Electroreduction of tetra-coordinate phosphonium derivatives; One-pot transformation of triphenylphosphine oxide into triphenylphosphine

Article abstract of DOI:10.1016/j.tet.2011.05.044

Electroreduction of triphenylphosphine dichloride in acetonitrile was performed successfully in an undivided cell fitted with an aluminium sacrificial anode and a platinum cathode, wherein Al³⁺, which was electrogenerated at the anode would react as a Lewis acid with triphenylphosphine dichloride to afford tetra-coordinate chlorotriphenylphosphonium species and subsequent two-electron reduction at the cathode would give triphenylphosphine. One-pot transformation of triphenylphosphine oxide to triphenylphosphine was achieved successfully by the treatment of triphenylphosphine oxide with oxalyl chloride and subsequent electroreduction. In a similar manner, some tetra-coordinate triphenylphosphonium species derived from triphenylphosphine oxide were reduced electrochemically to triphenylphosphine in moderate yields.

Full text of DOI:10.1016/j.tet.2011.05.044

Tetrahedron 67 (2011) 5825e5831
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Electroreduction of tetra-coordinate phosphonium derivatives; one-pot
transformation of triphenylphosphine oxide into triphenylphosphine
,
a,
*
Manabu Kuroboshi a, Tomotake Yano a, Shogo Kamenoue a, Hiromu Kawakubo b , Hideo Tanaka
*
a Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
b API Department, Asahi Kasei Chemicals Corporation, 1-105 Kanda-jinbocho, Chiyoda-ku, Tokyo 101-8101, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2011
Received in revised form 10 May 2011
Accepted 10 May 2011
Available online 17 May 2011
Electroreduction of triphenylphosphine dichloride in acetonitrile was performed successfully in an un-
divided cell fitted with an aluminium sacrificial anode and a platinum cathode, wherein Al3þ, which was
electrogenerated at the anode would react as a Lewis acid with triphenylphosphine dichloride to afford
tetra-coordinate chlorotriphenylphosphonium species and subsequent two-electron reduction at the
cathode would give triphenylphosphine. One-pot transformation of triphenylphosphine oxide to tri-
phenylphosphine was achieved successfully by the treatment of triphenylphosphine oxide with oxalyl
chloride and subsequent electroreduction. In a similar manner, some tetra-coordinate triphenylphos-
phonium species derived from triphenylphosphine oxide were reduced electrochemically to triphenyl-
phosphine in moderate yields.
Keywords:
Electroreduction
Triphenylphosphine oxide
Triphenylphosphine dichloride
Triphenylphosphine
Ó 2011 Elsevier Ltd. All rights reserved.
Tetra-coordinate phosphonium
Wittig reaction,
1. Introduction
Mitsunobu reaction,
In modern organic synthesis, triphenylphosphine 1 is an im-
portant reagent for various reactions, e.g., Wittig reaction,1 Mitsu-
nobu reaction,2 MukaiyamaeCorey lactonization,3 Appel reaction4
and Staudinger reaction.5 In these reactions, 1 is turned into tri-
phenylphosphine oxide 2, which is a stable and flame-resistant
chemical. As a result, a significant amount of 2 has been stored as
a troublesome waste. Facile methods for reducing 2 to 1 have been
in great demand from a viewpoint of treatment of the troublesome
waste and recycling of phosphine resources6,7 (Scheme 1). Indeed,
many procedures for the reduction of 2 to 1 have been reported
thus far. Direct reduction of 2 to 1 was performed with silyl
hydrides (HSiCl3,8 HSi(OMe)3/Ti(O-i-Pr)4,9 PhSiH3,10 etc.) and alu-
minium hydrides (LiAlH4,11 LiAlH4/CeCl3,12 AlH3,13 NaAlH4/
NaAlCl4,14 HAl(i-Bu)2,15 etc.) (Scheme 1, path A). Low-valent metals
(Si2Cl6,16 SmI2/HMPA,17 Cp2TiCl2/Mg,18 etc.) and non-metallic or-
ganic reagents (hydrocarbon/activated carbon,19 (Et2N)3P/POCl3,20
etc.) have also been used for the reduction of 2 to 1. However, the
procedures reported thus far are not practical since they always
require stoichiometric amounts of the reductants that are expen-
sive, explosive, not easy to handle and form a significant amount of
wastes.
etc.
[Path A]
Ph3P
Ph3P=O
2
Silyl Hydride,
Aluminium Hydride, etc.
1
(COCl)2
etc.
H2/Pd(Ph3P)4,
etc.
[Path B]
Ph3PX2
3
Scheme 1. Recycling use of triphenylphosphine 1.
Electrochemical reactions, which do not require any reductants/
oxidants and can be carried out under mild conditions, are highly
promising candidate21 for reagent-free, environmentally benign
and straightforward access, and several electrochemical processes
have been used successfully in industry. Upon electrochemical re-
duction of 2, however, the reductive cleavage of the PeC bond
mainly occurred to produce a complex mixture of diphenylphos-
phine oxide, diphenylphosphine, phenylphosphine oxide, benzene,
1,3-cyclohexadiene, cyclohexene, etc., and no appreciable amount
of 1 was obtained (Scheme 2).22 It is likely that the P]O bond of 2
(Cl3P]O 510 kJ/mol)23 is so strong to cleave that, accordingly, PeC
bond fission occurred predominantly to give diphenylphosphine
oxide, benzene, etc.
