Electroreduction of triphenylphosphine dichloride and the efficient one-pot reductive conversion of phosphine oxide to triphenylphosphine

Article abstract of DOI:10.1016/j.tetlet.2009.11.115

Electroreduction of triphenylphosphine dichloride in acetonitrile was performed successfully in an undivided cell fitted with an aluminium sacrificial anode and a platinum cathode. Further, the one-pot transformation of triphenylphosphine oxide to triphenylphosphine was achieved successfully by the treatment of triphenylphosphine oxide in acetonitrile with oxalyl chloride and subsequent electrochemical reduction.

Full text of DOI:10.1016/j.tetlet.2009.11.115

Tetrahedron Letters 51 (2010) 698–701
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Electroreduction of triphenylphosphine dichloride and the efficient one-pot
reductive conversion of phosphine oxide to triphenylphosphine
Tomotake Yano, Manabu Kuroboshi, Hideo Tanaka *
Department of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Electroreduction of triphenylphosphine dichloride in acetonitrile was performed successfully in an undi-
vided cell fitted with an aluminium sacrificial anode and a platinum cathode. Further, the one-pot trans-
formation of triphenylphosphine oxide to triphenylphosphine was achieved successfully by the
treatment of triphenylphosphine oxide in acetonitrile with oxalyl chloride and subsequent electrochem-
ical reduction.
Received 21 October 2009
Revised 20 November 2009
Accepted 26 November 2009
Available online 29 November 2009
Ó 2009 Elsevier Ltd. All rights reserved.
In modern organic synthesis, triphenylphosphine 1 is an impor-
Electrochemical reduction, which does not require any reduc-
tant and can be carried out under mild conditions, is a highly
promising procedure. Indeed, electroreduction of 2 provides re-
agents-free, cheap, environmentally benign and straightforward
access to 1. Although there have been a few reports on the electro-
reduction of 2, the reductive cleavage of the P–C bond mainly oc-
curred to produce a complex mixture of diphenylphosphine
oxide, phenylphosphine, benzene, 1,3-cyclohexadiene, etc.23 How-
1
tant reagent for various reactions, for example, Wittig reaction,
Mitsunobu reaction and Appel reaction.3 In these reactions, 1 is
converted to triphenylphosphine oxide 2, which is a stable and
flame-resistant chemical; as a result, a significant amount of 2
tends to be stored as troublesome waste. Facile methods involving
reduction of 2 to 1 have been in great demand from the viewpoints
of treatment of the troublesome waste and the use of recycled
phosphine resources (Scheme 1). Indeed, many procedures for
2
22
ever, the bond dissociation energy of the P–Cl bond (H
2
P–Cl,
the reduction of 2 to 1 have been reported thus far. Reduction of
315.1 kJ/mol) is less than that of the P–O bond (H P–OH,
2
4
24
2
with silyl hydrides (e.g., trichlorosilane, triethoxysilane/tita-
359.7 kJ/mol), suggesting that upon electrochemical reduction,
the P–Cl bond is cleaved more easily than the P–O bond to afford
1 efficiently.
5
6
nium tetra-iso-propoxide, and phenylsilane ) and aluminium hy-
7
drides (e.g., lithium aluminium hydride, lithium aluminium
hydride/cerium chloride,8 alane, sodium aluminium hydride/so-
9
In this Letter, we report the electroreduction of 3 as well as the
‘one-pot’ conversion of 2 to 1 through the chlorination of 2 to 3 and
subsequent electrochemical reduction of 3 to yield 1.
Electroreduction of triphenylphosphine dihalides 3, 4 and 5 was
carried out in a divided cell fitted with two Pt plate electrodes
10
dium aluminium tetrachloride and di-iso-butylaluminium hy-
dride ) provides the desired product 1 with good to excellent
yields (Scheme 1, Path A). Hexachlorodisilane, samarium io-
dide/hexamethylphosphoric triamide and titanocene dichloride/
magnesium have also been used for the reduction of 2. However,
the procedures reported thus far are not practical since they al-
ways require stoichiometric amounts of reducing agents that are
expensive and/or difficult to handle.
1
1
12
13
1
4
2
(1.5 Â 1.0 cm each). A typical procedure is as follows. A solution
2
5
of 3 (0.5 mmol) in acetonitrile (10 mL) containing tetrabutylam-
Silyl Hydride
Aluminium Hydride etc.
