A practical one-pot transformation of triphenylphosphine oxide to triphenylphosphine by reduction of in situ generated triphenylphosphine dichloride

Article abstract of DOI:10.1055/s-0029-1219194

One-pot transformation of triphenylphosphine oxide to triphenylphosphine was achieved by the reaction of triphenylphosphine oxide with oxalyl chloride, which led to the formation of triphenylphosphine dichloride, and subsequent reduction of triphenylphosphine dichloride with a combination of aluminum-catalytic metal salt.

Full text of DOI:10.1055/s-0029-1219194

LETTER
801
A Practical One-Pot Transformation of Triphenylphosphine Oxide to
Triphenylphosphine by Reduction of in situ Generated Triphenylphosphine
Dichloride
T
ransformation
o
of Tripheny
m
lphosphine Oxide to
o
Triphenylph
t
osphin
a
e
ke Yano, Masakatsu Hoshino, 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
Fax +81(86)2518079; E-mail: tanaka95@cc.okayama-u.ac.jp
Received 9 November 2009
direct reduction
Abstract: One-pot transformation of triphenylphosphine oxide to
triphenylphosphine was achieved by the reaction of triphenylphos-
phine oxide with oxalyl chloride, which led to the formation of
triphenylphosphine dichloride, and subsequent reduction of triphe-
nylphosphine dichloride with a combination of aluminum–catalytic
metal salt.
Si-H, Al-H, etc.
Ph
P
Ph
P
1
Wittig, Mitsunobu
Ph
Ph
Ph
Ph
Mukaiyama–Corey
Appel, Staudinger, etc.
2
O
Key words: triphenylphosphine, triphenylphosphine oxide, reduc-
tion, aluminum, multi redox system
Ph
COCl2
(COCl)2, etc.
Mtl,
Si-H, Al-H, etc.
Ph
Cl Cl
indirect reduction
P
Ph
3
Triphenylphosphine (1) is a versatile reagent and has been
frequently used in various organic reactions, for example,
Wittig reaction,1 Mitsunobu reaction,2 Mukaiyama–
Corey lactonization,3 Appel reaction,4 and Staudinger
reaction.5 In these reactions, 1 is converted into triphe-
nylphosphine oxide (2). Thus, a large amount of 2 is pro-
duced as waste material. Therefore, the development of a
facile method for reducing 2 to 1 is desired to render these
reactions environmentally benign and sustainable.
Scheme 1 Recycling use of triphenylphosphine
ed procedures and special care should be taken especially
when a large amount of 2 is treated.19
Aluminum is an ideal reducing agent because it is cheap,
air- and moisture-stable, and environmentally benign.
Aluminum releases three electrons per atom and acts as a
powerful reductant (standard redox potential = –1.662 V).
However, the use of aluminum in organic synthesis is
rather limited because it is easily deactivated in air by
forming a thin oxide layer on its surface. Hermeling car-
ried out the reduction of 3 to 1 by using a fine aluminum
powder (0.1–2 mm Ø).20 A fine aluminum powder is es-
sential for the efficient reduction of 3; however, the alu-
minum powder thus formed is explosive and special care
is required.
Direct reduction of 2 to 1 has been achieved successfully
using various reductants including metal hydrides such as
lithium aluminum hydride,6 trichlorosilane,7 and meth-
ylpolysiloxane,8 low-valent metals such as magnesium/
titanocene dichloride,9 metal salts such as samarium io-
dide,10 nonmetallic organic reagents such as hydrocarbon/
activated carbon11 and hexaethylphosphorus triamide/
phosphorus oxytrichloride,12 and electroreduction.13 The
reduction methods developed thus far are, however, not
practical because they always require stoichiometric
amounts of reducing agents that are expensive, explosive,
and/or are not easy to handle.
We have developed chemical transformations promoted
by a combination of aluminum and a catalytic metal salt,
wherein aluminum acts as an electron pool and the cata-
lytic amount of metal salt acts as a mediator for electron
transfer from aluminum to substrates.21 Indeed, various
combinations of aluminum and metal salts such as lead(II)
bromide, nickel(II) chloride, and chromium(III) chloride
have been developed to promote carbon–carbon bond for-
mation as well as highly selective functionalization. These
reactions have been successfully employed for the synthe-
sis of useful compounds such as b-lactam antibiotics and
b-lactamase inhibitors.
As an alternative method, indirect reduction of 2 to 1 via
triphenylphosphine dichloride 3 has also been examined
(Scheme 1). Triphenylphosphine dichloride (3) has been
prepared by the treatment of 2 with chlorination reagents
such as phosphorus pentachloride,6a phosgene,14 diphos-
gene,15 triphosgene,16 and oxalyl chloride.17 Reduction of
3 to 1 has been carried out by the use of metallic reduc-
tants such as sodium17 and metal hydrides such as
LiAlH4.18 Again, these methods do not necessarily pro-
duce satisfactory results because they involve complicat-
In this communication, we describe the one-pot reduction
of 2 to 1 by the chlorination of 2 to 3 with oxalyl chloride
and the subsequent reduction of 3 generated in situ using
an aluminum/metal salt redox system, thereby offering a
facile and simple recycling system of triphenylphosphine
(1).
SYNLETT 2010, No. 5, pp 0801–0803
1
6.
3
.2
0
1
0
Advanced online publication: 14.01.2010
DOI: 10.1055/s-0029-1219194; Art ID: U10909ST
© Georg Thieme Verlag Stuttgart · New York