* Corresponding authors. Tel./fax: þ81 86 251 8079; e-mail address: tanaka95@
cc.okayama-u.ac.jp (H. Tanaka).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.05.044

5826
M. Kuroboshi et al. / Tetrahedron 67 (2011) 5825e5831
electroreduction of 2 would not occur smoothly. On the other hand,
t
BuOH, [Bu3NMe]+OMs–
Ethylenediamine
Ph
P
Ph
P
an irreversible reduction wave is observed at ꢀ0.8 V in cyclic vol-
tammogram of triphenylphosphine dibromide (3b), suggesting that
two-electron reduction of 3b followed by PeBr bond fission would
occur to produce 1 (Fig.1c). These results prompted us to investigate
electroreduction of Ph3PX2 3 (X¼Cl (3a), Br (3b) and I (3c)).
Electroreduction of 3bwas carried out in a divided cell fitted with
a platinum plate anode and cathode (1.5ꢁ1.0 cm2 each) (Table 1,
entry 1). A solution of 3b (0.5 mmol) in MeCN (10 mL) containing
tetrabutylammonium tetrafluoroborate (Bu4NBF4) (0.05 M) and an
MeCN (10 mL) solution of Bu4NBF4 (0.05 M) were placed in the ca-
thodic and the anodic chambers, respectively. A constant current
(50 mA) was supplied until 2 F/mol-3b of electricity was passed. Gas
chromatography (GC) analysis of the reaction mixture in the ca-
thodic chamber showed the formation of triphenylphosphine 1
(38%) together with triphenylphosphine oxide 2 (62%), which would
be formed by hydrolysis of unreacted 3b during the course of elec-
troreduction and/or work-up process. Among thus far examined
solvents, MeCN was the only solvent effective for the present pur-
pose; thus, electroreduction of 3b to 1 hardly occurred in tetrahy-
drofuran (THF), dimethyl sulfoxide (DMSO), N,N-dimethyl
formamide (DMF) and 1,4-dioxane, resulting in the exclusive for-
mation of 2.
Ph
H
O
Ph
(GC)–(GC), Undivided Cell
CPE (50 V), 4 F/mol
Ph
2
Ph
1 (not detected)
Ph
Ph
P
Ph
P
+
+
+
benzene (25%)
P
O
+
H
H
O
+
Ph
(2%)
Ph
H
cyclohexadiene (35%)
cyclohexene (1%)
(4%)
(8%)
Scheme 2. Electroreduction of 2.22
Triphenylphosphine dichloride (Ph3PCl2, 3a) is easily prepared
by treatment of 2 with chlorinating reagents such as COCl2 and
(COCl)2.11,24,25 Compared with reduction of 2, the reduction of 3a to
cleave PeCl bond is expected to proceed more smoothly than that
of 2 to cleave P]O bond since PeCl bond is far weaker (H2PeCl,
315.1 kJ/mol)26 than P]O bond (Scheme 1, path B). Therefore, re-
duction of 3a has been intensively investigated as an alternative
access to 1. Indeed, reduction of 3a to 1 proceeded by hydrogena-
tion under high temperature and/or high pressure in the presence
of transition metal catalysts (Pt, Rh and Pd).27,28 Reduction of 3a
with several metals such as sodium,29 aluminium,30 silicon31 and
iron32 has also been reported. However, the reported procedures
are not necessarily satisfactory since each process requires tedious
pre-treatment of the metals and/or special care of the explosive
nature of the activated metals or hydrogen under vigorous
conditions.
Recently, we found that electrochemical reduction of triphe-
nylphosphine dihalides 3 (Ph3PX2, X¼Cl (3a), Br (3b) and I (3c))
proceeded smoothly to give 1 in an undivided cell fitted with an
aluminium sacrificial anode. We also developed one-pot trans-
formation of triphenylphosphine oxide 2 to 1 through 3.33,34 During
the study, we found that tetra-coordinate phosphonium species
[Ph3PeX]þ 4 would be a reactive intermediate of this reduction.
Herein, we describe the details on the electroreduction of 3 as well
as the ‘one-pot’ conversion of 2 to 1 and further applications to the
electroreduction of some tetra-coordinate phosphonium com-
pounds 5e7 derived from 2 in situ (Scheme 3).
Table 1
Electroreduction of Ph3PX2 in a divided cell
Bu4NBF4 (0.05 M)
MeCN (10 mL)
+
Ph3P
Ph3P=O
Ph3PX2
3
(Pt)-(Pt), Divided Cell
CCE, 50 mA, 2 F/mol
1
2
(0.5 mmol)
Entry
Ph3PX2
Yielda/%
X
1
2
1
2
3
3b
3c
3a
Br
I
Cl
38
40
2
62
60
98
a
Determined by GC.