[Path A]
As an alternative procedure, chlorination of 2 to form triphenyl-
1
5
phosphine dichloride 3 followed by the reduction of 3 to 1 has
been intensively investigated (Scheme 1, Path B). Reduction of 3
to 1 was performed by hydrogenation under high temperature
and/or high pressure in the presence of transition metal catalysts
Wittig reaction
Mitsunobu reaction
Appel reaction etc.
Ph
Ph
Ph
Ph
Ph
Ph
P
P
O
1
6,17
(
Pt, Rh, and Pd).
Reduction of 3 with several metals such as so-
1
8
19
20
21
1
2
Ph Cl
dium, aluminium, silicon and iron has also been reported.
However, the reported procedures are not necessarily satisfactory
in terms of operational simplicity and/or economic efficiency.
Ph
P
H /Pd(Ph P)
etc.
(COCl)2
2
3
4
Ph Cl
[Path B]
3
*
Corresponding author. Tel.:+81 86 251 8072; fax: +81 86 251 8079.
E-mail address: tanaka95@cc.okayama-u.ac.jp (H. Tanaka).
Scheme 1. System for recycling of triphenylphosphine 1 from triphenylphosphine
oxide 2.
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.115

T. Yano et al. / Tetrahedron Letters 51 (2010) 698–701
699
monium tetrafluoroborate (0.05 M) and an acetonitrile (10 mL)
3 4
0 ppm (H PO )
solution of tetrabutylammonium tetrafluoroborate (0.05 M) were
placed in the cathode and the anode chambers, respectively, and
a constant current (50 mA) was supplied. After passage of 2 F/
mol of electricity, the reaction mixture was poured into a mixture
of ice and 5% hydrochloric acid (5 mL), and extracted with ethyl
acetate (10 mL Â 3). The combined extracts were washed with
1
1
5
0
5
0
0
0
0
0
0
0
0
0
0
0
Ph PCl2
3
AlCl
3
(2.0 eq.)
64.9 ppm
p
p
m
(
f
1
)
5
0
0
2
0
0
0
0
0
0
0
1
1
5
0
5
0
0
0
0
0
0
0
0
Ph PCl2
3
6
5
4.6 ppm
8.2 ppm
3
AlCl (1.0 eq.)
brine, dried (Na
2
SO
4
), and concentrated under reduced pressure.
5
0
0
p
p
m
(
f
1
)
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
The residue was chromatographed (silica gel, hexane/ethyl ace-
tate = 3/1) to afford triphenylphosphine 1 (0.01 mmol, 2%) and 2
3
Ph PCl2
AlCl (0.5 eq.)
3
(
3
0.49 mmol, 98%) which would be derived from the hydrolysis of
during electrolysis and/or work-up process (Table 1, entry 1).
5
0
0
p
p
m
(
f
1
)
2
1
1
5
0
0
5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Ph
None AlCl
3 2
PCl
50.1 ppm
3
On the other hand, the electroreduction of triphenylphosphine
dibromide 4 and diiodide 5 was carried out in a similar manner
to provide 1 with a yield of 38% and 40%, respectively (entries 2
and 3). In all cases, no other by-products were detected.
p
p
m
(
f
1
)
5
0
0
Figure 1. 31P NMR spectra of Ph
PCl /AlCl .
3 2 3
The significant difference between the product yield in the elec-
troreduction of 3 and the corresponding yields in the electroreduc-
tion of 4 and 5 can be explained as follows: Triphenylphosphine
dihalides exist in five-coordinate and four-coordinate forms
3
(3.0 mmol)
AlCl3
3
1
(3.0 mmol)
(Scheme 2). A P NMR study of triphenylphosphine dihalides re-
AlCl4
Cl
Bu NBF (0.05 M)
4 4
vealed that 5 ionizes completely to form iodotriphenylphosphoni-
um iodide in a polar organic solvent, while 3 partially ionizes to
form an equilibrium mixture of 3 and chlorotriphenylphosphoni-
Ph
MeCN (10 mL)
Ph
Ph
P
1 (37%) + 2 (47%)
(
Pt)-(Pt), Divided Cell
5
0 mA, 2 F/mol
2
6
um chloride.
In this regard, Godfrey and co-workers reported the X-ray crys-
tal structure of triphenylphosphine dihalides.2 4 and 5 are ionic
four-coordinate phosphonium salts, and 3 is a five-coordinate
phosphorus compound, which has a trigonal bipyramidal struc-
ture. The electron density of the central phosphorus atom is lower
in the four-coordinate ionic structure than that in the five-coordi-
nate structure. Therefore, it is reasonable to assume that the ionic
forms of 5 and probably 4 are reduced more easily than the non-io-
nic form of 3.