802
T. Yano et al.
LETTER
A typical reaction procedure is described as follows Table 2 Effect of Solvent on Al/PbBr2 Systema
(Table 1, entry 1). To a mixture of 2 (1.4 g, 5.0 mmol),
Entry
Solvent
MeCN
DME
Yield of 1 (%)b
finely cut aluminum foil (137 mg, 5.0 mmol, ca. 1×1×0.012
mm3), and acetonitrile (5 mL) was added oxalyl chloride
(0.46 mL, 5.1 mmol) dropwise at room temperature under
argon atmosphere. The resulting mixture was stirred for
10 minutes at room temperature. 31P NMR analysis of the
aliquot of the mixture revealed that chlorination of 2 pro-
ceeded smoothly to give 3 (d = 50.1 ppm) quantitatively.
To the resulting mixture was added lead(II) bromide (19.9
mg, 0.05 mmol) in one portion, and the mixture was
stirred for 6 minutes at ambient temperature to give 1
(1276 mg, 4.9 mmol, 93%).22
1
2
3
4
5
6
95
79
78
57
7
CHCl3
PhCl
MeCOEt
DMF
1
a Conditions: 2 (5 mmol), (COCl)2 (100 mol%), finely cut aluminum
foil (400 mol%), PbBr2 (1 mol%), MeCN (5 mL), r.t., 0.1 h.
b Yields of isolated 1.
Table 1 One-Pot Reduction of 2 to 1 via 3 by Using Aluminum–
Catalytic Metal-Salt Redox Systema
(COCl)2
MeCN
Al
additive
ed in the presence of 2-butanone (MeCOEt) and N,N-di-
methylformamide (DMF, entries 5 and 6).
Ph
Ph
Ph
Ph Cl
Ph
Ph
P
O
Ph
P
P
r.t.
10 min
r.t.
time
Ph Cl
Ph
The reduction reaction was carried out using different
shapes of commercially available aluminum specimens
such as an aluminum foil, powder, a plate, and pellets
(Table 3). As the finely cut aluminum foil (1×1×0.012
mm3, entry 1) and aluminum powder (200 mesh, entry 2)
had a large surface area, reduction from 3 to 1 was com-
pleted using only 1 molar equivalent of aluminum. On the
other hand, the reduction did not proceed efficiently by
using aluminum plate and pellets because their surface
area was considerably small: indeed, a grain of an alumi-
num pellet (4 mmØ×6 mm, ca. 200 mg, 7.4 mmol) gave 1
in 49% yield after 1 hour (entry 4). The yield of 1 in-
creased by using a large excess amount of aluminum plate
and pellets: an aluminum plate (30×5×0.5 mm3, entry 3)
and pellets (4 mmØ×6 mm, 79 mmol, 16 mol equiv, entry
5) gave 1 in 94% and 96% yields, respectively, within 6
minutes.
2
3
1
Entry Additive mol%
Time
Yield (%)b Yield (%)b
of 1
of 2
1
2
3
4
5
6
7
PbBr2
none
1
6 min 93
4
0
24 h
1 h
1 h
1 h
1 h
1 h
7
82
SnCl2
BiCl3
FeCl3
ZnCl2
NiCl2
2
97
2
1
67
31
2 (5)d
2 (25)d
1
ND (46)d
ND (53)d
ND
94 (42)d
93 (43)d
97
c
a Conditions: 2 (5 mmol), (COCl)2 (100 mol%), finely cut aluminum
foils (400 mol%), additive, MeCN (5 mL), r.t.
b Yields of isolated compounds.
c Anhydrous.
Table 3 Shape of Al Specimen and Recycle Use of Al Pelletsa
d After 24 h.
Entry Al
mmol PbBr2 Yield 1
(mol%) (%)b
The metal salts showed considerable effects on the reac-
tion (Table 1). In the absence of lead(II) bromide, the re-
duction reaction hardly proceeded and only 7% of 1 was
obtained even after 24 hours (entry 2). Tin(II) chloride
and bismuth(III) chloride also effectively promoted the
reduction reaction, affording 1 in 97% and 67% yields, re-
spectively (entries 3 and 4). The reduction did not proceed
efficiently when iron(III) chloride, zinc(II) chloride, and
nickel(II) chloride were used (entries 5–7). Thus, 1 was
not detected and most of 2 (93–97%) was recovered after
1 hour, while 50% of 1 was obtained after 24 hours.
1
2
3
4
5
6
7
finely cut foil (1 × 1 × 0.012 mm3)
5
5
1
1
1
1
1
0
0
95
88
94
49
96
93
92
powder (200 mesh)
plate (30 × 5 × 0.5 mm3)
pellet (3–5 mm Ø)c
pellet (3–5 mm Ø)d
pellet (3–5 mm Ø)e,f
pellet (3–5 mm Ø)e,g
75
5
79
76
72
a Conditions: 2 (5 mmol), (COCl)2 (100 mol%), MeCN (5 mL), r.t.,
The proper choice of solvents was also important
(Table 2). Among the solvents examined, acetonitrile was
the best solvent to give 1 quantitatively (entry 1), whereas
1,2-dimethoxyethane (DME), chloroform (CHCl3), and
chlorobenzene (PhCl) gave 1 in moderate yields (79–
57%, entries 2–4). The reduction reaction hardly proceed-
6 min.
b Yields of isolated 1.
c 1 h.
d 1st run.
e In absence of additional PbBr2.
f 2nd run.
g 3rd run.
Synlett 2010, No. 5, 801–803 © Thieme Stuttgart · New York