It is of interest to note that electroreduction of 2 under similar
conditions gave no detectable amount of 1, resulting in the recovery
of most of 2. Electroreduction of triphenylphosphine diiodide (3c)
was similarly performed to give 1 and 2 in 40% and 60% yields,
respectively (entry 2), whereas electroreduction of Ph3PCl2 3a
hardly proceeded to give 1 in only 2% yield together with 2 (98%
yield) (entry 3). In all entries in Table 1, other by-products such as
diphenylphosphine oxide and diphenylphosphine were not detec-
ted, which were derived from the reductive CeP bond fission of 3.
The significant difference between the electroreduction of 3a
and that of 3b and 3c can be reasonably explained by assuming
that Ph3PX2 3 exists in penta-coordinate form 3-pen and/or
tetra-coordinate form 3-tet (Scheme 4).35 31P NMR studies of 3
revealed that 3c dissociated completely in a polar organic solvent
such as MeCN to form tetra-coordinate iodotriphenylphosphonium
iodide 3c-tet, whereas penta-coordinate 3a scarcely ionizes to form
an equilibrium mixture of 3a-pen and 3a-tet favouring 3a-pen. In
this regard, Godfrey and co-workers reported the X-ray crystal
structures of 3,36e38 showing that compounds 3b and 3c are ionic
tetra-coordinate tetrahedral phosphonium salts, whereas 3a is
a penta-coordinate trigonal bipyramidal phosphorus compound.
Above all, we assumed that the cationic forms (3b-tet and 3c-
tet) are reduced more easily than the neutral form (3a-pen). This
assumption, in turn, spurred us to investigate the formation of
tetra-coordinate phosphonium salt 4a from 3a by the aid of alu-
minium chloride (AlCl3) as a Lewis acid (Scheme 5).
[Ph3P–Cl]+[PO2Cl2]–
5
[Ph3P–OMe]+OTf – [Ph3P–OSiMe3]+OTf–
6
7
Scheme 3. Tetra-coordinate phosphonium salts 5e7.
2. Results and discussion
Cyclic voltammogram of triphenylphosphine oxide 2 in aceto-
nitrile (MeCN) (Fig. 1b) shows no detectable waves between ꢀ0.2
to ꢀ2.5 V versus Ag/Agþ (supporting electrolyte: 0.1 M of tetra-
butylammonium triflate (Bu4NOTf)), indicating that direct
Fig. 1. Cyclic voltammograms of Ph3P]O 2 and Ph3PBr2 3b. (a) Background, (b) 2
(10 mM), (c) 3b (10 mM) in Bu4NOTf (0.1 M)/MeCN; working electrode: Pt, counter
electrode: Pt wire, reference electrode: Ag/Agþ; scan rate: 300 mV/s.

M. Kuroboshi et al. / Tetrahedron 67 (2011) 5825e5831
5827
with AlCl3 (a hard acid), which would associate more efficiently
with 3a (a hard base) than with 1 (a soft base), as the results,
generating 4 again in situ.
Ph
Ph
Ph
Ph
Ph
Ph
X
X
P
X X
P
3-pen
3-tet
Ph Cl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
P
P
I I
P
Br Br
Ph Cl
3a-pen
Ph3PCl2
3a
tetra−coordinate
3b-tet
3c-tet
phosphonium
[Ph3PCl+][AlCl4
4a
penta–coordinate species
tetra–coordinate species
−
e−
Pt
e−
]
Scheme 4. Dissociation equilibrium of Ph3PX2 3ae3c.
AlCl3
AlCl3 + 2 Cl− + PPh3
1
Al
Ph Cl
Ph
Ph Cl
3a-pen
Ph
Ph
Ph
Sacrificial Anode
Anode: 2/3 Al
AlCl3
P
P
Cl Cl AlCl3
2/3 AlCl3 +
2 e-
[Ph3PCl]+[Cl−AlCl3]−
4a
+
+
2 Cl-
4a
Ph3PCl2
3a
AlCl3
Scheme 5. Tetra-coordinate complex 4a derived from 3a.
2 e-
PPh3 + AlCl3 +
2 Cl−
1
Cathode:
4a
+
The formation of ionic tetra-coordinate chlorotriphenyl-
phosphonium salt 4a was supported by 31P NMR spectra of 3a in
the absence and in the presence of AlCl3 (Fig. 2). In 31P NMR
spectrum of 3a alone in MeCN, a single peak appeared at 50.1 ppm
1
+ 2/3 AlCl3
Total: 2/3 Al + 3a
Fig. 3. Sacrificial anode: a source of Lewis acid.
(external standard H3PO4,
d¼0) (d). Significant low-field shift to
58.2 (Fig. 2c) and 64.6 ppm (Fig. 2b) was observed when 0.5 and
1 mol equiv of AlCl3 was added, respectively, and further addition of
AlCl3 did not result in appreciable low-field shift any more (Fig. 2a).
These results suggest that the electron density on the central
phosphorus atom of 3a decreased significantly by the formation of
a one-to-one tetra-coordinate complex 4a; thereby reducing the
reduction potential significantly.
In fact, the electroreduction of 3a proceeded without addition of
AlCl3; thus, a solution of 3a in MeCN containing Bu4NBF4 (0.05 M)
was electrolyzed in a beaker-type undivided cell fitted with an Al
plate anode (1.5ꢁ1.0 cm2) and a Pt plate cathode (1.5ꢁ1.0 cm2).