3 2 3
Scheme 3. Electroreduction of Ph PCl in the presence of AlCl .
7
Ph PCl
3
2
3
+
-
e-
e-
3 4
[Ph PCl ][AlCl ]
We assumed that the addition of a Lewis acid would facilitate the
AlCl3
ionization of 3 to form an ionic four-coordinate phosphonium salt.
-
Pt
Indeed, as shown in the 31P NMR spectrum of 3, a low-field shift
Al
Sacrificial Anode
AlCl3 + 2 Cl + PPh
3
1
was observed by the addition of aluminium chloride to the solution
of 3 in acetonitrile (Fig. 1). Without aluminium chloride, 3 showed a
singlepeakat 50.1 ppm downfieldfromthephosphoricacidexternal
standard. The peak shifted to 58.2 and 64.6 ppm when 0.5 and 1
equivalents of aluminium chloride were added, respectively, and
further addition of aluminium chloride did not result in a more
appreciable low-field shift. These results suggest that the electron
density on the central phosphorus atom of 3 decreased significantly
because of the formation of a 1-to-1 four-coordinate complex such
Figure 2. Sacrificial anode: a source of Lewis acid.
As aluminium chloride was generated during in situ electrolysis
from the aluminium sacrificial anode, we next examined the elec-
troreduction of 3 to 1 in an undivided cell without the addition of
aluminium chloride (Fig. 2).
In fact, the electroreduction of 3 in an undivided cell fitted with
a sacrificial anode was more efficiently and conveniently per-
formed. The electrolysis was carried out in a beaker-type undi-
+
À
as [Ph
3
PCl] [AlCl
4
]
6, which is expected to be reduced more easily
than the five-coordinate form of 3 (Scheme 3).
The above-mentioned observation prompted us to investigate
the electroreduction of 3 in the presence of aluminium chloride
2
vided cell fitted with an aluminium plate anode (1.5 Â 1.0 cm )
2
(
Scheme 3). A mixture of 3 and aluminium chloride (1 mol equiv)
in acetonitrile containing tetrabutylammonium tetrafluoroborate
0.05 M) was placed in the cathode compartment of a divided cell
and a platinum plate cathode (1.5 Â 1.0 cm ). A solution of 3
(0.5 mmol) in acetonitrile (10 mL) containing tetrabutylammo-
nium tetrafluoroborate (0.05 M) was placed in an undivided cell,
and a constant current (50 mA) was supplied. After the passage
of 2 F/mol of electricity and work-up procedure, 1 was obtained
with a 34% yield and 2 with a 46% yield (Table 2, entry 1).
(
and a constant current (50 mA, 2 F/mol) was supplied. As expected,
the yield of 1 significantly increased up to 37%.
Ph
Ph
Ph
I
I
Ph
Ph
Ph
Ph
Ph
When 3 F/mol of electricity was passed, the yield of 1 increased
to 46% (entry 2). Notably, when electrolysis was carried out by a
P
P
I
I
P
I
I
Ph
similar procedure using a magnesium rod or a zinc plate
2
5
(1.5 Â 1.0 cm ) as a sacrificial anode, the yields of 1 decreased to
2+ 2+
Cl
7–10% (entries 3 and 4). The Mg and Zn salts, generated
in situ from the sacrificial anodes, were rather weak Lewis acids
and did not efficiently form the four-coordinate phosphonium salts
Ph Cl
Ph
Ph
P
Ph Cl
3
Ph
Ph
P
Cl
+
À
+
À
3
(
[Ph PCl] [MgCl ] and [Ph PCl] [ZnCl ] ). In a similar manner, the
3 3 3
electroreduction of 4 and 5 proceeded smoothly to provide 1 with a
Scheme 2. Dissociation equilibrium of triphenylphosphine dihalide.
yield of 45% and 61%, respectively.

7
00
T. Yano et al. / Tetrahedron Letters 51 (2010) 698–701
The yield of 1 was significantly improved by the increase in the
Table 3
a
Effect of concentration of 3 and electrolyte
concentration of 3 (Table 3, entry 2). When the concentration of 3
increased to 1 M (a 20-fold increase over that in previous entries in
Tables 1 and 2), 1 was obtained with a yield of 68%. The effect of
the supporting electrolyte was not significant. Tetrabutylammo-
nium triflate and bromide could be used without appreciable
change in the yield of the product (entries 3 and 4).