LETTER
Transformation of Triphenylphosphine Oxide to Triphenylphosphine
803
It is interesting to note that after the reaction (entry 5), ex- A combination of aluminum and a catalytic amount of
cess aluminum pellets were recovered easily by filtration metal salt reduced triphenylphosphine dichloride to triph-
and they could be used repeatedly without adding lead(II) enylphosphine quantitatively under mild conditions. Alu-
bromide (entries 6 and 7). Thus, chlorination–reduction minum was activated by the metal salt, and the activated
(vide supra) of 2 (5 mmol) with aluminum pellets (79 aluminum was recovered and used repeatedly without fur-
mmol) in the presence of lead(II) bromide (0.5 mmol) ther activation. This indirect reduction offers an efficient
gave 1 in 96% yield (first run) with recovered aluminum regeneration system of triphenylphosphine.
pellets (76 mmol). The recovered aluminum pellets were
used again for the second run. It is notable that the second
run proceeded smoothly in the absence of lead(II) bro-
References and Notes
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2% yield.
A plausible reaction mechanism is explained as follows
(Scheme 2). As confirmed by 31P NMR, 2 was firstly con-
verted into 3 by the treatment with oxalyl chloride. Thus,
the in situ generated 3 was reduced with aluminum/cata-
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2 Al + 3 PbX2
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Pb0
PbX2
Ph
Ph
Ph
Ph
Ph
Ph
Ph Cl
Ph
P
Cl
(COCl)2
AlX3
P
P
O
Ph
P
Ph
Ph
Ph Cl
AlBr3Cl–
4
2
3
1
X = Cl and/or Br
Scheme 2 A plausible mechanism
(19) Recently, the authors reported electrochemical reduction of
triphenylphosphine dichloride to triphenylphosphine. Yano,
T.; Kuroboshi, M.; Tanaka, H. Tetrahedron Lett. in press.
(20) Hermeling, D.; Bassler, P.; Hammes, P.; Hugo, R.;
Lechtken, P.; Siegel, H. EP 638580, 1995; Chem. Abstr.
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(21) Tanaka, H.; Kuroboshi, M. Curr. Org. Chem. 2004, 8, 1027;
and references cited therein.
(22) Triphenylphosphine oxide 2 was obtained by the hydrolysis
of triphenylphosphine dichloride 3 in the workup procedure.
The surface of aluminum was activated by the treatment
of lead bromide. The activated aluminum surface also act-
ed as a reducing reagent for the dechlorination of 4. In-
deed, the aluminum pellets recovered in the previous run
promoted the reductive dechlorination of 3 without fur-
ther addition of lead bromide.
The treatment of triphenylphosphine oxide with oxalyl
chloride immediately gave triphenylphosphine dichloride.
Synlett 2010, No. 5, 801–803 © Thieme Stuttgart · New York

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