Passage of 2 F/mol-3a of electricity (50 mA) gave 1 and 2 in 34% and
46% yields, respectively (Table 2, entry 1). When 3 F/mol-3a of
electricity was passed, the yield of 1 increased to 46% (entry 2).
Notably, when the electrolysis was carried out by use of a magne-
sium rod (6 mm
f
) or a zinc plate (1.5ꢁ1.0 cm2) as a sacrificial anode,
0 ppm (85% H3PO4)
the yield of 1 decreased to 7e10% (entries 3 and 4). The Mg2þ
and Zn2þ salts, generated in situ from the sacrificial anodes,
are rather weak Lewis acids and would not efficiently form the tetra-
coordinate chlorotriphenylphosphonium salts ([Ph3PeCl]þ[MgCl3]ꢀ
and [Ph3PeCl]þ[ZnCl3]ꢀ). In a similar manner, the electroreduction
of 3b and 3c proceeded smoothly to provide 1 in 45% and 61% yields,
respectively.
15000
10000
(a)
64.9
50000
0
20000
(b)
(c)
(d)
15000
10000
50000
0
64.6
40000
30000
20000
10000
0
20000
15000
10000
50000
0
58.2
Table 2
50.1
Electroreduction of Ph3PX2 3 in an undivided cell using sacrificial anode
Bu4NBF4 (0.05 M)
MeCN (10 mL)
Ph3PX2
Ph3P
+
Ph3P=O
Fig. 2. 31P NMR spectra of Ph3PCl2 3a/AlCl3. (a) 3aþAlCl3 2 equiv, (b) 3aþAlCl3 1 equiv,
(c) 3aþAlCl3 0.5 equiv, (d) 3a.
a
−
(Anode) (Pt) , Undivided Cell
CCE, 50 mA, Q F/mol
1
2
3 (0.5 mmol)
Entry
Ph3PX2
Anode
Q (F/mol-3)
Yieldb/%
Electroreduction of 3a in the presence of AlCl3 was performed in
a divided cell in a manner similar to that described above. A mix-
ture of 3a and AlCl3 (1 mol equiv) in MeCN containing Bu4NBF4
(0.05 M) was electrolyzed under a constant current condition
(50 mA). Electricity (2 F/mol-3a) was passed to give 1 in 37% yield
together with 2 (47%), suggesting that electroreduction of 3a pro-
ceeded smoothly through the in situ generated tetra-coordinate
chlorotriphenylphosphonium species 4a.
Electroreduction of 3a through 4a was performed more effi-
ciently and conveniently in an undivided cell fitted with an Al
sacrificial anode, wherein the anodic reaction would supply the
required Al salts (Fig. 3) and in turn give tetra-coordinate phos-
phonium 4a by the reaction with 3a. Two-electron reduction at the
cathode of thus formed cation species 4a would give 1 together
X
1
2
1
2
3
4
5
6
3a
3a
3a
3a
3b
3c
Cl
Cl
Cl
Cl
Br
I
Alc
2
3
2
2
2
2
34
46
7
10
45
61
46
33
55
88
39
20
Alc
Mgd
Znc
Alc
Alc
a
Pt: 1.5ꢁ1.0 cm2, 33.3 mA/cm2.
Yields of the isolated products.
Plate (1.5ꢁ1.0 cm2).
b
c
d
Rod (6 mmf).
The yield of 1 was significantly improved by the increase of the
concentration of 3a (Table 3, entry 2). When the concentration of 3a

5828
M. Kuroboshi et al. / Tetrahedron 67 (2011) 5825e5831
Table 5
increased from 0.05 M to 1 M, 1 was obtained in 68% yield. In place
of Bu4NBF4, Bu4NOTf and tetrabutylammonium bromide (Bu4NBr)
were used as supporting electrolytes without appreciable change in
the yield of 1 (entries 3 and 4).
Electroreduction of triarylphosphine oxides
(COCl)2 (5 mmol)
Bu4NOTf (0.5 mmol)
(Al)-(Pt), Undivided Cell
Current, 3 F/mol
Ar3P=O
[Ar3PCl2]
Ar3P
MeCN (5 mL)
3
1
2b (Ar= p-tolyl)
Table 3
Effect of concentration of 3a and supporting electrolytea
2c (Ar = m-tolyl)
(5 mmol)
Entry 3a (mmol) MeCN (mL) Concn
Supporting
3a (mol/L) electrolyte (0.1 M)
Yieldb/%
1
2
Entry
Ar
Current/mA
Yielda/%
1
2
3
4
0.5
5.0
5.0
5.0
10
5
5
0.05
1.0
1.0
Bu4NBF4
Bu4NBF4
Bu4NOTf
Bu4NBr
46
68
68
62
33
11
7
1
1
2
3
4
p-Tolyl (2b)
p-Tolyl (2b)
m-Tolyl (2c)
m-Tolyl (2c)
50
200
50
1b, 60
1b, 59
1c, 76
1c, 72
5
1.0
16
a
(Al)-(Pt): 1.5ꢁ1.0 cm2, undivided cell, 50 mA, 3 F/mol.
200
b
Yields of the isolated compounds.
a
Yields of the isolated compounds.
