Coord. Chem., 1996, 22 (Engl. Transl.)].
7. R. A. Cherkasov, Metalloorg. Khim., 1989, 2, 13 [Organomet.
Chem. USSR, 1989, 2 (Engl. Transl.)].
8. P. M. Bersier, L. Carlsson, and J. Bersier, Top. Curr. Chem.,
1994, 170, 113.
9. E. Leonel, E. Dolem, M. Devaud, P. Paugam, and J.ꢀY.
Nedelec, Electrochim. Acta, 1997, 42, 2125.
10. M. Pilo, G. Manca, M. A. Zoroddu, and R. Sceber, Inorg.
Chim. Acta, 1991, 180, 225.
11. A. Cinquantini, R. Cini, R. Seeber, and P. Zanello,
J. Electroanalyt. Chem., 1981, 121, 301.
12. P. Zanello, P. A. Vigato, and G. A. Mazzocehin, Transition
Met. Chem., 1982, 7, 291.
13. R. Bhalla, M. Hellivell, R. L. Beddoes, D. Collison, and
C. D. Garner, Inorg. Chim. Acta, 1998, 273, 225.
14. A. Domenech, E. GarciaꢀEspana, S. V. Luis, V. Marcellino,
and J. F. Miravet, Inorg. Chim. Acta, 2000, 299, 238.
15. E. Franco, E. LopezꢀTorres, M. A. Mendiola, and T. Sevilla,
Polyhedron, 2000, 19, 441.
16. T. V. Magdesieva, K. P. Butin, and O. A. Reutov, Usp.
Khim., 1988, 57, 1510 [Russ. Chem. Rev., 1988, 57 (Engl.
Transl.)].
17. Yu. G. Budnikova, O. E. Petrukhina, and Yu. M. Kargin,
Zh. Obshch. Khim., 1996, 66, 610 [Russ. J. Gen. Chem., 1996,
66 (Engl. Transl.)].
18. K. P. Butin, V. V. Strelets, and O. A. Reutov, Metalloorg.
Khim., 1990, 3, 814 [Organomet. Chem. USSR, 1990, 3 (Engl.
Transl.)].
19. Y. Okamoto, T. Kusano, and S. Takamuku, Bull. Chem. Soc.
Jpn., 1988, 61, 3359.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos. 01ꢀ03ꢀ33210
and 01ꢀ15ꢀ99353) and the INTAS (Grant 00ꢀ0018).
Received April 22, 2002;
in revised form January 5, 2003