Finally, we examined the one-pot conversion of 2 to 1 through
the chlorination of 2 with oxalyl chloride and the subsequent elec-
trochemical reduction. A typical procedure is as follows. To a solu-
tion of 2 (5.0 mmol) and tetrabutylammonium triflate (200 mg,
Entry
3
Electrolyte
Yield of 1 b (%)
Recovery of 2b (%)
(
mmol)
(M)
1
0.5
0.05
Bu
4
NBF
4
4
46
33
2
3
4
5.0
5.0
5.0
1.00
1.00
1.00
Bu
Bu
Bu
4
4
4
NBF
68
68
62
11
7
16
NOTf
NBr
a
2
2
Pt: 1.5 Â 1.0 cm , 33.3 mA/cm .
b
Yields of the isolated compounds.
0
.5 mmol) in acetonitrile (5 mL) was added oxalyl chloride
(
0.43 mL, 1 equiv) at room temperature, and the mixture was stir-
red for a few minutes. An aluminium anode and a platinum cath-
ode were immersed in the mixture, and a constant current
Table 4
3 3
One-pot conversion of Ph PO 2 to Ph P 1
(
50 mA) was supplied. After the passage of 3 F/mol of electricity
and usual work-up, triphenylphosphine 1 (3.7 mmol, 74%) and 2
0.3 mmol, 6%) were obtained, respectively (Table 4, entry 1). In
(
COCl)2 (5.0 mmol)
Ph Cl
2
MeCN (5 mL)
Electrolyte (0.10 M)
1
(
5.0 mmol)
Ph
P
(
(Al)-(Pt)
Ph Cl
place of tetrabutylammonium triflate, aluminium chloride and
bromide were used efficiently as a supporting electrolyte to pro-
duce 1 with a yield of 74% and 84%, respectively (entries 2 and
Undivided Cell
50 mA, 3 F/mol
3
Entry
1
Electrolyte
Bu NOTf
AlCl
AlBr
None
Yield of 1a (%)
Recovery of 2a (%)
3
). It is interesting to note that even without the addition of the
4
74
6
supporting electrolytes, electroreduction proceeded smoothly to
give 1 with an overall yield of 73% (entry 4).
It is likely that in the initial stage of electrolysis, chloride con-
taminants, for example, hydrogen chloride and aluminium chlo-
2
3
3
74
12
84 (72)b
12 (8)b
3
4
73
14
a
Determined by GC.
Yields of the isolated compounds.
b
Table 1
Electroreduction of triphenylphosphine dihalides 3–5 to triphenylphosphine 1 in a
divided cella
Bu NBF (0.05 M)
ride, derived from oxalyl chloride and/or a small amount of the
ionic form of 3, acted as a supporting electrolyte, and aluminium
salts generated in situ from the sacrificial anode then worked as
the supporting electrolyte. These results demonstrate the feasibil-
ity of the one-pot procedure for the conversion of 1 to 2; thereby,
offering a practical recycling system for 1.
4
4
Ph
Ph
Ph
Ph
Ph
MeCN (10 mL)
Ph
0.5 mmol)
X = Cl : 3
3
PX
2
P
+
P
O
(
Pt)-(Pt), Divided Cell
(
Ph
50 mA, 2 F/mol
room temp.
1
2
=
=
Br : 4
I : 5
In conclusion, the electrochemical reduction of 3 was per-
formed efficiently in an undivided cell fitted with an aluminium
sacrificial anode and a platinum cathode, wherein aluminium chlo-
ride generated in situ from the sacrificial anode reacted with 3 and
Entry
Ph
3
PX
2
Yield of 1b (%)
Recovery of 2b (%)
x
1
2
3
3
4
5
Cl
Br
I
2
38
40
98
62
60
+
À
the thus formed four-coordinate ionic species ([Ph
3 4
PCl] [AlCl ] )
underwent two-electron reduction at the cathode to provide 1 in
moderate yields. The one-pot conversion of 2 to 1 through 3 was
achieved successfully by the treatment of 2 with oxalyl chloride
in acetonitrile and the subsequent electrochemical reduction of
the mixture with an aluminium anode and a platinum cathode.
a
2
2
Pt: 1.5 Â 1.0 cm , 33 mA/cm .
b
Determined by GC.
Table 2
a
Electroreduction of triphenylphosphine dihalides in an undivided cell
References and notes
Bu NBF (0.05 M)
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4
4
Ph
Ph
Ph
X
Ph
Ph
Ph
Ph
Ph
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