Since Ph3PX2 3 is highly moisture sensitive, and is easily hydro-
lyzed to give 2, we next examined the one-pot conversion of 2 to 1 in
which chlorination of 2 to 3a with oxalyl chloride in situ and sub-
sequent electrochemical reduction of 3a were performed. A typical
procedure is as follows. To a solution of 2 (5 mmol) and Bu4NOTf
(0.5 mmol) in MeCN (5 mL) was added oxalyl chloride (0.43 mL,
5 mmol) at ambient temperature, and the mixture was stirred for
10 min. An Al anode (1.5ꢁ1.0 cm2) and a Pt cathode (1.5ꢁ1.0 cm2)
were immersed in the mixture, and a constant current (50 mA) was
supplied until 3 F/mol-2 of electricity was passed. Usual work-up
gave 1 (3.7 mmol, 74%) and 2 (0.3 mmol, 6%) (Table 4, entry 3).
Current did not affect on the yields of 2 (entries 1e3). Ph3P 1 was
obtained from Ph3P]O 2 in 74, 72 and 82% yields when the current
was set at 50, 100 and 200 mA, respectively. In place of Bu4NOTf,
AlCl3 and AlBr3 were used efficiently as supporting electrolytes to
produce 1 in 74% and 84% yields, respectively (entries 4 and 5). It is
interesting to note that even without addition of the supporting
electrolytes, the electroreduction proceeded smoothly to give 1 in
73% yield (entry 6). In the initial stage of the electrolysis, chloride
contaminants, e.g., hydrogen chloride and aluminium chloride,
which were derived from oxalyl chloride and/or a small amount of
the ionic form of 3a, would act as a supporting electrolyte, and then
aluminium salts generated in situ from the sacrificial anode would
work as a supporting electrolyte and Lewis acid. These results
demonstrated the feasibility of the one-pot procedure for the con-
version of 2 to 1; thereby, offering a practical recycling system for 1.
Electroreduction of 5e7 was also investigated. To an MeCN
(2 mL) suspension of 2 (2.0 mmol) was added POCl3 (2.0 mmol),
and the mixture was stirred at room temperature for 1 h to give
a clear solution of [Ph3PCl]þ[PO2Cl2]ꢀ 5 (Scheme 6).39 To the solu-
tion were immersed an Al anode (1.0ꢁ1.5 cm2) and a Pt cathode
(1.0ꢁ1.5 cm2), and the mixture was electrolyzed under constant
current conditions (50 mA, 93 min, 3 F/mol-2) to give 1 in 62%
yield.
O
POCl3 (2.0 mmol)
MeCN (2 mL)
Ph
Ph
Ph
Ph3P=O
P
Cl Cl
P
Cl
2 (2 mmol) room temp., 1 h
O
5
(Al) - (Pt)
Ph3P
1 (62%)
+
2
Undivided cell
3 F/mol, 50 mA
room temp.
(38%)
Scheme 6. Synthesis and electroreduction of [Ph3PCl]þ[Cl2PO2]ꢀ 5.
Methoxytriphenylphosphonium triflate [Ph3PeOMe]þ[OTf]ꢀ
6
was generated from 2 by treatment with methyl triflate (MeOTf)
(Scheme 7). A mixture of MeCN (2 mL), 2 (2 mmol) and MeOTf
(2 mmol) was stirred at room temperature for 1 h. 31P NMR of the
Table 4
mixture showed a single peak of 6 (31P NMR:
d 66.2 ppm, 100%),
One-pot conversion of 2 to 1 via 3a
suggesting that 6 was obtained as a tetra-coordinate phosphonium
species. To the mixture were immersed an Al anode (1.0ꢁ1.5 cm2)
and a Pt cathode (1.0ꢁ1.5 cm2), and the mixture was electrolyzed
(50 mA, 93 min, 3 F/mol-2) to give 1 and 2 in 11% and 77% yields,
respectively.40
(COCl)2 (5 mmol)
Electrolyte (0.5 mmol)
Ph3P=O
2
[Ph3PCl2]
3a
Ph3P
1
MeCN (5 mL)
room temp.
10 min.
−
(Al) (Pt)
Undivided Cell
(5 mmol)
Current, 3 F/mol
Entry
Electrolyte
Current/mA
Yielda/%
MeOTf (2.0 mmol)
1
Ph
Ph3P=O
MeCN (2 mL)
room temp.,1 h
Ph
P
O
Me
1
2
3
4
5
6
Bu4NOTf
Bu4NOTf
Bu4NOTf
AlCl3
AlBr3
None
200
100
50
50
50
82
72
74
OTf
Ph
2 (2.0 mmol)
6 (66.2 ppm)
74
(Al) - (Pt)
84 (72)b
73
Ph3P
1 (11%)
+
2
50
Undivided cell
3 F/mol, 50 mA
room temp.
(77%)
a
Determined by GC.
Yields of the isolated 1.
b
Scheme 7. Synthesis and electroreduction of [Ph3PeOMe]þ[OTf]ꢀ 6.
Electroreduction of other triarylphosphine oxides such as tri(p-
tolyl)phosphine oxide (Ar¼p-MeC6H4) 2b and tri(m-tolyl)phosphine
oxide (Ar¼m-MeC6H4) 2c proceeded smoothly in a similar manneras
described above to give tri(p-tolyl)phosphine 1b and tri(m-tolyl)
phosphine 1c in 60% and 76% yields, respectively (Table 5).
[Ph3PeOeSiMe3]þ[OTf]ꢀ 741 was prepared by treatment of 2
with Me3SiOTf. A mixture of MeCN (2 mL), 2 (2.0 mmol) and
Me3SiOTf (2.0 mmol) was stirred at room temperature for 1 h to

M. Kuroboshi et al. / Tetrahedron 67 (2011) 5825e5831
5829
give a solution of 7 whose 31P NMR showed a single peak at
d
solvents under argon atmosphere. MeCN and DMF were distilled
from CaH2. THF and 1,4-dioxane were distilled from sodium ben-
zophenone ketyl under argon atmosphere.
52.1 ppm. The mixture was electrolyzed (Al anode (1.0ꢁ1.5 cm2),
Pt cathode (1.0ꢁ1.5 cm2), 50 mA, 93 min, 3 F/mol-2) to give 1 and 2
in 35% and 47% yields, respectively (Table 6, entry 1). By increasing
of Me3SiOTf to 6 mmol, yield of 1 increased up to 65% (entry 3).
In conclusion, the electrochemical reduction of triphenylphos-
phine dihalides 3 was performed efficiently in an undivided cell
fitted with an aluminium sacrificial anode and a platinum cathode,
wherein in situ generated AlCl3 interacted with 3 and thus formed
tetra-coordinate ionic species ([Ph3PCl]þ[AlCl4]ꢀ) suffered two-
electron reduction at the cathode to provide 1 in good yields. The
one-pot conversion of 2 to 1 through 3a (Ph3PCl2) was achieved
successfully by the treatment of 2 with oxalyl chloride in MeCN and
the subsequent electrochemical reduction of the mixture with an Al
anode and a Pt cathode.
3.2. Electroreduction of Ph3PCl2 3a in a divided cell (Table 1,
entry 3)
In the cathodic chamber of an H-type divided cell equipped with
a stirring bar was placed an MeCN (10 mL) solution of Ph3PCl2 (3a,
167 mg, 0.5 mmol) and Bu4NBF4 (165 mg, 0.5 mmol), and in the
anodic chamber was placed an MeCN (10 mL) solution of Bu4NBF4
(165 mg, 0.5 mmol). In each chamber, Pt plate electrodes
(1.5ꢁ1.0 cm2) were immersed. The solutions were electrolyzed at
room temperature under constant current conditions (50 mA,
33 mA/cm2) until 2.0 F/mol-3a of electricity was passed (32 min).
The mixture was poured into 5% aq HCl (5 mL) and extracted with
ethyl acetate (AcOEt) (10 mLꢁ3). The combined extracts were
washed with satd aq NaHCO3 and brine, successively, dried
(Na2SO4) and concentrated under reduced pressure. The residue
was diluted with acetone (10 mL) containing 1,4-di-iso-pro-
pylbenzene (92 mg, 0.6 mmol) as an internal standard and ana-
lyzed by GC, showing that Ph3P (1, 2%) and Ph3P]O (2, 98%) were
obtained.
Table 6
Synthesis and electroreduction of [Ph3PeOSiMe3]þ[OTf]ꢀ 7a
In a similar manner, electroreduction of Ph3PX2 3 in a divided
cell was carried out under several electroreduction conditions. The
results are summarized in Table 1.
3.3. Electroreduction of 3a in an undivided cell (Table 2,
entry 1)
Entry
Me3SiOTf (mL, mmol)
Ratio,b 1/2
1
2
3
4
0.37, 2.0
0.73, 4.0
1.11, 6.0
1.47, 8.0
35:47
48:44
65:35
66:34
Into an undivided cell equipped with an Al plate anode
(1.5ꢁ1.0 cm2), a Pt plate cathode (1.5ꢁ1.0 cm2) and a stirring bar
was placed an MeCN (10 mL) solution of Ph3PCl2 (3a, 167 mg,
0.5 mmol) and Bu4NBF4 (165 mg, 0.5 mmol). The solution was
electrolyzed at room temperature under constant current condi-
tions (50 mA, 33 mA/cm2) until 2 F/mol-3a of electricity was passed
(32 min). After work-up as described above, purification by column
chromatography (SiO2, hexane/AcOEt¼5:1, 1:1) afforded 1 (45 mg,
0.17 mmol, 34%) as colourless solids.
a
Compound 2 (2.0 mmol), room temp.
b
Determined by 31P NMR.
3. Experimental section
3.1. General
1H and 13C NMR spectra were recorded on a Varian MERCURY
300 (1H: 300 MHz, 13C: 75 MHz) spectrometer in CDC13 (1H: Me4Si
3.3.1. Triphenylphosphine (1). Colourless solids; Rf¼0.80 (hexane/
AcOEt¼1:1); 1H NMR (300 MHz, CDCl3)
d
7.28e7.35 (m, 15H); 13C
(d
¼0), 13C: CDCl3
(d
¼77.0)). 31P NMR spectra were recorded on
NMR (75 MHz, CDCl3)
d
128.45 (d, J¼6.8 Hz), 128.66, 133.69 (d,
J¼19.5 Hz), 137.10 (d, J¼9.8 Hz); 31P NMR (120 MHz, CDCl3)
ꢀ4.91;
d
a Varian MERCURY 300 (120 MHz) spectrometer in CDC13 and/or
IR (KBr) 3066, 1638, 1308, 1281, 1155 cmꢀ1
.
CH3CN/C6D6 (85% H3PO4
(
d¼0) as an external standard). Infrared
In a similar manner, electroreduction of Ph3PX2 in an undivided
cell was carried out under several conditions. The results are
summarized in Tables 2 and 3.
(IR) spectra were recorded on a JASCO FT/IR-4100 spectrometer.
Elemental analysis was executed on PerkineElmer PE 2400 Series II
CHNS/O analyzer. Gas chromatography (GC) was performed with
SHIMADZU GC-2014 instrument equipped with two packed
columns (Silicon SE-52 (5%), Chromosorb W 80-100 AW-DMCS,
2.1 mꢁ3.2 mm I.D.) using N2 gas as an eluent. Analytic thin layer
chromatography was performed on Merck, pre-coated plate silica
gel 60 F254 (0.25 mm thickness). Column chromatography was
3.4. One-pot conversion of 2 to 1 by chlorination/
electroreduction (Table 4, entry 1)
Into an undivided cell equipped with an Al plate anode
(1.5ꢁ1.0 cm2), a Pt plate cathode (1.5ꢁ1.0 cm2) and a stirring bar
was placed an MeCN (5 mL) solution of 2 (1391.1 mg, 5 mmol) and
Bu4NOTf (195.8 mg, 0.5 mmol). To the solution was added oxalyl
chloride (0.43 mL, 5 mmol) at room temperature, and the mixture
was stirred for 10 min. The solution was electrolyzed at room
temperature under constant current conditions (50 mA, 33 mA/
cm2) until 3 F/mol-2 of electricity was passed (8 h). Work-up and
purification by column chromatography (SiO2, hexane/AcOEt¼5:1,
1:1) afforded 1 (970 mg, 3.7 mmol, 74%) as colourless solids, and 2
(83 mg, 0.3 mmol, 6%) was recovered.
performed on KANTO CHEMICAL silica gel 60N (40e50
Cyclic voltammetry (CV) was measured on the BAS 100B/W
potentiostat (BAS, Tokyo, JAPAN). A platinum electrode (3 mm ),
a platinum wire electrode (1 mm
) and an Ag/Agþ electrode (Ag
mm).
f
f
wire in 0.01 M MeCN solution of AgNO3, and Bu4NClO4) were used
as a working, a counter and a reference electrode, respectively. The
working electrode was polished with 5
with 0.5 m alumina slurry, successively, and washed with deion-
ized water and acetone.
mm diamond slurry and
m
Unless otherwise noted, all materials were purchased from
commercial suppliers, and reagent grade materials were used
without further purification. All reactions were performed in dry
In a similar manner, one-pot conversion of 2 was carried out
under several conditions. The results are summarized in Table 4.

5830
M. Kuroboshi et al. / Tetrahedron 67 (2011) 5825e5831
3.5. Synthesis and electroreduction of [Ph3PCl]D[PCl2O2]L
5
and aq 5% HCl, and extracted with AcOEt (3ꢁ10 mL). The combined
organic layers were washed with brine, dried over Na2SO4 and
concentrated under reduced pressure. The residue was analyzed by
31P NMR (120 MHz, CDCl3) to find that 1 and 2 were obtained in
66:34 ratio.
Into a Schlenk flask equipped with a magnetic stirring bar,
a three-way cock and a rubber septum was placed triphenylphos-
phine oxide 2 (565 mg, 2.0 mmol). The whole apparatus was
purged with argon. To the mixture were added MeCN (2 mL) and
POCl3 (0.18 mL, 2.0 mmol), successively, at room temperature. The
mixture was stirred at room temperature for 1 h. The electrolysis
was carried out using an aluminium plate anode (1.5ꢁ1.0 cm2) and
a platinum plate cathode (1.5ꢁ1.0 cm2) under constant current
(50 mA, 93 min, 3 F/mol-2) conditions. The reaction mixture was
poured into a mixture of ice and aq 5% HCl, and extracted with
AcOEt (3 ꢁ10 mL). The combined organic layers were washed with
brine, dried over Na2SO4 and concentrated under reduced pressure.
The residue was analyzed by 31P NMR (120 MHz, CDCl3) to find that
3.8. Cyclic voltammetry of Ph3PBr2 3b (Fig. 1b)
The working, counter and reference electrodes were dipped into
an MeCN solution of Ph3PBr2 3b (10 mM) containing Bu4NOTf
(0.1 M) as a supporting electrolyte in a beaker-type electrochemical
cell. The solution was degassed with argon for 10 min before
measurement. Cyclic voltammogram was measured in a potential
range from 0.0 V to ꢀ2.8 V versus Ag/Agþ at a scan rate of 300 mV/s.
triphenylphosphine 1 (
62:38 ratio.
d
ꢀ5.02) and 2 (d 29.6) were obtained in
References and notes
1. (a) Maercker, A. Organic Reactions; John Wiley & Sons: New York, NY, 1965;
Vol. 14, Chapter 3; (b) Boutagy, J.; Thomas, R. Chem. Rev. 1974, 74, 87.
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Corey, E. J.; Nicolaou, K. C. J. Am. Chem. Soc. 1974, 96, 5614; (c) Corey, E. J.;
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3.5.1. Tris(p-tolyl)phosphine (1b). Colourless solids; Rf¼0.63 (hex-
ane/AcOEt¼5:1); 1H NMR (300 MHz, CDCl3)
d
7.55e7.13 (m, 12H),
2.34 (s, 9H); 13C NMR (75 MHz, CDCl3)
d 138.38, 134.63, 134.38,
134.14, 132.46, 130.18, 128.19, 20.82; 31P NMR (120 MHz, CDCl3)
d
ꢀ4.73; IR (KBr) 3068, 3013, 2971, 2920, 2865, 2732, 2359, 1917,
1814, 1658, 1595, 1496, 1450, 1394, 1352, 1307, 1272, 1211, 1186, 1118,
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4. (a) Appel, R. Angew. Chem., Int. Ed. Engl. 1975, 14, 801; (b) Calzada, J. G.; Hooz, J.
Org. Synth. 1974, 54, 63.
476 cmꢀ1
.
5. Staudinger, H.; Meyer, J. Helv. Chim. Acta 1919, 2, 635; Gololobov, Y. G.;
Zhmurova, I. N.; Kasukhin, L. F. Tetrahedron 1981, 37, 437; Scriven, E. F. V.;
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7. As an another approach, catalytic Wittig reaction using 3-methyl-1-
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3.5.2. Tris(m-tolyl)phosphine (1c). Colourless solids; Rf¼0.63 (hex-
ane/AcOEt¼5:1); 1H NMR (300 MHz, CDCl3)
d
7.36e7.17 (m, 12H),
2.41 (s, 9H); 13C NMR (75 MHz, CDCl3)
d 137.74, 137.64, 137.16,
137.01, 134.44, 134.15, 130.69, 130.46, 129.31, 128.23, 128.15, 21.26;
31P NMR (120 MHz, CDCl3)
ꢀ7.38; IR (KBr) 3074, 3046, 3020, 2994,
2951, 2917, 2360, 1952, 1884, 1693, 1589, 1474, 1449, 1400, 1305,
1269, 1216, 1169, 1103, 1038, 998, 884, 781, 696, 542, 519 cmꢀ1
d
.
8. Fritzsche, H. Chem. Ber. 1965, 98, 171.
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10. Marsi, F. M. J. Org. Chem. 1974, 39, 265.
3.6. Synthesis and electroreduction of [Ph3POMe]DOTfL
6
11. Horner, L.; Hoffmann, H.; Beck, P. Chem. Ber. 1958, 91, 1583.
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Into a Schlenk flask equipped with a magnetic stirring bar,
a three-way cock and a rubber septum was placed triphenylphos-
phine oxide 2 (568 mg, 2.0 mmol). The whole apparatus was
purged with argon. To the mixture were added MeCN (2 mL) and
MeOTf (0.22 mL, 2.0 mmol), successively, at room temperature to
give compound 6 (31P NMR
d 66.2 ppm). The mixture was stirred at
17. Handa, Y.; Inanaga, J.; Yamaguchi, M. J. Chem. Soc., Chem. Commun. 1989, 298.
18. Mathey, F.; Maillet, R. Tetrahedron Lett. 1980, 21, 2525.
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room temperature for 1 h. The electrolysis was carried out using an
aluminium plate anode (1.5ꢁ1.0 cm2) and a platinum plate cathode
(1.5ꢁ1.0 cm2) under constant current (50 mA, 93 min, 3 F/mol-2)
condition. The reaction mixture was poured into a mixture of ice
and aq 5% HCl, and extracted with AcOEt (3ꢁ10 mL). The combined
organic layers were washed with brine, dried over Na2SO4 and
concentrated under reduced pressure. The residue was analyzed by
31P NMR (120 MHz, CDCl3) to find that triphenylphosphine 1, 2 and
Reduction Synthesis; Kodansha
& Wiley-VCH: Tokyo and Weinheim, 2006;
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ratio.
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3.7. Synthesis and electroreduction of [Ph3POSiMe3]DOTfL
7
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821; (b) Hermeling, D. H. R.; Lechtken, P.; Rotermund, G. W.; Siegel, H. Ger.
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Into a Schlenk flask equipped with a magnetic stirring bar,
a three-way cock and a rubber septum was placed triphenylphos-
phine oxide 2 (568 mg, 2.0 mmol). The whole apparatus was
purged with argon. To the mixture were added MeCN (2 mL) and
Me3SiOTf (1.47 mL, 8.0 mmol), successively, at room temperature to
give compound 7 (31P NMR 51.3).40 The mixture was stirred at
d
room temperature for 1 h. The electrolysis was carried out using an
aluminium plate anode (1.5ꢁ1.0 cm2) and a platinum plate cathode
(1.5ꢁ1.0 cm2) under constant current (50 mA, 93 min, 3 F/mol-2)
conditions. The reaction mixture was poured into a mixture of ice
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Elguero, J. J. Phys. Chem. A 2005, 109, 4359.
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phosphine oxide together with substitution products CH3eNu, while
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